The Morphology of Xenarthrous Vertebrae (Mammalia: Xenarthra)

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Geology NEW SERIES, NO. 41

Timothy J. Gaudin

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September 30, 1999 Publication 1505

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Geology NEW SERIES, NO. 41

The Morphology of Xenarthrous Vertebrae (Mammalia: Xenarthra)

Timothy J. Gaudin

Department of Biological and Environmental Sciences University of Tennessee at Chattanooga 615 McCallie Avenue Chattanooga, Tennessee 37403-2598 U.S.A. 1

Research Associate Department of Geology Field Museum of Natural History Roosevelt Road at Lake Shore Drive Chicago, Illinois 60605-2496 U.S.A.

1 Address to which correspondence should be sent.

Accepted September 3, 1998 Published September 30, 1999 Publication 1505

PUBLISHED BY FIELD MUSEUM OF NATURAL HISTORY © 1999 Field Museum of Natural History ISSN 0096-2651 PRINTED IN THE UNITED STATES OF AMERICA Table of Contents lumbar vertebrae in anterior and posterior view 8

4. Zaedyus pichiy, stereophotographs of

Abstract 1 thoracic and lumbar vertebrae in right

Introduction 1 lateral view 10 List of Abbreviations 4 5. Tolypeutes matacus (juvenile), thoracic Descriptive Anatomy 4 and lumbar vertebrae in left lateral Cingulata 4 view 14 Euphracta (esp. Zaedyus pichiy) 4 6. Tolypeutes matacus (juvenile), sacral Tolypeutes matacus 13 vertebrae in dorsal, ventral, and right Other Cingulates 15 lateral views 16 Vermilingua 17 7. Priodontes maximus, first lumbar verte- Tamandua mexicana 17 bra in anterior view 17 Other 19 Vermilinguas 8. Tamandua mexicana, stereophoto- 19 Tardigrada graphs of posterior thoracic vertebrae 19 Bradypus variegatus in right lateral view 18 21 Hapalops 9. Bradypus variegatus, stereophotographs Other 24 Tardigrada of thoracic and lumbar vertebrae in Conclusions 26 right lateral view 20 Morphological Summary 26 10. Hapalops sp., posterior thoracic verte- and Evolution of Xenarthrous Phylogeny brae in dorsal view 22 Vertebrae 27 11. Hapalops sp., posterior thoracic verte- Relationship of Xenarthra to Early Ceno- brae in left lateral view 22 zoic Fossil Taxa 30 12. Pronothrotherium isolated Acknowledgments 33 typicum, mid-thoracic vertebra in dorsal view 24 Literature Cited 33 of Appendix: Summary of morphological 13. Diagrammatic representation typical Data 36 xenarthran intervertebral facets in anterior and posterior view 26 14. Distribution of vertebral character states on a phylogeny of the Xenarthra 28 List of Illustrations

Mephitis mephitis, thoracic and lumbar List of Tables vertebrae in anterior, posterior, and left lateral views Zaedyus pichiy, Tamandua mexicana, and Bradypus variegatus, thoracic and 1 . Ratio of maximum width to anteropos- lumbar vertebrae in dorsal view terior length of anterior zygapophyseal Zaedyus pichiy, Tamandua mexicana, facets in anterior thoracic vertebrae of and Bradypus variegatus, thoracic and Xenarthra 23

The Morphology of Xenarthrous Vertebrae (Mammalia: Xenathra)

Timothy J. Gaudin

Abstract

The presence of supplementary intervertebral articulations termed "xenarthrales" in the posterior dorsal vertebrae has been considered perhaps the most important diagnostic feature of the mammalian order Xenarthra. Xenarthrales are poorly understood, however, and substantial confusion exists in the literature over which facets are supplementary and which are not. Furthermore, much of the variation that exists in these joints, both within taxa and among the various xenarthran lineages, has gone unnoticed. Finally, the structural evolution of these facets has been inadequately treated. The goal of the present study is to describe the morphology of xenarthrous vertebrae in juvenile and adult extant xenarthrans and in extinct xenarthrans, to develop a model for the structural evolution of the supernumerary joints, and to use this information to assess the affinities of several enigmatic groups of early Cenozoic taxa (Palaean- odonta, Ernanodon, and Eurotamandua) with purported ties to the Xenarthra. Vertebral morphology is described in detail for two armadillo species, one species of anteater, and one extant and one extinct species of sloth, with brief comments on other xenarthran taxa. The results suggest that all xenarthrans are characterized by two sets of zygapophyseal facets in the postdiaphragmatic vertebrae, one medial and one lateral to the metapophysis. In addition, the Xe- narthra is characterized primitively by a pair of xenarthrous facets on each side of the vertebra between the dorsal surface of the anapophysis and the ventral surface of the metapophysis of the succeeding vertebra. Other xenarthrous joints evolve within various xenarthran lineages. It is suggested that the supplementary facets developed initially in the diaphragmatic region of the vertebral column by means of a progressive widening of the zygapophyseal facets in the thoracic vertebrae and an increase in size of the metapophysis, which subdivided the zygapophyseal facets into medial and lateral facets. Hypertrophy of the anapophyses and their contact with the metapophyses led to the formation of true xenarthrous facets. A review of vertebral morphology in the Palaeanodonta, Ernanodon, and Eurotamandua revealed few resemblances to undoubted xenarthrans beyond hypertrophy of the metapophyses and anapophyses—characteristics known to occur in many different groups of mammals. No supplementary intervertebral articulations could be documented unequivocally in any of these taxa. Thus, on the basis of vertebral morphology there is little evidence that would suggest a close phylogenetic relationship between true xenarthrans and palaeanodonts, Ernanodon, or Eurotamandua.

Introduction The distinctive nature of the vertebral column in the mammalian order Xenarthra was recog- The most character- single important osteological nized in the earliest osteological descriptions of istic of Xenarthra is the of ar- presence accessory , ^ ( Cuyi 836a) In most mammals ticular processes or anapophyses, which articulate • ~7 . • \ i«_ successive vertebrae are not an in- ventral to the metapophyses, or between them and jotned only by tervertebral but also a of the transverse processes, of the following vertebrae disc, by single pair sy- (Rose & Emry, 1993, p. 87). novial joints carried on more or less distinct ar-

FIELDIANA: GEOLOGY, N.S., NO. 41, SEPTEMBER 30, 1999, PP. 1-38 1 ns

B ns ns

Fig. 1. Mephitis mephitis, utcm 521: thoracic and lumbar vertebrae shown in anterior, posterior and left lateral = views (proceeding left to right). A, T4, B, L3. Scale bar 1 cm. Abbreviations: ap, anapophysis; az, anterior zygapophyseal facets; dp, diapophysis; la, lamina; mp, metapophysis; ns, neural spine; pe, pedicel; pz, posterior zygapophyseal facet; rf, rib facet; sn, notch for emergence of spinal nerve; tp, transverse process; vc, vertebral centrum.

ticular processes of the neural arches termed zyg- these xenarthrous articulations, their almost uni- apophyses (Fig. 1). In addition to the typical zyg- versal presence among living and fossil xenar- apophyseal articulations, all xenarthrans possess thrans, and the near universal absence of similar one or more pairs of supplementary intervertebral supplementary intervertebral joints in other mam- articulations that are usually present between all mals (but see Scutisorex: Lessertisseur & Saban, lumbar and a variable number of posterior thorac- 1967; Kingdon, 1984; Cullinane & Aleper, 1998; ic vertebrae. The supernumerary articulations, Cullinane et al., 1998), the presence or absence of termed "xenarthrales," are well developed in xe- xenarthrales has been used as a "litmus test" for narthrans that span a wide range of locomotory determining phylogenetic relatedness to the Xe- habits, including a subterranean armadillo (Chla- narthra. The pangolins and aardvarks were origi- myphorus), fossorial armadillos and anteaters nally included with xenarthrans under the taxo- (e.g., Dasypus, Euphractus, Myrmecophaga), ar- nomic grouping Edentata, but they were subse- boreal climbing anteaters (Tamandua, Cyclopes), quently removed to separate orders largely be- and semiarboreal (e.g., Hapalops; White, cause they lacked xenarthrales (Weber, 1904; see 1993a,b) to fully terrestrial (e.g., Mylodon, Me- Hoffstetter, 1982, and Glass, 1985, for history of gatherium) extinct ground sloths. Xenarthrales are edentate classification). Several enigmaic groups present in the oldest well-known fossil xenarthran of early Cenozoic mammals have been linked to skeleton, the Casamayoran armadillo Utaetus the Xenarthra on the basis of "incipient" devel- (Simpson, 1948). The supplementary joints are opment of xenarthrales. These include the Pa- strongly reduced only in the suspensory tree laeanodonta (Simpson, 1931), a group known sloths, and they are absent only in the glypto- from Paleocene to Oligocene deposits of North donts; in the latter the dorsal portions of the back- America and Europe, and Ernanodon (Ding, bone are fused into a bony tube used to support 1987), a Late Paleocene genus from China. Both the massive carapace of the animals (Hoffstetter, Ernanodon and the palaeanodont Metacheiromys 1958; Gillette & Ray, 1981). have enlarged anapophyses in the posterior dorsal Because of the peculiar and complex nature of vertebrae. Similarly enlarged anapophyses can be

FIELDIANA: GEOLOGY found, however, among a number of unrelated 1975; Engelmann, 1985; Vaughn, 1986; Rose & groups of mammals, e.g., felids and geomyid ro- Emry, 1993). The joints have also been referred dents (Rose & Emry, 1993). Hence such process- to by the noun "xenarthrales" (e.g., Frechkop, es are not necessarily structural antecedents of 1949; Grasse, 1955; Hoffstetter, 1958; DeBlase & 3 true xenarthrous articulations. The purported Mid- Martin, 1981; Storch 1981), and the condition of dle Eocene anteater Eurotamandua, from the possessing such joints has been termed "xenar- Messel fauna of Germany, allegedly possesses thry" (e.g., Lessertisseur & Saban, 1967; Hoff- true xenarthrous articulations (Storch, 1981). Un- stetter, 1982; Novacek & Wyss, 1986). The first fortunately, several subsequent authors have been detailed morphological description of xenarthrous unable to verify the presence of accessory inter- intervertebral articulations (Owen, 1851a) preced- vertebral articulations in this taxon (Rose & ed Gill's work by some 20 years. Owen described Emry, 1993; Szalay & Schrenk, 1994). Novacek the morphology of the supplementary interverte- and his colleagues (Novacek, 1986; Novacek & bral articulations in at least two extant species Wyss, 1986; Novacek et al., 1988) resurrected the from each of the three major xenarthran subor- taxonomic grouping Edentata, including pango- ders, the Cingulata (armadillos), Vermilingua lins, palaeanodonts, and xenarthrans, in a com- (Neotropical anteaters), and Tardigrada (sloths). mon supraordinal cohort based on the results of Moreover, Owen described regional variation in morphological studies of eutherian interordinal the morphology of the extra intervertebral joints phylogeny. Their work has led them to suggest along the backbone of individual species. Owen that the phylogenetic significance of xenarthrales (1851a) began each description with the anterior- may be overemphasized. most xenarthrous vertebra and then described how Part of the difficulty in determining the phylo- the morphology of the facets changed as one genetic significance of xenarthrales, and in iden- moved posteriorly along the spine. tifying xenarthrales or incipient xenarthrales in Flower's (1885) description was similar but early Cenozoic taxa potentially allied to Xenar- much briefer than Owen's. He described the mor- thra, lies in the fact that the morphology of these phology of the xenarthrous articulations along the articulations among unquestioned xenarthrans is whole length of the vertebral column, but only in not well understood. the vermilinguan Myrmecophaga (Flower, 1885). As stated above, the presence of accessory in- He also briefly summarized the form of the xe- tervertebral articulations in xenarthrans has been narthrous facets in the sloth genus Bradypus. In- noted since the early nineteenth century. Cuvier terestingly, Rower differed from Owen in decid-

1 (1836a) wrote the first brief description of such ing which facets to designate as the normal zyg- joints. They were not formally named, however, apophyseal facets and which to designate as sup- 2 until Gill (1886, p. 66) coined the term "xenar- plementary. Owen (1851a) consistently = = thral" (Gk., xenos "strange," arthron recognized the medialmost pair of intervertebral "joint") in order to distinguish xenarthran verte- facets, those lying medial to the metapophyses, as brae from normal, "nomarthral" vertebrae. Simp- the normal zygapophyseal facets. Flower (1885, son (1931, 1948) used the adjective "xenar- figs. 22-24) designated a set of facets lying lateral throus" to refer to these accessory joints. This to the metapophyses as the homologues of the adjective is the one most commonly used in the typical mammalian zygapophyses. recent literature (e.g., Emry, 1970; McKenna, As noted by Rose and Emry (1993), the confusion over which set of facets constitutes supple- articulations and which are the normal 1 mentary Curiously, the joints are not described in a contem- facets has to the poraneous edition of Cuvier's Recherches sur les osse- zygapophyseal persisted present. mens fossiles, where, e.g., the vertebrae of the anteater The designation of a lateral facet as the zygapo- are described as "unremarkable" Myrmecophaga (Cu- physeal facet by Flower is followed by Grasse vier, 1836b, p. 208). and iden- 2 (1955) Vaughn (1986). Owen's (1851a) Glass (1985) cites Gill (1872) as the source of the tification of the as more me- term. I can find no mention of the term, however, in my zygapophyses lying copy of Gill's mammalian classification. Indeed, Gill's 1 classification is incomplete, because it contains neither Gaudin and Biewener (1992) and Gaudin (1993) re- a discussion of family and subfamily characteristics nor fer to the joints as "xenarthrae," a term which I had a list of genera for any of the Ineducabilia, a group in- assumed was standard usage. Unfortunately, I can now cluding the Bruta (= Xenarthra + Pholidota + Tubuli- find no historical source for the term. Although I do not dentata). Vertebral morphology is not mentioned in the believe I invented the term, I have chosen to abandon it ordinal level description of Bruta. for the apparently more widely used "xenarthrales."

GAUDIN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE dially is followed by Hoffstetter (1958, 1982), ground sloth genera from other North American Lessertisseur and Saban (1967), and Gaudin and museums. The descriptions provided below, how- Biewener (1992). Jenkins (1970) notes that in Ta- ever, will focus on five representative taxa: the mandua the distinct facets designated as "zyg- extant armadillos Zaedyus and Tolypeutes, the ex- apophyses" by Owen (1851a) and Flower (1885) tant anteater Tamandua, the extant sloth Brady- are distinct only in the lumbar vertebrae. In the pus, and the extinct sloth Hapalops. Unlike the posterior thoracic vertebrae, the medial and lateral study reported by Owen (1851a), the present anal- facets become confluent. ysis also incorporates ontogenetic data from ju- The problem of identifying which facets are xe- venile specimens, as well as paleontological in- narthrous and which represent the typical mam- formation unavailable to Owen. Based on this de- malian zygapophyseal facets is exacerbated by scriptive information as well as on functional variability in the number and form of the inter- (Gaudin & Biewener, 1992; Gaudin, 1993; Gau- vertebral joints among xenarthrans and by the din & Fortin, unpubl. data) and phylogenetic (En- lack of any modern comprehensive study of the gelmann, 1978, 1985; Gaudin, 1993, 1995) infor- problem. It is known that in some living xenar- mation garnered from other sources, a scenario is thrans, e.g., the giant armadillo Priodontes, there postulated for the structural evolution of xenar- are as many as six pairs of intervertebral articu- throus intervertebral facets. Finally, this scenario lations (Grasse, 1955; see below). In others the is used to evaluate the phylogenetic affinity of number of facets is reduced, so that in many several extinct early Cenozoic taxa with purported ground sloths, e.g., Hapalops, there are only two ties to Xenarthra. pairs of intervertebral facets, one set of zygapophyses, and one set of supplementary joints (Scott, 1903-1904; see below). This variability has gone largely unnoticed, as most modern studies that List of Abbreviations make any mention of the morphology of xenarthrales illustrate only a single joint in a single spe- The following abbreviations will be utilized cies, and describe this joint in a few brief sen- throughout the text: amnh, American Museum of tences (Grasse, 1955; Hoffstetter, 1958, 1982; Natural History, New York; fmnh, Field Museum

Vaughn, 1986; Gaudin & Biewener, 1992). Jen- of Natural History, Chicago; LI, L2, L3 . . . first kins (1970) describes regional variation in the lumbar vertebra, second lumbar vertebra, third morphology of xenarthrales, but only in Taman- lumbar vertebra, respectively, etc.; SI, S2, . . . dua tetradactyla. first sacral vertebra, second sacral vertebra, etc.;

Finally, the problem of the origin of xenarthra- Tl, T2, . . . first thoracic vertebra, second thoracic les has been inadequately treated in the literature. vertebra, etc.; utcm, University of Tennessee at Gaudin and Biewener (1992), Gaudin (1993), and Chattanooga Natural History Museum, Chatta- Gaudin and Fortin (unpubl. data) considered the nooga. functional reasons for the evolution of these facets but did not address their actual structural antecedents. MacPhee (1994, p. 174) suggested that the facets arose through the "sacralization" of the Descriptive Anatomy lumbar and posterior thoracic vertebrae, but he noted the paucity of evidence to substantiate this Cingulata claim. To date no study has examined the struc- Euphracta (esp. Zaedyus pichiy) tural origin of xenarthrales from either a devel- opmental or paleontological perspective. The vertebral columns of six euphractan ar- The present study reexamines the morphology madillos (sensu Engelmann, 1985) were exam- of xenarthrous vertebrae throughout the Xenar- ined, two from the species Zaedyus pichiy (fmnh thra. Following Owen (1851a), both regional in- 23809, 104817), three from Chaetophractus vil- traspecific variation and interspecific variation losus (fmnh 60467, 122623, 134611), and one across the various xenarthran suborders are ad- from Euphractus sexcinctus (fmnh 152051). The dressed. Nearly all of the extant xenarthran genera description below is based primarily upon Z p. have been examined in detail, as well as the fossil caurinus, fmnh 104817 (Figs. 2A, 3 A, 4A-D). It genera housed in the Field Museum of Natural should be noted at the outset that vertebrae are History (fmnh) collections, plus an assortment of bilaterally symmetrical structures. Nearly all of

FIELDIANA: GEOLOGY the vertebral facets, foramina, and processes de- opisthocoelous centra are found in all thoracic scribed in this paper are paired, with the midline vertebrae except the last two. neural spine constituting the single major excep- The pedicels of the anterior thoracic vertebrae tion. In order to simplify the descriptions and are extremely low and broad anteroposteriorly. avoid confusion, however, each vertebra is de- Beginning with the third thoracic vertebra, these scribed unilaterally, with the implicit assumption long, low pedicels occlude the intervertebral fo- that the same structures are present and exhibit ramina from which the spinal nerves typically the same morphology on opposite sides of the ver- emerge. Hence the neural arch of each thoracic tebra unless otherwise stated. Following Walker vertebra from T3 through T10 is perforated by and Homberger (1992) and Wake (1979), the tho- two foramina: one for the ventral branch of each racic vertebrae are defined as those vertebrae pos- spinal nerve, perforating the pedicel itself, and sessing articulations with movable ribs, the sacral one for the dorsal branch of each spinal nerve, vertebrae as those vertebrae that articulate directly perforating the lamina of the vertebra just behind with the ilium (or are fused to these vertebrae pos- the root of the diapophysis (Fig. 4B). The latter teriorly), and the lumbar vertebrae as those ver- process is much higher in euphractans than is typ- tebrae that lie in between the thoracic and sacral ical for mammals; this is due at least in part to vertebrae. the vertically depressed nature of the neural arch. All of Rower's (1885) euphractan armadillo The articular facet for the head of the rib is like- specimens had 14 dorsal vertebrae—3 lumbar and wise more dorsally situated. It lies between the 11 thoracic. The fmnh specimens are somewhat anterodorsolateral part of the vertebral centrum more variable. Although most have 14 dorsal ver- and the posterolateral surface of the pedicel of the tebrae, one Chaetophractus specimen (fmnh preceding vertebra. 60467) and the sole Euphractus specimen have Finally, the zygapophyseal facets of the anterior 15, the former with 10 thoracic and 5 lumbar and thoracic vertebrae are unusual in certain respects. the latter with 1 1 thoracic and 4 lumbar. The an- The anterior zygapophysis bears a typical flat, terior thoracic vertebrae of extant euphractans are ovate facet situated immediately to one side of the very similar to those of other mammals, with midline. The long axis of this facet runs antero- small, depressed centra and elongated, posteriorly posteriorly, and the facet faces dorsally. The an- inclined neural spines (Fig. 1; see also Slijper, terior zygapophyseal facet articulates with a sim- 1946; Walker & Homberger, 1992). As in many ilar ventrally directed facet on the underside of other mammals, the neural spines of the anterior the lamina of the preceding vertebra. Both the an- thoracic vertebrae are dramatically longer than terior and posterior zygapophyseal facets, how- those of more posterior thoracic vertebrae. The ever, are contiguous or confluent with a second anterior thoracic vertebrae of euphractans are un- set of facets laterally (Figs. 2A, 3A). These atyp- usual, however, in several respects. The centra are ical lateral zygapophyseal articulations are also functionally opisthocoelus. The main anterior ar- flat and ovate, but with their long axes oriented ticular surface of each centrum is nearly flat, but transversely. Their presence dramatically widens it is flanked by two small lateral facets (Fig. 3A). the zygapophyseal articular surface. The anterior These facets face anterolaterally, creating a con- lateral zygapophyseal facet faces dorsally and vex profile for the whole anterior surface of the slightly laterally. It articulates with a ventrally centrum. Similarly, the posterior surface of each facing facet borne on the underside of a rudimen- centrum bears two small lateral facets that face tary anapophysis (or accessory process) that pro- posteromedially (Fig. 3A). This creates a concave jects posteriorly from the root of the diapophysis posterior articular surface to receive the convex (Fig. 3A). The anterior and posterior lateral zyg- anterior surface. In Zaedyus such functionally apophyseal facets are present in all euphractan

Fig. 2. Thoracic and lumbar vertebrae shown in dorsal view. A, Zaedyus pichiy caurinus, fmnh 104817—T6, T7, T8, LI, L2, proceeding from right to left. B, Tamandua mexicana, fmnh 69597—T12, T13, T14, LI, proceeding from right to left. C, Bradypus variegatus, fmnh 69589—T14, T15, LI, L2, proceeding from right to left. All scale = bars 1 cm. Abbreviations as in Figure 1, plus alz, anterior lateral zygapophyseal facet; amz, anterior medial zygapophyseal facet; ax, anterior xenarthrous facet; ax/alz, fused anterior lateral zygapophyseal facet and anterior xenarthrous facet; D, diaphragmatic vertebra; plz, posterior lateral zygapophyseal facet; pmz, posterior medial zygapophyseal facet; px, posterior xenarthrous facet.

GAUDIN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE B

amz

ax/a I z

L1 T14

L2 L1

Fig. 2.

FIELDIANA: GEOLOGY dp pmz r* .alz ns ^ -A-amz ,4 /^ yv px

T8 T7(D) T6

ap r>^P

pmz . ^

ns ns

T 13(D) T12

,dp ^mp

az ns

T 15(D) T14

Fig. 2. Continued.

GAUDIN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE ns

mp px

T7(D) T7(D) caud cran

ns B mp '"As*

T 13(D) caud

Fig. 3. Thoracic and lumbar vertebrae shown in cranial and caudal views. A, Zaedyus pichiy caurimis, fmnh 104817—T6 (cran.), T6 (caud.), T7 (cran.), T7 (caud.), T8, (cran.), proceeding from right to left. B, Tamandua mexicana. fmnh 69597—T12 (caud.), T13 (cran.). T13 (caud.), T14 (cran.). proceeding from right to left. C, Bradypus variegatus. fmnh 69589—T15 (cran.), T15 (caud.), LI (cran.), LI (caud.), L2 (cran.), proceeding from right to left.

All = 1 scale bars cm. Abbreviations as in Figures 1 and 2, plus cran, cranial view; caud, caudal view: lea, lateral centrum articulation: If. lateral foramen for ventral branch of spinal nerve: rf/px, fused rib facet and posterior xenarthrous facet.

FIELDIANA: GEOLOGY T6 T6 caud cran

T13(D) T12 cran caud

C^-.

T15(D) T15(D) caud cran

Fig. 3. Continued.

specimens examined. In Zaedyus they occur from is actually borne on a small anterior projection T3 posteriorly. that emerges from the base of the metapophysis The anteriormost xenarthrous articulations are but lies well above the lamina of the neural arch. between the sixth and seventh thoracic vertebrae. The anterior xenarthrous facet of T7, coupled with The small anapophysis of the sixth thoracic ver- the normal horizontal zygapophyseal facet on the tebra not only bears a lateral zygapophyseal facet lamina of the neural arch, forms a slot that re- on its underside, but also a small, flat, ovate facet ceives the anapophysis of T6 (Fig. 2A). Owen on its dorsal surface (Figs. 2A, 3A, 4A). This lon- (1851a) analogized this interlocking of vertebrae gitudinally elongated dorsal facet articulates with to a carpenter's "mortise and tenon" joint. a ventrally directed facet carried on the underside The seventh thoracic vertebra is the diaphrag- of the metapophysis of T7, forming the first true matic vertebra, defined by Slijper (1946) as that xenarthrous joint (Figs. 3A, 4A). The facet on T7 in which the anterior zygapophyseal facets are

GAUDIN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE a px pmz P mp ax/px

mp ax/px

Fig. 4. Zaedyus pichiy caurinus. fmnh 104817: thoracic and lumbar vertebrae shown in right lateral view. A, = stereophotographs of T6 and T7. B, stereophotographs of T7 and T8. C, T8 and T9. D, Tl 1 and LI. Scale bar 1 cm. Abbreviations as in Figures 1 and 2, plus ax/px, xenarthrous intervertebral joint; ax 1/alz, fused anterior lateral zygapophyseal facet and anterior xenarthrous facet; ax 2/px 2, xenarthrous intervertebral joint between secondary anterior and posterior xenarthrous facets; px 1/plz, fused posterior lateral zygapophyseal facet and posterior xenarthrous facet; px 2, secondary posterior xenarthrous facet; spn, foramen/foramina for spinal nerve roots.

horizontally oriented and the posterior zygapoph- The portion of the anterior zygapophyseal facet yseal facets are roughly vertical. T7 is the anter- that is medial to the base of the metapophysis iormost vertebra to bear a distinct, although small, (i.e., the anterior median zygapophysis) strongly metapophysis. The metapophyses become pro- resembles the postdiaphragmatic anterior zyg- gressively elongated posteriorly (Figs. 2A, 4B- apophyseal facets of other mammals (Pick & D). By the ninth thoracic vertebra, the metapoph- Howden, 1977; Walker & Homberger, 1992) and ysis is as long as the neural spine, and by the first is homologized with these facets by Owen lumbar vertebra, the metapophysis exceeds the (1851a) and others (Hoffstetter 1958, 1982; Les- neural spine in height. On the seventh thoracic sertisseur & Saban, 1967; Gaudin & Biewener, vertebra, the base of the metapophysis lies pos- 1992; Rose & Emry, 1993). I concur with this terior to the anterior zygapophyseal facet. On T8, homology. The facet is concave and transversely however, the base of the metapophysis contacts elongated. The medial half of the facet is hori- the anterior edge of the lamina (Figs. 2A, 3A). zontal and faces dorsally. The lateral half is ver- This more fully divides the anterior zygapophyse- tical, rolling up onto the base of the metapophysis al facet into medial and lateral components. and facing medially. This facet articulates with a

10 FIELDIANA: GEOLOGY spn

pmz

px2

spn ax2/px2

convex, ventrolaterally oriented facet on the pos- facet of T7. It is, however, oriented much more terior edge of the lamina of T7 (Fig. 3A). obliquely (indeed, it is nearly vertical) and lies The portion of the zygapophyseal facet lateral further laterally on the neural arch. It abuts a third to the base of the metapophysis in T8 is similar intervertebral facet on its dorsomedial edge (Fig. in position to the lateral anterior zygapophyseal 3A). The third facet, also positioned lateral to the

GAUDIN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE 11 base of the metapophysis, is nearly identical to The anapophysis of T7 is enlarged relative to the anterior xenarthrous facet on T7. It is situated that of T6, a trend that continues posteriorly (Fig. on the ventral surface of a small anterior projec- 4A-D). As noted by Owen (1851a), this enlarge- tion at the base of the metapophysis (Figs. 3A, ment primarily represents an increase in vertical 4B). The facet faces primarily ventrally, although thickness, accompanied by a more modest in- it bears a concave medial lappet that extends onto crease in length. As the anapophysis becomes the lamina and faces laterally. The two facets lat- deeper, it participates in bearing, laterally, the fac- eral to the metapophysis articulate with the an- et for the head of the rib (starting at T8; Fig. 4C). apophysis of T7, forming the medial wall of a Not only is the anapophysis of T8 larger than that deep pocket that receives the anapophysis in mor- of T7, but the two facets it carries on its inner tise and tenon fashion (Figs. 3A, 4B). The an- surface, the lateral zygapophyseal and the xenar- apophysis on T7 bears a convex dorsal facet, the throus facet, are confluent (Fig. 3A). The lateral xenarthrous facet, as well as a flat, ventromedially zygapophyseal and xenarthrous facets are similar- oriented facet that appears to be the serial ho- ly confluent on the anterior edge of T9. The in- mologue of the lateral posterior zygapophyseal tervertebral joints of T9/T10 and T10/T1 1 are vir- facet of T6. Unlike T6, however, this lateral pos- tually identical to those of T8/T9. In each case terior zygapophyseal facet is not in contact with there are two sets of intervertebral joints, one me- the medial posterior zygapophyseal facet. Rather, dial to the base of the metapophysis, representing the two are divided by a distinct notch (Figs. 2A, the medial zygapophyseal joint, and one lateral to 3A). the base of the metapophysis, representing the Because of its apparent serial homology with conjoined lateral zygapophyseal and xenarthrous 4 portions of the more anterior zygapophyseal fac- joints. ets, I have not applied the term "xenarthrous" to The intervertebral joint between Til and LI the distinct lateral zygapophyseal articulation of differs from the T10/T1 1 intervertebral joint in a T7/T8 and more posterior vertebrae in Zaedyus number of important respects. The diapophysis, and other euphractans (as well as other xenar- which is progressively reduced posteriorly begin- thrans described below). Rather, in order to en- ning with T8 (Fig. 4A-C), is rudimentary on Tl 1 hance precision and avoid confusion, I will re- (Fig. 4D). Concomitant with the reduction of the strict the term "xenarthrous articulation" to those diapophysis, the foramen for the dorsal branch of accessory intervertebral joints that are clearly dis- the spinal nerve, which lies between the diapoph- tinct from the zygapophyseal system of facets. It ysis and anapophysis anteriorly, occupies a pro- should be noted that this usage differs markedly gressively more caudal position on the side of the from that of most previous authors, who identify anapophysis of T8-T10 (compare Fig. 4B with a single set of "normal" zygapophyseal facets, C). It also changes orientation, from a nearly ver- either medial or lateral, and then several sets of tical course to a horizontal, posteriorly directed "xenarthrous" facets (Owen, 1851a; Flower, course. At the 1 1th and last thoracic vertebra, the 1885; Grasse, 1955; Hoffstetter, 1958, 1982; Les- foramen reaches the caudal edge of the anapophy- sertisseur & Saban, 1967; Vaughn, 1986; Gaudin sis (Fig. 4D). The groove leading to this opening & Biewener, 1992; Rose & Emry, 1993). This us- divides the articular facets carried on the anapo- age also differs from that of Jenkins (1970), who physis into separate dorsal and ventral facets. In merely identifies facets as dorsal, ventral, or in- Chaetophractus a similar dorsal articulation on termediate pre- and postzygapophyses without the ultimate thoracic vertebra contains two facets, distinguishing "normal" from "xenarthrous" fac- a lateral zygapophysis and a xenarthrous facet ets. I believe there are two sets of "normal" fac- (see footnote 4). In Zaedyus and Euphr actus, ets in xenarthrans, i.e., facets that can be homol- these facets are fused. As in more anterior xenar- ogized with zygapophyseal facets in more anterior vertebrae. One set lies medial to the metapophy- 4 The point at which the lateral facets fuse in euphrac- sis, the other lateral. Previous authors on disagree tans is somewhat variable. In Euphractus, the two facets which set is normal because they assume that only are never separate. In Chaetophractus, however, they re- one set can be normal. I believe that in addition main separate all the way back to the second lumbar vertebra. Moreover, in the latter the lateral to these two sets of "normal" zygapophyseal fac- genus zygapophysis is split into two facets starting at T7. These ets, xenarthrans possess extra "xenarthrous" fac- two lateral zygapophyseal facets remain separate until ets that lack serial homologues in more anterior T10. Similar facets have also been observed in adult vertebrae. specimens of Tolypeutes (fmnh 121540, 153773).

12 FIELDIANA: GEOLOGY throus joints, the anapophyseal facet(s) above the Chaetophr actus, the giant armadillo Priodontes, spinal nerve contacts the ventrolateral surface of and in certain extinct genera of mylodontid sloths the metapophysis and the lateral surface of the (Owen, 1842; Stock, 1925). neural arch. The serial homology of the ventral The anterior thoracic vertebrae of Tolypeutes facet below the spinal nerve is more difficult to differ somewhat from those of Zaedyus. They lack ascertain. Its articulation posteriorly with the dor- the functionally opisthocoelus centra. Although sal surface of the transverse process (a pleur- the pedicels are low and longitudinally elongated, apophysis, not parapophysis, contra Owen, only the ventral spinal foramen is present (Fig. 1851a; see Tolypeutes below) suggests, however, 5 A). The dorsal branch of the spinal nerve emerg- homology with the facet on the lateral surface of es through a notch between the large diapophysis the anapophysis that receives the head of the rib and the rudimentary anapophysis. Only a single in the thoracic vertebrae. pair of zygapophyseal facets is present, and the The anapophysis of Til appears much more zygapophyseal surface is narrower mediolaterally elongate than that of T10 (Fig. 4C, D). This can in Tolypeutes than in Zaedyus, especially in adult be attributed to the reappearance of the interver- specimens. tebral foramen at Til and the concomitant nar- In fmnh 124569, the diaphragmatic vertebra is rowing of the pedicel. The intervertebral foramen T7, and the anteriormost xenarthrous articulation is present in all the lumbar vertebrae. The mor- lies between this vertebra and the preceding one, phology of the intervertebral articulations is vir- T6. As in Zaedyus, this joint is formed between tually unchanged from Tll/Ll to L3/S1. the dorsal surface of a small anapophysis and the ventral surface of the metapophysis, which is rudimentary in this specimen (Fig. 5A). Unlike this xenarthrous between T6 and Tolypeutes matacus (fmnh 124569 [juv.]) Zaedyus, joint T7 occurs only on the right side of the specimen. The of this first xenarthrous is This specimen is a neonate. The neural arches position joint apparently somewhat variable in Tolypeutes. In are still unfused to the centra. Similarly, the cer- 5 fmnh 1 the first xenarthrous also oc- vical and lumbar ribs remain unfused. However, 24540, joint curs between T6 and T7, but on the left rather the left and right halves of the neural arches are than the side. Moreover, the fused in all but the cervical vertebrae. The spec- right diaphragmatic vertebra in this is T8 rather than T7. In imen has 15 dorsal vertebrae— 11 thoracic and 4 specimen fmnh the vertebra is lumbar. The numbers of thoracic and lumbar ver- 124570, diaphragmatic T7, but the first xenarthrous occurs between T7 tebrae are variable in adult members of the genus joint and T8. Tolypeutes. The fmnh collections include individ- As was the case with in uals with 1 1 thoracic and 4 lumbar vertebrae Zaedyus, Tolypeutes the vertebra the vertebra (fmnh 121540, 124570, 153773) as well as indi- following diaphragmatic bears a whose base reaches viduals with 12 thoracic and 3 lumbar vertebrae large metapophysis the anterior of the lamina. This creates (fmnh 122233). Flower (1885) characterizes To- margin three sets of intervertebral joints between T7 and lypeutes as possessing only 14 dorsal vertebrae T8: (1) a joint medial to the base of the metapoph- (11 thoracic, 3 lumbar), but I suspect that he the medial a failed to include the last lumbar vertebra in his ysis, zygapophyseal joint; (2) joint lateral to the base of the metapophysis, formed by count. The last lumbar vertebra is typically fused facets on the dorsal surface of the anapophysis to the first sacral vertebra in adult specimens. This and the ventral surface of the and condition is not uncommon in xenarthrans. It has metapophysis; (3) a lateral to the base of the metapophysis, been observed in the euphractans Euphractus and joint formed by facets on the ventromedial surface of the anapophysis and the lateral surface of the neu- ' This confirms that the transverse processes of the ral arch (Fig. 5A). It is not possible to determine lumbar vertebrae in adult armadillos (like those of other in this juvenile specimen whether or not these two mammals) are pleurapophyses, i.e., rib attachments of lateral surfaces are confluent in the thoracic the vertebra plus a fused rib (contra Owen, 1851a). The joint neonatal lumbar vertebrae lack any lateral projections, vertebrae. They are separate, however, in the tho- precluding the possibility that the adult transverse pro- racic vertebrae of at least one adult specimen cesses are i.e., lateral of the parapophyses, projections (fmnh 153773), becoming confluent in the lumbar vertebra that serve as the site of attachment for the ven- vertebrae of both the adult and tral head of two-headed ribs (Wake, 1979; Kardong, juvenile speci- 1995). mens. As in Zaedyus, these two joints presumably

GAUDIN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE 13 B

ax/px

spn

Fig. 5. in left lateral Tolypeutes matacus, fmnh 124569 (j uv -)- A, series of thoracic vertebrae (T5-T8) shown = view. B, series of lumbar vertebrae (L1-L3) shown in left lateral view. Scale bar 1 cm. Abbreviations as in Figures 1, 2, and 4, plus lrf, lumbar rib facet.

represent xenarthrous and lateral zygapophyseal racic vertebrae differ significantly from that of articulations. Tll/Ll and those of the lumbar vertebrae. The

As occurs in Zaedyus, the anapophysis and me- ventral spinal foramen resumes a typical interver- tapophysis become progressively larger in the tebral condition in Tl 1 and all subsequent lumbar more caudal dorsal vertebrae in fmnh 124569. vertebrae (Fig. 5B). The dorsal branch of the spi- Also, the intervertebral articulations of the tho- nal nerve, which emerges ventral to the anapoph-

14 FIELDIANA: GEOLOGY ysis in the majority of thoracic vertebrae, emerges phology of the xenarthrous articulations among medial to the anapophysis of Til and the lumbar living adult armadillos is remarkably constant. vertebrae, i.e., between the anapophysis and the The anteriormost xenarthrous facets typically oc- pedicel of the following vertebra. Below the open- cur in the vicinity of the diaphragmatic vertebra, ing for the dorsal branch of the spinal nerve, the between the diaphragmatic vertebra and either the anapophysis develops a laterally directed facet for preceding or the succeeding vertebra (although articulation with the last thoracic and the lumbar occasionally xenarthrous joints occur between the ribs (Fig. 5B). This facet is absent in a slightly first and second prediaphragmatic vertebrae; see younger specimen (fmnh 124568, in which the Tolypeutes above). The first xenarthrous joints right and left neural arches of the posterior tho- form between the dorsal surface of a small an- racic vertebrae are still unfused). In adult speci- apophysis and the ventral surface of a small me- mens it becomes a facet between the anapophysis tapophysis. As the metapophysis enlarges poste- and transverse process. riorly and its base reaches the anterior edge of the MacPhee (1994, p. 174) suggested that xenar- lamina (as almost always occurs in the first post- throus intervertebral articulations are produced diaphragmatic vertebra), the zygapophyseal facet through "a kind of 'sacralization' of the lower is divided in two. One portion comes to lie medial part of the free spine," a hypothesis that he be- to the base of the metapophysis. It articulates with lieved could be confirmed by the discovery of ac- a facet on the posteromedial portion of the lamina cessory processes or transitory supplementary in- of the diaphragmatic vertebra. The second portion tervertebral articulations in the early developmen- comes to lie lateral to the base of the metapophy- tal stages of some xenarthran or even non-xenar- sis, on the most lateral portion of the lamina. It thran mammal. The sacral vertebrae of juvenile articulates with a facet borne on the medial sur- specimen fmnh 124569 and the younger fmnh face of the anapophysis of the diaphragmatic ver- 1 24568 were examined for evidence of such pro- tebrae. It usually becomes confluent with the xe- cesses or articulations. In the anterior sacral ver- narthrous facet in the first few postdiaphragmatic tebrae the pedicels of successive vertebrae are vertebrae. The xenarthrous facet is carried on the broadly fused lateral to the metapophyses and ventrolateral surface of the metapophysis and ar- dorsal to the intervertebral foramina (Fig. 6). It is ticulates with the dorsomedial surface of the an- unclear, however, whether these lateral areas of apophysis. These three types of joints—medial fusion represent anapophyses, fused sacral ribs, or zygapophyseal, lateral zygapophyseal, and xenar- some other type of structure. Moreover, the xe- throus—are present in all the postdiaphragmatic narthrous articulations 1 thoracic vertebrae. second of xenarthrous between L4 and S , typical A type of older Tolypeutes (including fmnh 124569), had joint is also usually present in armadillos. It is not yet developed in fmnh 124568. This suggests found in the lumbar vertebrae, where it forms be- that sacral fusion and the development of xenar- tween the ventral surface of the anapophysis and thrales in the thoracic and lumbar vertebrae are the dorsal surface of the transverse process of the unrelated. following vertebra. A very similar joint is found in the ultimate or penultimate thoracic vertebra, formed by the ventral surface of the anapophysis Other Cingulates and the dorsal surface of the rib or transverse process of the following vertebra. The number of dorsal vertebrae is somewhat Although the condition of the xenarthrous ar- more variable among dasypodid (sensu Engel- ticulations cannot be ascertained in glyptodonts mann, 1985) armadillos than in euphractans. The because of extensive fusion among the dorsal ver- total number of dorsal vertebrae varies between tebrae (Hoffstetter, 1958; Gillette & Ray, 1981), 13 and 16. Thoracic counts range from 13 in Prio- the dorsal vertebrae remain unfused in several dontes (fmnh 25271) to 9 in Dasypus hybridus close relatives (following Engelmann, 1985; Pat- (Flower, 1885). The number of lumbar vertebrae terson et al., 1989) of the glyptodonts, the pam- varies from 2 in Priodontes (perhaps 3, given the patheres and eutatine armadillos. In pampatheres fusion of the last lumbar to the sacrum in this typical xenarthrous articulations "begin to appear species) to 5 in Dasypus novemcinctus and some at the anterior end of the thoracic section, and are Dasypus hybridus (Flower, 1885; Gaudin & Biew- well developed in the posterior thoracic and lum- ener, 1992). bar vertebrae" (Edmund, 1985, p. 88). A posterior Despite variation in vertebral number, the mor- thoracic vertebra illustrated by Edmund (1985,

GAUDIN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE 15 6. in and dorsal Fig. Tolypeutes matacus, fmnh 124569 (j uv -)- A-C, sacral vertebrae shown right lateral, ventral, views. Scale bar = 1 cm.

fig. 9) shows confluent xenarthrous and lateral phragmatic vertebra and the first prediaphragmatic zygapophyseal facets on the anapophysis and vertebra, formed by the ventral metapophysis of what is perhaps an anapophyseal facet for artic- the former and the dorsal anapophysis of the lat- ulation with the head of the rib. A specimen of ter. Facets medial and lateral to the metapophyseal Proeutatus (fmnh PI 29 12), a eutatine armadillo base are present in the postdiaphragmatic verte- from the late early to early middle Miocene Santa brae, with a third set of facets present between the Cruz Formation of Patagonia (Scott, 1903-1904), anapophysis and transverse process in the last tho- also has a typical cingulate pattern, fmnh PI 29 12 racic and first lumbar vertebrae. includes the last five thoracic and first lumbar ver- Priodontes constitutes the only major exception tebrae in articulation. The diaphragmatic vertebra to the cingulate pattern of xenarthrous articula- is the second in the series. The most anterior xe- tions. The giant armadillo possesses all of the ar- narthrous articulation occurs between the dia- ticulations described above. It is characterized,

16 FIELDIANA: GEOLOGY ax1/alz

ax 3, 4, 5

= Fig. 7. Priodontes maximus, fmnh 25271; first lumbar vertebra shown in cranial view. Scale bar 1 cm. Ab-

as in 1 breviations Figures , 2, and 4, plus ax 3, 4, 5, third, fourth, and fifth anterior xenarthrous facets, respectively.

however, by several additional sets of facets in the Flower (1885) cite specimens of Tamandua with posterior thoracic and lumbar vertebrae (Fig. 7). 17 thoracic and 3 lumbar vertebrae. Anteriorly, these vertebrae have a single midline The anterior thoracic vertebrae of Tamandua facet on the dorsal surface of the lamina, flanked are less strongly modified than those of cingu- by two pairs of facets also situated on the antero- lates, with much taller, narrower pedicels and nor- dorsal edge of the lamina, cranial to the zyg- mal intervertebral foramina. The diapophyses, apophyseal facets. The extra facets articulate with though large, are less elevated than in the arma- corresponding facets on the posteroventral edge dillos, and the head of the rib articulates exclu- of the lamina of the preceding vertebra. The pres- sively with the anterodorsal portion of a single ence of the extra facets is clearly a derived feature vertebral centrum (Fig. 8). These vertebrae are of Priodontes. unusual, however, in four respects. First, the large neural spines are of uncommonly uniform height throughout the thoracic region, decreasing only slightly in height posteriorly. Second, these neural Vermilingua spines are markedly robust and elongated anteroposteriorly (Fig. 8). Third, the zygapophyseal ar- Tamandua mexicana ticular facets are widely separated from the midline of the vertebral lamina (Fig. 2B). On the an- The vertebral columns of five specimens of the terior edge of the lamina this separation is pro- species Tamandua mexicana were examined duced by a broad, rounded, midline indentation. the facets themselves are (fmnh 22398, 58545 |juv.], 69597, 93095 [juv.], Fourth, zygapophyseal wide as can be seen and 93176 [juv.]). The description below is based quite mediolaterally. Indeed, primarily on T. mexicana, fmnh 69597 (Figs. 2B, in anterior and posterior views (Fig. 3B), the fac- 3B, 8). ets extend further laterally than the vertebral cen- The number of dorsal vertebrae tends to be tra, a morphology in sharp contrast to that found higher in pilosans than in cingulates, particularly in the armadillos (Fig. 3A) and to the generalized in the thoracic portion of the column. The actual mammalian condition (Fig. 1; Flower, 1885; Jen- numbers of thoracic and lumbar vertebrae in Ta- kins & Parrington, 1976; Jenkins & Schaff, 1988; mandua vary, fmnh 69597 has 17 thoracic and 2 Walker & Homberger, 1992). lumbar vertebrae; fmnh 22398 has 18 thoracic and All of the thoracic vertebrae have distinct me- 2 lumbar vertebrae. Both Cuvier (1836a) and tapophyses that increase in size posteriorly. There

GAUDIN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE 17 pmz

px ap plz

Fig. 8. Tamandua mexicana, fmnh 69597: stereophotographs of thoracic vertebrae shown in right lateral view. = A, T12 and T13, B, T13 and T14. Scale bar 1 cm. Abbreviations as in Figures 1, 2, and 4, plus alz/plz, lateral zygapophyseal joint.

is no trace, however, of an anapophysis in any a mortise-and-tenon-style articulation is formed prediaphragmatic vertebra (Figs. 2B, 8). The di- with the well-developed anapophysis of T13. In aphragmatic vertebra is T13. The first xenarthrous anterior view, the facet lateral to the base of the articulation occurs between the diaphragmatic metapophysis of T14 is parabolic in shape (Fig. vertebra and T14. As in Zaedyus, the base of the 3B), with its dorsal portion facing ventrally and enlarged metapophysis on the first postdiaphrag- its ventral portion facing dorsally. The ventral matic vertebra reaches the anterior edge of the portion is nearly identical in position, size, shape, lamina, dividing the wide anterior zygapophyseal and orientation to the lateral half of the anterior facet in half (Fig. 2B). The portion remaining me- zygapophysis of T13, which is likewise borne on dial to the metapophysis is a concave medial zyg- a lateral extension of the anterior vertebral lamina. apophyseal facet that faces dorsomedially. It ar- I believe it to be the serial homologue of the outer ticulates with a convex, ventrolaterally oriented half of the anterior zygapophyseal facet. The dor- facet on the posterior edge of the lamina of T13 sal portion is a true xenarthrous facet borne on (Fig. 3B). This medial joint appears identical to the ventral surface of a small anterior process pro- the relatively vertical zygapophyseal articulations jecting from the base of the metapophysis. The found in the posterior thoracic and lumbar verte- anapophysis of T13 has dorsal and ventral facets brae of non-xenarthran mammals. that are continuous medially. The dorsal facet is Lateral and ventral to the metapophysis of T14, a xenarthrous facet. The ventral facet, though sep-

18 FIELDIANA: GEOLOGY arated from the medial zygapophyseal facet by a lations. The sacral vertebrae of this specimen are deep notch, is apparently a lateral zygapophyseal joined not by synovial joints but by three areas of facet (Figs. 3B, 8). rugose bone. One joins the laminae near the mid- Nearly identical intervertebral articulations oc- line, representing the aforementioned sacral joints cur between the vertebrae caudal to T13/T14. The of the younger Tamandua specimen. The second major morphological changes in these more pos- unites the vertebral centra. The third zone of at- terior vertebrae include a gradual increase in the tachment is a massive area of rugose bone situated size of the metapophyses and a gradual decrease lateral to the metapophysis and dorsal to the in- in size of the diapophyses. Unlike the situation in tervertebral foramina, presumably representing a Zaedyus, the anapophyses of the posterior thorac- broad zone of contact between the sacral ribs and ic vertebrae do not articulate with the ribs. It is their attendant anapophyses. It does not involve therefore not surprising that articulations between the metapophyses or vertebral laminae, and thus the anapophyses and transverse processes at T17/ it bears little resemblance to the xenarthrous con- LI and L1/L2 are absent. There is, however, a nections of the dorsal vertebrae. third pair of intervertebral articulations that forms between L2 and S 1 . The anapophysis of L2 bears Other a ventrolaterally directed facet on its lateral edge. Vermilinguas It articulates with a dorsomedially oriented facet As in its close relative Tamandua, the number on the dorsal surface of the sacral rib of S 1 . of thoracic and lumbar vertebrae in the giant ant- Several early juvenile specimens of Tamandua eater Myrmecophaga is variable— 15 thoracic and were available, including one very young speci- 3 lumbar or 16 thoracic and 2 lumbar (Cuvier, men (fmnh 58545) in which the left and right neu- 1836b; Owen, 1851a; Flower, 1885). The pygmy ral arches of the sacral vertebrae are unfused (in anteater Cyclopes has 16 thoracic and 2 lumbar contrast to both juvenile Tolypeutes specimens de- vertebrae (Flower, 1885; fmnh 61853, 69969, scribed above). This specimen has fairly typical 69971). The vertebrae of Myrmecophaga are vir- adult-style xenarthrous articulations between the tually identical to those described above for Ta- posterior thoracic and the lumbar vertebrae, be- mandua, except that the joint between the an- ginning at the diaphragmatic vertebra and extend- apophysis of the last lumbar and the sacral rib is ing back to the joint between LI and L2. How- also represented between the ultimate and penul- ever, the lumbosacral joint differs markedly from timate lumbar vertebrae. In these vertebrae, the the adult morphology. The last lumbar vertebra joint lies between the ventrolateral surface of the bears a large, anteroposteriorly broad transverse anapophysis and the dorsal surface of the trans- process. This in turn carries a very small an- verse process of the succeeding vertebra (Flower, apophysis that lies immediately lateral to a notch 1885). for the dorsal branch of the spinal nerve. The an- The vertebrae of Cyclopes are likewise similar apophysis does not contact the fused transverse to those of Tamandua. Cyclopes lacks the joint process/sacral rib of S 1 . The last lumbar and first between the anapophysis of L2 and the sacral rib. sacral vertebrae are joined by a single pair of wide More interestingly, rudimentary anapophyses are facets. These facets extend laterally from the base present in the first prediaphragmatic vertebra (and of the neural spine to the medial margin of the sometimes in the second), and xenarthrous artic- notch for the dorsal spinal nerve. In SI the facet ulations occur between the diaphragmatic and pre- passes around the base of the metapophysis, but diaphragmatic vertebrae. These xenarthrales are it is much more extensive medial to this process. formed between the dorsal surface of the rudi- The intervertebral connections between subse- mentary anapophysis and the ventral surface of quent sacral vertebrae (S1-S3) are virtually iden- an anterior projection of the metapophysis. tical to those described at the lumbosacral joint. Although the sacral ribs of all three sacral vertebrae appear to carry small anapophyses, there is no indication that these participate in supplemen- Tardigrada tary articulations. A slightly older juvenile Tamandua (fmnh Bradypus variegatus 93176) with unfused sacral vertebrae but fused neural arches was examined to ascertain ontoge- The vertebral columns of 2 specimens of Bra- netic changes in the sacral intervertebral articu- dypus variegatus were examined (fmnh 68919,

GAUDIN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE 19 pmz plz I ap tp

ap tp

Fig. 9. Bradypus variegatus, fmnh 69589. Stereophotographs of thoracic and lumbar vertebrae shown in right = lateral view. A, T15 and LI. B, LI and L2. Scale bar 1 cm. Abbreviations as in Figures 1 and 2.

69589). The description below is based primarily the posterodorsal edge of the vertebral centrum on fmnh 69589 (Figs. 2C, 3C, 9), which has 15 and the anterior edge of the pedicel of the suc- thoracic and 3 lumbar vertebrae. The numbers of ceeding vertebra. The zygapophyseal facets of the thoracic and lumbar vertebrae within the genus anterior thoracic vertebrae of Bradypus strongly vary; there are between 14 and 16 thoracic ver- resemble those of Tamandua. They are trans- tebrae and either 3 or 4 lumbar vertebrae (Flower, versely oval and widely separated by a broad, 1885). rounded, midline notch on the anterior edge of the The anterior thoracic vertebrae of Bradypus re- lamina (Fig. 2C). As in anteaters, these zygapoph- semble those of anteaters in that the pedicels are yseal facets extend lateral to the vertebral centra 6 tall and narrow and normal intervertebral foram- in anterior view. The lateral extension of the zyg- ina are present. Similarly, no dramatic difference apophyses becomes increasingly prominent in in the height of the neural spines exists between more posterior thoracic vertebrae. the anterior and posterior thoracic vertebrae, al- As in Tamandua, there is no trace of an an- though the height of the neural spines does grad- apophysis in any anterior thoracic vertebra. There ually diminish posteriorly. The neural spines are are rudimentary metapophyses present in most much shorter in Bradypus than in armadillos and thoracic vertebrae (from at least T3 posteriorly). anteaters (Figs. 3C, 9). The rib articulations occur These are situated on the anterodorsal edge of the at roughly the same level as was observed in Ta- mandua. The head of the rib articulates between 6 This is not the case in posterior view, due to the successive vertebrae, as in armadillos but not ant- presence of elongated tubercles that extend dorsolater- eaters. The lower rib facet is shifted somewhat al^ from the caudal edge of the centrum and articulate anteriorly relative to that in armadillos, lying on with the head of each rib.

20 FIELDIANA: GEOLOGY diapophyses, causing the diapophyses to be lon- mentary articulations found in other xenarthrans. gitudinally elongated in dorsal view (Fig. 2C). The anapophysis of L3 in fmnh 68919 is situated This is also quite reminiscent of the anteater con- well medially and forms a typical lateral zyg- dition. The first distinct metapophysis is small and apophyseal articulation. borne by T14. The metapophyses of succeeding vertebrae are progressively larger (Fig. 2C). T15 is the diaphragmatic vertebra. It differs Hapalops from that of other xenarthrans in lacking supplementary intervertebral articulations. T15 bears a The vertebral columns of four specimens of small anapophysis that extends posteriorly from Hapalops from the Miocene Santa Cruz Forma- the base of the fused diapophysis/15th rib (Figs. tion of South America (Scott, 1903-1904) were 2C, 3C, 9). The anapophysis closely approximates examined. These specimens included Hapalops a lateral projection of the lamina of LI. This ex- longipalatus, fmnh PI 3 146, a mounted skeleton tension lies lateral to the base of the metapophysis with a nearly complete series of vertebrae; Ha- of LI. As in the postdiaphragmatic vertebrae of palops sp., fmnh PI 3 145, a partially prepared other xenarthrans, the base of the metapophysis (ventral surface only) block of articulated lumbar of LI reaches the anterior margin of the lamina. and sacral vertebrae; Hapalops sp., fmnh PI 3 133, In contrast to other xenarthrans, however, a true a specimen with several discontinuous strings of synovial joint does not occur between the small articulated thoracic vertebrae; and Hapalops sp., anapophysis and the area lateral to the base of the fmnh PI 53 18, an articulated series of vertebrae metapophysis (Fig. 9A). composed of the first lumbar and ultimate and The anapophysis of LI is much larger than that penultimate thoracic elements. The latter speci- of T15, and it bears a flat, circular, ventromedially men is the best preserved and is emphasized in directed facet on its medial edge (Figs. 2C, 3C). the description below (Figs. 10, 11). This facet articulates with a similar facet located Scott (1903-1904) estimated the number of lateral to the base of the metapophysis of L2. This thoracic vertebrae to be between 21 and 22 in the lateral facet on L2 is carried on a lateral extension type specimen of Hapalops longiceps, with 3 lum- of the neural arch, and it is narrowly divided from bar vertebrae present, fmnh PI 3 146 has 22 tho- the curved, upright medial zygapophyseal facet racic and 3 lumbar vertebrae, although the column by the base of the metapophysis (Fig. 2C). The is incomplete, with several thoracic vertebrae and articulation between the anapophysis of LI and their accompanying ribs reconstructed in plaster. the lamina of L2 is nearly identical in position, The anterior thoracic vertebrae are typically pi- shape, and orientation to the lateral zygapophy- losan. They have tall pedicels and well-developed seal joints of armadillos and anteaters (Fig. 9), intervertebral foramina. The zygapophyseal facets and it is hence homologized with these joints. In- are remarkably wide, even more than in living terestingly, a dorsal facet on the anapophysis of pilosans. In pilosans and euphractan armadillos, LI, or an anterior projection or ventrally directed the maximum width of the zygapophyseal facets facet on the metapophysis of L2, is absent in Bra- is about twice the maximum anteroposterior dypus. There is apparently no true xenarthrous length. In Hapalops the width is two-and-a-half joint between these vertebrae. times the length (Table 1 ). The anterior zygapoph- The intervertebral joints between L2 and L3 yseal facets are separated from one another by a and between L3 and SI are virtually identical to rounded midline notch somewhat narrower than that between LI and L2 in fmnh 69589. fmnh that observed in Bradypus and Tamandua. As in 68919, however, possesses an additional set of ar- the living anteater and sloth, these facets extend ticular facets between L3 and SI. The transverse further laterally than the vertebral centra. The fac- process of L3 in this specimen bears a wide, shal- ets become even wider posteriorly and are slightly low, ovate articular facet at its distal end. The fac- curved in a horizontal plane, with the medial por- et forms a synovial joint with a similar facet lying tion extending out from the midline anterolater- on the anterior edge of the transverse process/sa- al^ and the lateral part oriented almost directly cral rib of SI. This joint provides the only evi- laterally (Fig. 10). dence of nonzygapophyseal supplementary inter- The anterior thoracic vertebrae of Hapalops are vertebral articulations in this genus. The joint is more reminiscent of anteaters than Bradypus in of further significance because it does not involve several respects. The neural spines are quite tall the anapophysis at all, in contrast to the supple- and elongated anteroposteriorly. They are of rel-

GAUDIN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE 21 az atively uniform height, decreasing only slightly from the middle of the thoracic series posteriorly. As in Tamandua, the articulation for the head of the rib is positioned on the anterodorsal corner of a single vertebral centrum (Fig. 11). The diapophyses of Hapalops are much taller than those of the Bradypus. They are longitudinally elongated, as are those of other pilosans, due to the presence of rudimentary metapophyses extending from the anterior of the The ns edge diapophyses. metapophyses become progressively larger posteriorly. The first freestanding metapophysis occurs on T15 in

fmnh P13146 (T18 in Scott, 1903-1904, pi. 30). As in Bradypus and Tamandua, anapophyses are absent on all anterior thoracic vertebrae. pmz The diaphragmatic vertebra of fmnh PI 3 146 is T19, as in Scott's (1903-1904) specimens of Ha- 1 palops longiceps and H. elongatus. Like Taman- dua, the first supplementary intervertebral articulation occurs between the diaphragmatic and post- ?dp/tp diaphragmatic vertebrae. In addition, the diaphragmatic vertebra is the anteriormost thoracic

7 There may be some variation in the position of the diaphragmatic vertebra in Hapalops. In fmnh P15318, the third and last vertebra in the series, the presumed 21st thoracic vertebra, has broken transverse processes and a very weak depression on the anterolateral surface Fig. 10. Hapalops sp., fmnh P15318; 19th through of the centrum for articulation with the head of the rib. 21st thoracic vertebrae shown in dorsal view (cranial = These rib facets are very strongly developed on T20 and end toward the top of the page). Scale bar 1 cm. T19 (Fig. 1 1). This raises the possibility that the vertebra Abbreviations as in Figures 1 and 2, plus ?dp/tp, pos- labeled T2 1 is in fact the first lumbar, and the diaphrag- sible diapophysis or transverse process. matic vertebra is the penultimate thoracic vertebra.

pmz

?dp/tp az

Fig. 11. Hapalops sp., fmnh P15318; 19th through 21st thoracic vertebrae shown in left lateral view. Scale bar 1 cm. Abbreviations as in Figures 1, 2, and 10, plus rh, rhachitomous foramen.

22 FIELDIANA: GEOLOGY Table 1 . Ratio of maximum width (parallel to long axis) to maximum anteroposterior length (orthogonal to long axis) of the anterior zygapophyseal facets of the prediaphragmatic thoracic vertebrae in Xenarthra. the metapophysis and the dorsal surface of the anapophysis are sporadically present in fmnh inter- PI 3 146 (e.g., on the right side only of the vertebral joint between T22 and LI, and on both sides of L2/L3). Xenarthrous joints are clearly absent in the thoracic intervertebral joints, even those bearing supplemental articulations. The presence or absence of xenarthrous joints could not be confirmed at L1/L2 or L3/S1 because of poor preservation.

Other Tardigrada

The extant tree sloth Choloepus and the extinct ground sloth families Mylodontidae, Megatheri- idae, Nothrotheriidae, and Megalonychidae are, like other pilosans, characterized by a relatively large number of thoracic vertebrae and a small number of lumbar vertebrae. Thoracic vertebral Fig. 12. Pronothrotherium fmnh PI 4467: counts among the extinct forms range from 15 in typicum, isolated mid-thoracic vertebra shown in dorsal view Glossotherium and Paramylodon (Flower, 1885; (cranial end toward the bottom of the page). Scale bar to 22 in The num- = Stock, 1925) Hapalops. highest 1 cm. Abbreviations as in Figure 1. ber among non-Santacrucian ground sloths is 19, which occurs in the Pliocene nothrotheriid Prono- throtherium (fmnh P14503) and the Pleistocene shape. They are, however, very widely separated megalonychid Megalocnus (Matthew & Paula from one another on either side of the midline

Couto, 1959, pi. 26). The number of thoracic ver- and, at least in Scelidotherium, their lateral mar- tebrae in Choloepus ranges between 22 and 24, gins lie lateral to the outer margins of the centra. the highest among living mammals (Flower, The anterior zygapophyseal facets of the Pleisto- 1885). The tardigrades characteristically possess cene West Indian megalonychid Megalocnus, al- three lumbar vertebrae, although some Choloepus though retaining the elongated ovate shape, are and some Hapalops have four (Flower, 1885; oriented anterolaterally at an angle of approxi- Scott, 1903-1904), and the Santacrucian genus mately 45° to the midsagittal plane. The facets are Schismotherium has five (amnh 9244; Scott, also more widely separated on either side of the 1903-1904). In several genera, one or more of the midline than is usual for ground sloths. In the ex- lumbar vertebrae are fused to the sacral vertebrae tinct Puerto Rican genus Acratocnus (Anthony, {Glossotherium, Owen, 1842; Paramylodon, 1918) and the extant genus Choloepus (fmnh Stock, 1925; Megalocnus, Matthew & Paula Cou- 147993, 127422), the facets are oriented almost to, 1959; Acratocnus, amnh 177616-177619). directly anteriorly and separated by a very broad, The anterior thoracic vertebrae among tardi- rounded, midline notch very reminiscent of that grades are typically much like those described in in Bradypus. The thoracic vertebrae of these two Hapalops, with tall, anteroposteriorly elongated taxa, particularly in Choloepus, also resemble neural spines and diapophyses, and wide anterior Bradypus in the degree of reduction of the ver- and posterior zygapophyseal facets that extend tebral processes. further laterally than the vertebral centra and are One other noteworthy modification of the tho- separated by a small midline notch. The zyg- racic vertebrae appears in the anterior and middle apophyseal facets appear to be particularly wide thoracic vertebrae of certain large mylodontid and among the Santacrucian megalonychimorphs megatheriid ground sloth genera. In these forms (sensu Gaudin, 1993) and the nothrotheriids (Fig. some of the thoracic vertebrae bear an additional 12). The zygapophyseal facets of Eremotherium articular facet both anteriorly and posteriorly. The (Paula Couto, 1978) and Scelidotherium (Gervais, facets are unpaired and lie on the midline. The 1855) are unusual in that they are not elongated anterior facet lies on the dorsal surface of the lam- mediolaterally but instead are almost circular in ina, with its anterior margin situated approximate-

24 FIELDIANA: GEOLOGY ly at the level of the posterior margin of the an- lani, where these angled medial and lateral facets terior zygapophyseal facets. The posterior facet is are variably expressed between the prediaphrag- borne on the undersurface of the neural spine. matic and diaphragmatic vertebrae. The angled These midline facets have been noted in Megathe- facets may appear bilaterally, on one side only, or rium (Owen, 1851b), Eremotherium (Paula Couto, be absent. These angled medial and lateral facets the Santacrucian Plan- 1978), large-bodied genus have also been observed in Megalonyx, including 1961), the scelidotheres ops (Hoffstetter, (Mc- amnh FLA- 103- 1986, in which the abutting, an- Donald, 1987), and the mylodontines Paramylo- gled medial and lateral posterior zygapophyses don harlani (Stock, 1925) and gar- "Mylodon" are present on one side, while on the other side mani (Allen, 1913). They are absent, however, in the two posterior facets are widely separated, with the related Glossotherium 1842). closely (Owen, the vertical medial facet and horizontal lateral fac- In almost all tardigrades accessory interverte- et typical of diaphragmatic vertebrae in other bral articulations make their initial in appearance sloths. the posterior thoracic vertebrae. As in Hapalops, The accessory articulations are smaller and accessory articulations usually occur as a single fewer in number in sloths compared to those of articular surface between the ventral surface of a other xenarthrans. However, the articulations are rather indistinct anapophysis and the dorsal sur- reduced even further in several tardigrade gen- face of an extension of the vertebral lamina lying era. In the North American Pleistocene genus lateral to the base of the metapophysis. These ar- Nothrotheriops, the morphology of the articula- ticulations extend from the joint between the di- tions is much like that described above for Ha- aphragmatic and postdiaphragmatic vertebrae to palops. The diaphragmatic vertebra occurs in the the lumbosacral joint. In the majority of taxa, this lumbar vertebrae, however, rather than lateral zygapophyseal articulation is flat, although posterior in the thoracic vertebrae. As in other in Planops (Hoffstetter, 1961) and Paramylodon posterior has three lumbar verte- harlani (Stock, 1925) it curves dorsomedially sloths, Nothrotheriops onto the lateral surface of the metapophysis and brae, the second of which is the diaphragmatic the medial edge of the anapophysis. vertebra, so that the accessory intervertebral ar- A second accessory articulation, a true xenar- ticulations occur only between L2 and L3 and throus facet between the dorsal surface of the between L3 and SI (Stock, 1925). In Choloepus anapophysis and the ventral portion of the me- the reduction is even more significant. The dia- tapophysis, occurs only rarely among tardi- phragmatic vertebra of Choloepus hoffmani grades. It has, however, been observed in three (fmnh 127422, 147993) is the 21st, or antepen- genera from the Miocene Santa Cruz formation ultimate, thoracic, a fairly typical position for a of where it be vari- Patagonia; Hapalops, may tardigrade. Metapophyses are well developed on as noted ably present, above; Schismotherium, T21 and more posterior vertebrae. The anapo- where its is variable in again presence (observed physis first appears as a small nubbin on T23 and fmnh PI 3 137 but not in amnh 9244 or in speci- becomes progressively larger on LI, L2, and L3. mens described Scott, 1903-1904); and Pre- by Nevertheless, the only accessory articulation prepotherium, a genus closely related to Planops sent occurs at the lumbosacral joint, and even (Scott, 1903-1904). then only on one side of that joint (fmnh 127422 In most tardigrades separate medial and lateral = = left; fmnh 147993 right). In a juvenile Chol- zygapophyseal facets do not occur anterior to the oepus specimen (fmnh 127421) only slightly diaphragmatic vertebra, but several taxa exhibit at younger than the juvenile Tolypeutes (fmnh least incipient separation of the facets in the pre- 124569) described above (based on the degree of diaphragmatic vertebra. In the Santacrucian genus fusion of the neural arches in the midline), the Pelecyodon (amnh 9240), the anterior zygapophy- accessory articulations are absent, as are meta- seal surface of the diaphragmatic vertebra is and When the formed by two contiguous facets oriented approx- pophyses anapophyses. present, intervertebral of re- imately 120° to one another. The medial facet is accessory joint Choloepus sembles that of other the form oriented horizontally, and the lateral facet slopes tardigrades, taking of a lateral articulation between ventrolaterally. The posterior zygapophyseal fac- zygapophyseal the ventral surface of the and the ets on the prediaphragmatic vertebra are similarly anapophysis constructed. A very similar morphology has been dorsal surface of a facet situated immediately lat- described by Stock (1925) in Paramylodon har- eral to the base of the metapophysis.

GAUDIN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE 25 B

2. Medial zygapophyseal facet

3. Lateral 4. Xenarthrous zygapophyseal facet facet

Fig. 13. Diagrammatic representation of the morphology of typical xenarthran intervertebral facets in the posterior thoracic or lumbar vertebrae. A, anterior view; B, posterior view. Abbreviations as in Figure 1.

Conclusions thrans (except glyptodonts, in which the vertebrae are fused), the postdiaphragmatic vertebrae bear Morphological Summary on each side a curved, vertically oriented intervertebral facet that lies medial to the metapoph- The data described above are morphological ysis and adjacent to the midline anteriorly (Fig. summarized in Table 1 The in- Appendix (p. 36). 13). Posteriorly, the corresponding facet lies at the tervertebral articulations between the thoracic and base of the neural spine adjacent to the midline. lumbar vertebrae of xenarthrans can be placed As noted above, the facets are homologized with into four distinct categories. the zygapophyseal facets of other mammals by 1. Normal zygapophyseal facets. The interver- Owen (1851a) and others (Hoffstetter, 1958, tebral articulations between the anterior thoracic 1982; Lessertisseur & Saban, 1967; Gaudin & vertebrae in most xenarthrans differ little from the Biewener, 1982; Rose & Emry, 1993; contra thoracic zygapophyseal facets found in other Flower, 1885; Grasse, 1955; Vaughn, 1986). mammals (Fig. 1; Walker & Homberger, 1992). These facets are structurally identical to those pre- Morphological departures from the normal mam- sent in the postdiaphragmatic vertebrae of other malian pattern include widening of the anterior mammals (see Fig. 1). Given this fact, and their and posterior zygapophyses in pilosans and eu- near-universal distribution within Xenarthra, I see phractan armadillos (Table 1, Fig. 3B), so that the no reason to doubt this homology. zygapophyses extend further laterally than the In several xenarthran taxa, the medial portion vertebral centra. In addition, pilosans possess a of the zygapophyseal facet of some prediaphrag- broad midline notch that separates the anterior matic vertebrae is separated from the lateral por- zygapophyseal facets (Fig. 2B,C). tion by a small groove or ridge. This separation Typical mammalian zygapophyses are present occurs in the first vertebra of only in the prediaphragmatic vertebrae of the ma- prediaphragmatic several sloth taxa. In the euphractan ar- jority of xenarthrans. The exceptions are the liv- ground madillos it occur as far forward as the third ing tree sloths Bradypus and Choloepus, in which may the normal mammalian morphology extends from thoracic vertebra. 3. Lateral A of one {Bradypus) to six vertebrae {Choloepus) pos- zygapophyseal facets. majority xenarthran taxa an anterior facet on each terior to the diaphragmatic vertebrae. In addition, possess the vertebrae that lies in euphractan armadillos and some extinct sloths, side of postdiaphragmatic the zygapophyses of some prediaphragmatic tho- on a lateral extension of the vertebral lamina, sit- lateral to the base of the me- racic vertebrae begin to show evidence of incipi- uated immediately ent division into distinct medial and lateral facets tapophysis (Fig. 13). A corresponding facet is (see below). found posteriorly, on the ventromedial surface of 2. Medial zygapophyseal facets. In all xenar- the anapophysis. As noted by Jenkins (1970) for

26 FIELDIANA: GEOLOGY Tamandua, these facets closely resemble in struc- a ventrolateral articular facet that apparently cor- ture and position the lateral portion of the zyg- responds to the facet just described for more anapophysis of the prediaphragmatic vertebrae. I terior vertebrae. In the ultimate thoracic vertebra therefore suggest that these facets are serially ho- it articulates with a facet carried on the dorsal mologous with the lateral parts of the zygapophy- surface of the transverse process of LI (Fig. 4D). seal articulations of the prediaphragmatic verte- This type of articulation continues posteriorly brae. These facets are considered accessory zyg- through the lumbar vertebrae to the lumbosacral apophyseal articulations, termed "lateral zyg- joint. A similar facet is present between the ven- apophyseal facets," and are distinct from true tral portion of the anapophysis and the dorsal por- "xenarthrous" facets. tion of the transverse process/sacral rib in Myr- The homology of these lateral zygapophyseal mecophaga (at L1/L2 and the lumbosacral joint) articulations is less secure than that of the medial and Tamandua (at the lumbosacral joint only), but zygapophyseal facets because their taxonomic not in Cyclopes. This type of facet is unknown in distribution is not entirely congruent with the tax- sloths, although some specimens of Bradypus are onomic distribution of the medial facet. Two taxa, characterized by an articulation formed between the tree sloths Bradypus and Choloepus, lack lat- the distal tip of the transverse process of L3 and eral zygapophyseal articulations in at least some the anterior edge of the sacral rib. of the postdiaphragmatic vertebrae. However, the Finally, the vertebrae of several groups of xe- normal zygapophyseal articulations are poorly de- narthrans possess unpaired midline facets that veloped in both of these taxa, with successive ver- may be generally categorized as xenarthrous. The tebrae fitting together only loosely. Moreover, the anterior facet is typically found on the dorsal sur- vertebral processes are weak in both taxa. It there- face of the vertebral lamina, posterior to the zyg- fore seems likely that the absence of lateral zygapophyseal articulations and anterior to the base apophyseal facets in these taxa represents a sec- of the neural spine. The posterior facet lies on the ondary reduction. undersurface of the neural spine. These facets are 4. True xenarthrous facets. There are several particularly characteristic of large-bodied xenar- different types of facets found in various xenarthrans, such as the giant armadillo Priodontes thran taxa that can be categorized as true "xenar- (Fig. 7) and megatheriid, scelidotheriine, and throus" accessory intervertebral articulations. some mylodontine ground sloths. The most common is a facet borne, on each side, on the ventrolateral surface of an extension projecting from the base of the metapophysis; the Phylogeny and Evolution of Xenarthrous corresponding posterior facet lies on the dorsal Vertebrae surface of the anapophysis (Fig. 13). Such facets are found on the diaphragmatic and postdiaphrag- When the data on intervertebral articulations matic vertebrae of all cingulates and vermilin- are plotted on a phylogeny of Xenarthra (Fig. 14), guas, and may extend as far forward as the second a number of features may be identified that appear prediaphragmatic vertebra in these groups. These to be primitive characteristics of xenarthrous ver- facets are also variably present in a few genera of tebrae. The prediaphragmatic thoracic vertebrae early megalonychimorph sloths (Hapalops, Schis- probably have abnormally wide zygapophyses, al- motherium, Prepotherium), and hence they likely though this feature is absent in many cingulates. represent the primitive condition for tardigrades The metapophyses are large, especially in the pos- as well. They are often fused to the lateral zyg- terior thoracic and lumbar vertebrae. In the post- apophyseal facets. diaphragmatic vertebrae, these large metapophy- A second type of facet occurs on the ventrolat- ses split the zygapophyseal articulations into sep- eral surface of the anapophysis of some thoracic arate medial and lateral joints. In addition, each vertebrae in armadillos for articulation with a rib. metapophysis bears a ventrolateral articular facet Such facets are found on T8-T10 (the penultimate that articulates with the dorsal surface of the an- thoracic vertebra) in Zaedyus (Fig. 4C) and T6 or apophysis of the preceding vertebra. Large an- T7-T9 (again the penultimate thoracic vertebra) apophyses may also be a primitive characteristic in Dasypus, but have not been observed in any of xenarthrous vertebrae. However, the anapo- pilosan. These articular surfaces do not serve an physes are only weakly developed in sloths and intervertebral function. However, the anapophysis in the anterior xenarthrous vertebrae of cingulates. of the last thoracic vertebra of cingulates carries The goal of the present study was not only to

GAUDIN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE 27 Cingulata Vermilingua Tardigrada l)Wide zygapophyseal facets l)Wide zygapophyseal facets l)Wide zygapophyseal facets (only in euphractans) 2) Enlarged metapophyses 2) Enlarged metapophyses 2) Enlarged metapophyses (in most taxa) 3) Enlarged anapophyses 3) Enlarged anapophyses 3) Weak anapophyses (only in post. thor. and lumb. vert.) 4) Medial and lateral zygapophyseal 4) Medial and lateral zygapophyseal 4) Medial and lateral zygapophyseal facets in post-diaphragmatic facets in post-diaphragmatic facets in post-diaphragmatic vertebrae vertebrae vertebrae (in most taxa) 5) Xenarthrous articulations between: 5) Xenarthrous articulations between: 5) Xenarthrous articulations between: metapophysis and anapophysis metapophysis and anapophysis metapophysis and anapophysis anapophysis and rib (thor. vert.) anapophysis and transverse (several Santacrucian genera) transverse neural anapophysis and process (lumbo-sacral joint, spine and lamina process (lumb. vert.) Tamandua & Myrmecophaga (large ground sloths only) neural spine and lamina only) (Tolypeutes & Priodontes only)

PilOSa l)Wide zygapophyseal facets 2) Enlarged metapophyses 3) ? anapophyses 4) Medial and lateral zygapophyseal facets in post-diaphragmatic vertebrae 5 Xenarthrous articulations XENARTHRA ) metapophysis and DVVide zygapophyseal facets ^^ anapophysis 2) Enlarged metapophyses 3) Enlarged anapophyses, (in post thor. and lumb. vert.) 4) Medial and lateral zygapophyseal facets in post-diaphragmatic vertebrae 5) Xenarthrous articulations between metapophysis and anapophysis

Fig. 14. Phylogeny of the Xenarthra (following Engelmann, 1985; Gaudin, 1993), showing the distribution of vertebral character states among the three major suborders, and the inferred character states at the ancestral nodes "Pilosa" and "Xenarthra." For further description of each character state, see text. Abbreviations: lumb. vert., lumbar vertebrae; post. thor. vert., posterior thoracic vertebrae; thor. vert., thoracic vertebrae.

analyze the morphology of xenarthrous vertebrae mediate condition between nomarthry and xenar- across the Xenarthra as a whole, but also to un- thry." As mentioned above, however, MacPhee derstand the structural evolution of these acces- (1994, p. 174) suggested that the development of sory intervertebral facets. Although determining accessory intervertebral facets may be the result the primitive morphology of xenarthrous verte- of a process of "sacralization," whereby the pos- brae is an important step in understanding their terior dorsal vertebrae become more solidly at- evolution, a more complete explanation requires tached to one another in a manner analogous to information on intermediate conditions leading to the intimate union characteristic of mammalian the appearance of fully developed xenarthrales. sacral vertebrae. Such an analogy seems particu- The difficulty in determining the structural gen- larly apt for an animal such as Scutisorex (Les- esis of xenarthrous intervertebral facets stems sertisseur & Saban, 1967; Kingdon, 1984), in

from what MacPhee (1994, p. 174) described as which the posterior thoracic and lumbar vertebrae an "apparent absence of any recognizable inter- are united laterally by closely interlocking bony

28 FIELDIANA: GEOLOGY 8 excrescences. It is less clear whether the analogy zygapophyseal facets reminiscent of those in the can be usefully applied to xenarthrans, in which anterior thoracic vertebrae. In a young juvenile the vertebrae are not synostosed laterally or con- Choloepus specimen, the sacral vertebrae lacked nected by tight fibrous joints but rather are con- anapophyses altogether. It would seem at present nected by extra synovial joints that limit interver- that there is little evidence in support of sacral- tebral mobility (Gaudin & Biewener, 1992). ization as the process through which xenarthrous Whereas MacPhee (1994) noted that the sacral el- articulations originated. ements in some mammals remain unfused In contrast to MacPhee's (1994) claim, I sug- throughout life and hence might serve as an ap- gest that documented structural intermediates be- propriate xenarthran analogue, the fused condition tween xenarthrous and nomarthrous vertebrae do is certainly primitive for xenarthrans, and perhaps exist. Among tardigrades, the xenarthrous articu- for mammals as a whole (Jenkins & Schaff, 1988; lations are reduced relative to the primitive con- Kielan-Jaworowska & Gambaryan, 1994; but see dition for the order as whole. The reduction in- also unfused sacral vertebrae in Jenkins & Par- cludes not only a diminution in the number of tington, 1976; Krebs, 1991; Marshall et al., 1995). intervertebral joints that possess accessory artic- In support of MacPhee 's (1994) hypothesis is ulations (see Bradypus, Nothrotheriops, and Chol- the observation that in several xenarthran taxa oepus, above), but also a simplification of the ac- some or all of the lumbar vertebrae are fused to cessory articulations themselves. The vast major- the sacral vertebrae. This lumbosacral fusion oc- ity of sloths lack a xenarthrous joint between the curs in the dasypodid armadillos Priodontes and ventral surface of the metapophysis and the dorsal Tolypeutes, in the euphractan armadillos Euphrac- surface of the anapophysis. Weakly developed an- tus and Chaetophractus, in glyptodonts (Gillette apophyses are universally characteristic of sloth & Ray, 1981), in some mylodontine sloths (Owen, vertebrae. Sloths retain only the wide predia- 1842; Stock, 1925), and in some West Indian me- phragmatic zygapophyseal facets, large met- galonychid sloths (Matthew & Paula Couto, apophyses, and medial and lateral postdiaphrag- 1959). Nevertheless, the phylogenetic distribution matic zygapophyseal facets characteristic of prim- of lumbosacral fusion suggests that it is a derived itive xenarthran vertebrae. Even these medial and feature of these lineages and not a primitive fea- lateral postdiaphragmatic zygapophyseal facets ture of Xenarthra. Moreover, in the juvenile spec- may be restricted to the last few lumbar vertebrae imens of Tolypeutes and Tamandua described in several taxa, most notably the extant tree sloths. above, the lumbosacral joint is the last to develop The presence of xenarthrous joints between the accessory intervertebral joints. This observation metapophysis and anapophysis in several relative- would seem to argue against a claim of some ly early sloth genera, e.g., Hapalops and Prepo- structural continuity between sacral and more an- therium, suggests that the xenarthrales in sloths terior vertebrae. are reduced secondarily. Although the manner in MacPhee (1994) asserted that the demonstra- which these facets are reduced need not parallel tion of accessory intervertebral joints in the sa- the process by which they originate, I believe that crum of developing xenarthrans (or indeed non- the sloth condition, as the only known structural xenarthran mammals) would strongly support the intermediate between xenarthry and nomarthry, sacralization hypothesis. No evidence of acces- represents the best available tool for understand- sory synovial joints is present, however, in the ing the evolutionary origin of xenarthrous verte- skeletal remains of very young juvenile individ- brae. uals representing all three major subgroups of xe- Using the condition in sloths as a model, I sug- narthrans (armadillos, anteaters, tree sloths) in gest that the first step in the evolution of xenar- fmnh collections. Indeed, in the youngest Taman- throus articulations was a widening of the zyg- dua specimen examined the sacral anapophyses apophyseal facets and enlargement of the met- do not contact the succeeding vertebrae. Rather, apophyses, both becoming more extensive in pro- the sacral vertebrae were joined through wide gressively more posterior thoracic and lumbar vertebrae. The widening of the zygapophyses should increase the lateral moment of resistance 8 Such excrescences are also in several bony present of the joints and hence enhance the stability of fmnh specimens of Tamandua, e.g., fmnh 137419, the vertebral column in lateral bending (see be- 140912, and 150733. All, however, are zoo specimens. The would It therefore seems likely that this condition is an age- low). enlarged metapophyses provide related pathological state. a larger area for origin of the epaxial transverso-

GAUDIN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE 29 spinalis muscles. At the joint between the dia- tem has a particularly important role to play in phragmatic and first postdiaphragmatic vertebrae, resisting the large dorsal and lateral reaction forc- the metapophyses would become sufficiently es generated by digging. The axial skeleton of xenarthrans is stiffened relative to the large to split the widened zygapophyseal facets primitive perforce into separate medial and lateral zyg- mammalian condition in dorsal and lateral bend- The of wide in ear- apophyseal joints. These medial and lateral zyg- ing. appearance zygapophyses xenarthrans would increase the lat- apophyseal joints would then continue posteriorly ly effectively eral moment of resistance of the stiff- to the lumbosacral joint. vertebrae, the intervertebral in lateral The second step in the evolution of xenarthrales ening joints bending. would involve the development of the other two Because xenarthrans dig primarily with their fore- the vertebral column is loaded as a canti- primitive features of xenarthrous vertebrae, an en- limbs, with loads This larged anapophysis and a xenarthrous joint be- lever, increasing posteriorly. would account for the increase in tween the dorsal surface of the anapophysis and progressive width of the facets in more caudal the ventrolateral surface of the metapophysis. zygapophyseal vertebrae. would result from These two structurally linked modifications would Large metapophyses hypertrophy of transversospinalis muscles, which, serve to further enhance the stability of the spinal according to Gaudin and Fortin (unpubl. data), column, particularly in lateral and dorsal bending play an important role in stabilizing the vertebral (Gaudin & Biewener, 1992). The final steps in the column dorsal these mus- evolution of the xenarthrous vertebrae would in- during bending. Again, cles would be expected to increase in size poste- volve the appearance of the various types of spe- in concert with the increase in dorsal bend- cialized xenarthrous facets in the major xenar- riorly ing forces. thran subgroups, e.g., the joints between anapoph- It is of particular interest that the above model yses and transverse processes in cingulates and of xenarthrous vertebral evolution does not ini- some anteaters. These presumably function to tially include enlargement of the anapophyses. provide additional stiffness to the vertebral col- Enlarged anapophyses have been considered by umn while preserving a certain degree of mobil- several authors (Simpson, 1931; Ding, 1987; ity. Secondary loss of some of these facets in Storch, 1981; see below) as indicating incipient sloths might be the result of their switch from a xenarthry. However, Gaudin and Fortin (unpubl. digging to a more terrestrial or semiarboreal hab- data) note that the longissimus dorsi muscles, itus (Gaudin, 1993; White, 1993a,b). which take their origin from the anapophyses, are The above model contrasts with that of Mac- not enlarged in xenarthrans relative to the primi- Phee (1994) in postulating that xenarthrales de- tive mammalian condition and likely play little or from the thoracic vertebrae back velop posterior no role in enhancing dorsal and lateral stiffness. rather than from the sacral vertebrae forward. The Further, Gaudin and Biewener (1992) were able new model accords well with the observation that to cut the anapophyses in the posterior lumbar xenarthrous articulations in xenarthrans living vertebrae of Dasypus without significantly de- tend to become more and that complex caudally, creasing either dorsal or lateral stiffness. Gaudin these more complex caudal articulations appear and Biewener (1992) presented evidence from later in development (see Tolypeutes and Taman- strain gauge analyses suggesting that enlarged an- dua above). It conforms with what is known about apophyses serve to reduce shear stress in dorsal the mechanics of the mammalian spine. The mam- bending, to augment the lateral moment of resis- malian backbone tends to exhibit maximum flex- tance in lateral bending, and to transmit forces ibility in the mid-dorsal region, in the vicinity of from the forelimbs to the robust pelvic girdle and the diaphragmatic vertebra (Slijper, 1946; Jenkins, hind limbs in both dorsal and lateral bending. 1974). Thus one might predict that adaptations to Nevertheless, the functional and phylogenetic ev- reduce spinal mobility might first appear in this idence appears to indicate that enlarged anapoph- are structural antecedent of region. Finally, the model fits well with what is yses not a necessary known about the functional morphology of xe- xenarthrous intervertebral articulations. narthrous vertebrae. Gaudin and Biewener (1992; see also Gaudin, 1993) have reaffirmed recently Relationship of Xenarthra to Early Cenozoic the idea that Xenarthra an offshoot of represents Fossil Taxa early placental mammals that were primitively specialized for digging. They note further that in The final goal of the present study was to utilize digging mammals, the axial musculoskeletal sys- the above conclusions on the structural evolution

30 FIELDIANA: GEOLOGY of xenarthrales to evaluate the taxonomic affinity 20028; Gaudin, pers. observ.) and the diaphrag- of several enigmatic early Cenozoic taxa with al- matic vertebra of Metacheiromys (usnm 26132; legedly close ties to Xenarthra. Of particular con- Gaudin, pers. observ.). They are not so broad, cern are the Palaeanodonta, a group of fossorial however, as in xenarthrans, in which width often mammals with reduced dentitions that is known exceeds anteroposterior length by a factor of two from Paleogene strata of North America (Rose et or more (Table 1). Simpson (1931, fig. 12c) fig- al., 1991, 1992; Gunnell & Gingerich, 1993) and ured narrow anterior zygapophyseal facets on the Europe (Hessig, 1982); Ernanodon, a taxon based 10th or 11th thoracic vertebra of Metacheiromys, on a single skeleton found in Late Paleocene sed- but these facets are situated well lateral to the iments from China (Ding, 1987); and Eurotaman- midline. In no case, however, are distinct medial dua, a purported anteater from the Middle Eocene and lateral zygapophyseal facets observed in any Messel fauna of Germany (Storch, 1981). palaeanodont. The anapophyses of palaeanodonts Simpson (1931) proposed a close phylogenetic are enlarged (Simpson, 1931; Rose et al., 1992), link between palaeanodonts and xenarthrans again in the vicinity of the diaphragmatic verte- based on various lines of evidence, including the bra. Although relatively deep on T12 and T13 of morphology of the vertebral column (see Emry, Alocodontulum (Rose et al., 1992, fig. 3), the an- 1970, for contrasting interpretations). In his de- apophyses of palaeanodonts tend to be dorsoven- scription of the vertebrae of the Eocene palaean- trally narrow and spine-like. In this respect they odont Metacheiromys, Simpson (1931, p. 334) more closely resemble the anapophyses of carni- stated that "While xenarthrous articulations are vores than those of xenarthrans (Rose et al., 1992; not definitely incipient in Metacheiromys, . . . [its Rose & Emry, 1993). Furthermore, there is no morphology] seems ... an ideal point of departure indication of articular surfaces between the ana- for the origin of the secondary articulations." Yet pophyses and the succeeding metapophyses or Matthew (1918, p. 629), in his description of the transverse processes. Late Paleocene (or Early Eocene; see Gunnell & On balance, it would appear that palaeanodont Gingerich, 1993) genus Palaeanodon, could find vertebrae share few if any of the derived charac- "no recognizable foreshadowing of the peculiar teristics of xenarthran vertebrae unequivocally. 'xenarthral' articulations" characteristic of true The strongest similarity between the vertebrae of xenarthrans. the two groups is the enlargement of the meta- In part the discrepancy results from the poor pophyses. This specialization, however, is known state of preservation of the vertebrae of Palaean- to occur in many groups of fossorial mammals odon. Other Paleocene palaeanodonts lack pre- (Gaudin & Fortin, unpubl. data), and hence may served vertebrae (Rose, 1978, 1979), and hence likely be convergently acquired. shed no light on the problem. Better preserved Among other supposed similarities to Xenar- material exists from the Early and Middle Eocene, thra, Ding (1987, p. 92) noted the posterior tho- representing both families of palaeanodonts racic vertebrae of Ernanodon, which bear "lon- (Epoicotheriidae and Metacheiromyidae). In the gitudinally elongated projections under the mam- epoicotheriid Alocodontulum and the metacheiro- millary process [= metapophysis], which are in- myid Metcheiromys, certain resemblances to xe- cipiently developed xenarthrous articulations." As narthran vertebrae can be observed (Simpson, in xenarthrans and palaeanodonts. the posterior 1931; Rose et al., 1992). The metapophyses are thoracic and lumbar vertebrae of Ernanodon are enlarged, particularly in the posterior thoracic ver- characterized by well-developed metapophyses. A tebrae in the vicinity of the diaphragmatic verte- pair of large vertebral processes extend lateral to bra. Unlike xenarthrans, however, the metapophy- the metapophyses, but do not form joints with the ses become progressively smaller in more poste- metapophyses. These lateral processes (Ding, rior lumbar vertebrae. It is unclear whether or not 1987, fig. 7) apparently represent large anapophy- the thoracic zygapophyseal facets of palaeano- ses. In the posterior thoracic vertebrae the ana- donts are particularly enlarged transversely. Rose pophyses arise anteriorly from the back of rather et al. (1992, p. 226) describe the lumbar prezy- low, weakly developed diapophyses, a condition panophyses of Alocodontulum as "broad." Broad that only vaguely resembles that characteristic of antero- true xenarthrans. In the lumbar , mediolateral width much greater than most vertebrae, .terior length) zygapophyseal facets have also the anapophyses arise from short, anteroposteri- ;n observed in the lumbar vertebrae of the Ear- orly elongated transverse processes, the latter ly Eocene epoicotheriid Pentapassulus (usnm reminiscent of those found in extinct sloths. The

GAUDIN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE 31 anapophyses are relatively deep in the thoracic anapophysis and metapophysis or a combined an- vertebrae, but are oriented more dorsally than apophyseal/metapophyseal xenarthrous facet and among unequivocal xenarthrans. The anapophyses a lateral zygapophyseal facet. The ventral articu- of the lumbar vertebrae are not illustrated in lat- lation would then represent a xenarthrous joint be- eral view, and the width of the zygapophyseal fac- tween the anapophysis and the transverse process ets in Ernanodon is unclear. They are described of the first lumbar vertebra. Note that a joint be- as "oval" in the anterior thoracic vertebrae, in- tween the anapophysis and transverse process is creasing in size posteriorly (Ding, 1987, p. 92). not a primitive characteristic of xenarthran or ant- However, the zygapophyseal joints of the lumbar eater vertebrae, according to the phylogenetic dis- vertebrae are enrolled, as are those of pholidotans tributions presented above (Fig. 14). Storch (Emry, 1970); such a condition is uncommon (1981) suggests that less elaborate accessory in- among true xenarthrans, where enrollment occurs tervertebral articulations can also be observed be- in only a few derived taxa (e.g., Euphractus, Prio- tween the diaphragmatic and the preceding ver- dontes). There is no evidence of any accessory tebra, and perhaps even at the next anterior inter- intervertebral articulations in Ernanodon, be they vertebral junction. lateral zygapophyses or xenarthrous joints be- Unfortunately, there remain a number of diffi- tween anapophyses, metapophyses, and transverse culties that render Storch's (1981) morphological processes. assessments open to question. Several subsequent As with the palaeanodonts, the case for strong workers have been unable to verify the presence shared derived similarity between the vertebrae of of the xenarthrous articulations in Eurotamandua Ernanodon and undoubted xenarthrans is weak. (Rose & Emry, 1993; Szalay & Schrenk, 1994). The most notable resemblances are the enlarged Szalay and Schrenk (1994, p. 48A) state that "xe- metapophyses and anapophyses. However, the narthry cannot be corroborated (and not only be- former are widespread among digging forms, as cause of the nature of preservation)." The lumbar noted above. The latter differ slightly in their mor- vertebrae, which on the basis of comparison with phology from those of xenarthrans. Moreover, it undoubted xenarthrans would be expected to is not clear that enlarged anapophyses are char- show the most well-developed xenarthrous artic- acteristic of Xenarthra primitively. ulations, are preserved with only the dorsalmost Eurotamandua presents the most striking claim portions of the vertebrae exposed. This prevents for derived resemblance between the vertebrae of observation of any lateral accessory joints. In ad- undoubted xenarthrans and a non-South American dition, the morphology of the accessory joints early Cenozoic taxon. As in palaeanodonts and themselves, particularly those of the prediaphrag-

Ernanodon, the vertebrae of Eurotamandua pos- matic vertebrae, is unusual (Storch, 1981, fig. 8). sess large metapophyses, beginning three to four The two vertebrae anterior to the diaphragmatic vertebrae anterior to the diaphragmatic vertebra vertebra have narrow, spine-like anapophyses, and extending posteriorly through the lumbar ver- very similar to those found in palaeanodonts and tebrae (Storch, 1981). Similarly, enlarged an- unlike anything known in Xenarthra. These anapophyses are present in the diaphragmatic region apophyses allegedly articulate with a small con- of the vertebral column. Unlike what is found in cave facet or depression on the pedicel of the suc- palaeanodonts and Ernanodon, the anterior tho- ceeding vertebra. This latter facet lies midway be- racic vertebrae of Eurotamandua are very remi- tween the base of the metapophysis dorsally and niscent of armadillo (but not anteater) vertebrae, the base of a second process ventrally. The more with elongated, anteroposteriorly narrow neural ventral process apparently carries a facet for the spines, broad laminae, and elevated diapophyses. rib tubercle distally, and hence it must represent 9 The most remarkable aspect of the vertebral mor- a ventrally situated diapophysis. This type of ar- phology in this genus, however, is its purported ticulation, between the anapophysis anteriorly and possession of true accessory intervertebral artic- the metapophysis and diapophysis posteriorly, is ulations. not present in any known xenarthran. The diaphragmatic vertebra of Eurotamandua, 9 the last thoracic according to Storch (1981), has Storch (1981) labels it a parapophysis, but I am not aware of mammal that a true parapophysis a large anapophysis that apparently bears both any possesses in the posterior thoracic vertebrae. Hence I believe it dorsal and ventral articular facets. By analogy more likely to represent a ventrally displaced diapoph- with the former would unequivocal xenarthrans, ysis, articulating with the tubercle rather than the head represent either a xenarthrous facet between the of the rib.

32 FIELDIANA: GEOLOGY In summary, the case for derived resemblance tions in this report. For providing access to spec- between the vertebrae of Eurotamandua and un- imens under their care, including the loan of the doubted xenarthrans remains less than compel- figured specimens, I am grateful to Larry Heaney, ling. Like palaeanodonts and Ernanodon, Euro- Bruce Patterson, and Bill Stanley of the Division tamandua does possess enlarged metapophyses of Mammals, Field Museum of Natural History, of and anapophyses, but these are equivocal phy- and John Flynn and Bill Simpson of the Depart- of the logenetic utility. The width zygapophyses ment of Geology, Field Museum of Natural His- in Eurotamandua cannot be assessed from the tory. I thank Gerry Deluliis, Laura Panko, and published descriptions and figures. Storch (1981) two anonymous reviewers for their comments on claims that intervertebral articulations accessory an earlier draft of this manuscript. The research are a lateral present, including possible zygapoph- on which this report is based was supported by a and xenarthrous between the an- yseal joint joints UC Foundation Faculty Research Grant from the and the and apophysis metapophysis, anapophysis University of Tennessee at Chattanooga. transverse process, and the anapophysis/ metapophysis and diapophysis. However, the latter joint is unknown in undoubted xenarthrans, Literature Cited and the presence of all such joints has been ques- G. M. 1913. A new Memoirs of the tioned by subsequent workers. Allen, Mylodon. Museum of Comparative Zoology, 40: 318-346. On the basis of vertebral morphology, there is Anthony, H. E. 1918. The indigenous land mammals of at present little evidence that would clearly sug- Porto Rico, living and extinct. Memoirs of the Amer- gest a close phylogenetic relationship between ican Museum of Natural History, 2: 331-435. true xenarthrans and Ernanodon, palaeanodonts, Cullinane, D. M., and D. Aleper. 1998. The functional or Eurotamandua. Assessment of the vertebral and biomechanical alterations of the spine of Scuti- morphology in the latter two taxa is hindered by sorex somereni, the hero shrew: Spinal musculature. Journal of 244: 453-458. the paucity of available material and the incom- Zoology, London, plete preservation of the existing material. More Cullinane, D. M., D. Aleper, and J. E. A. Bertram. 1998. The functional and biomechanical modifications complete information on the vertebral column of of the spine of Scutisorex somereni, the hero shrew: either Ernanodon or Eurotamandua (in particular Skeletal scaling relationships. Journal of Zoology, the would allow a more reliable assessment latter) London, 244: 447-452. to be made of their potential ties with Xenarthra. Cuvier, G. 1836a. Lecons d'anatomie comparee, vol. 1, Good material is available for Eocene and Oli- 3rd ed. H. Dumont, Bruxelles, 655 pp. The Paleocene material in gocene palaeanodonts. . 1836b. Recherches sur les ossemens fossiles, this group is still poorly known, and thus it is vol. 8, Part VI. Des ossemens d'edentes. 4th ed. E. difficult to arrive at a more complete understand- D'Ocagne, Paris, 461 pp. ing of any phylogenetic link between this group DeBlase, A. E, and R. E. Martin. 1981. A Manual of with to Families of the World. Wil- and the Xenarthra. In light of the present study, it Mammalogy Keys liam C. Brown Publishers, Dubuque, la., 436 pp. is possible that restudy of the younger palaeano- Ding, S.-Y. 1987. A Paleocene edentate from Nanxiong dont material, focusing in particular on the mor- Basin, Guangdong. Palaeontologia Sinica, 173: 1- of the addi- phology zygapophyses, might yield 118. tional derived characteristics linking xenarthrans Edmund, G. 1985. The fossil giant armadillos of North and For these = palaeanodonts. now, however, early America (Pampathcriinae, Xenarthra Edentata), pp. Cenozoic taxa provide little help in elucidating the 83-93. In Montgomery, G. G., ed., The Ecology and and evolutionary history of the peculiar intervertebral Evolution of Armadillos, Sloths, Vermilinguas. Institution 451 articulations that characterize Xenarthra. Our best Smithsonian Press, Washington, D.C. pp. guide to understanding the structural evolution of Emry, R. J. 1970. A North American Oligocene pan- xenarthrous vertebrae would appear to be contin- golin and other additions to the Pholidota. Bulletin of ued of the functional, and study ontogenetic, phy- the American Museum of Natural History, 142: 459- logenetic history of these articulations among the 510. xenarthrans themselves. Engelmann, G. 1978. The logic of phylogenetic analysis and the phylogeny of the Xenarthra. Ph.D. thesis, Co- lumbia University, New York, 329 pp.

. 1985. The phylogeny of the Xenarthra, pp. 51- Acknowledgments 64. In Montgomery, G. G., ed., The Ecology and Evo- lution of Armadillos, Sloths, and Vermilinguas. My thanks go first and foremost to Julia Scott, Smithsonian Institution Press, Washington, D.C. 451 who so skillfully executed all the artistic rendi- pp.

GAUDLN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE 33 Flower, W. H. 1885. An Introduction to the Osteology Jenkjns, F. A., Jr. 1970. Anatomy and function of ex- of the Mammalia. Macmillan and Co., London. 382 panded ribs in certain edentates and primates. Journal pp. of Mammalogy, 51: 288-301.

Frechkop. S. 1949. Replication biologiquc fournie par . 1974. Tree shrew locomotion and the origins of les Tatous. d'un des caracteres distinctifs des X6nar- primate arborealism. pp. 85-1 15. In Jenkins. F. A., Jr., thres et d'un caractere adaptif analogue chez les Pan- ed.. Primate Locomotion. Academic Press, New York. golins. Institut Royal des Sciences Naturelles de Bel- 390 pp. gique, 25: 1-12. Jenkins, F. A., Jr., and E R. Parrington. 1976. The Gal din. T. J. 1993. Phylogeny of the Tardigrada (Mam- postcranial skeleton of the Triassic mammals Eozos- malia. Xenarthra) and the evolution of locomotor trodon, Megazostrodon, and Erythrotherium. Philo- function in the Xenarthra. Ph.D. thesis. University of sophical Transactions of the Royal Society of London, Chicago, Chicago, 111., 226 pp. B273: 387-431.

. 1995. The ear region of edentates and the phy- Jenkins. F A., Jr., and C. R. Schaff. 1988. The Early logeny of the Tardigrada (Mammalia, Xenarthra). Cretaceous mammal Gobiconodon from the Cloverly Journal of Vertebrate Paleontology. 15: 672-705. Formation in Montana. Journal of Vertebrate Paleon- 8: 1-24. Galdin, T J., and A. A. Bihwener. 1992. The functional tology, morphology of xenarthrous vertebrae in the armadillo Kardong, K. V 1995. Vertebrates. Comparative Anat- Dasypux novemcinctus (Mammalia, Xenarthra). Jour- omy, Function, Evolution. William C. Brown Publish- nal of Morphology, 214: 63-81. ers, Dubuque, la., 777 pp. Gervais. M. P. 1855. Part VII. Ordre des Edentes., pp. Kjelan-Jaworowska, Z., and P. P. Gambaryan. 1994. 44-53. In Animaux Nouveaux ou Rares Recueillis Postcranial anatomy and habits of Asian multituber- Pendant 1* Expedition dans las Parties Centrales de culate mammals. Fossils & Strata, 36: 1-92. LAmerique du Sud, de Rio de Janeiro a Lima, et de Kingdon, R. 1984. East African Mammals. An Atlas of Lima au Para. Vol. 1, Anatomic Chez P. Bertrand. Evolution in Africa. Vol. IIA. Insectivores and Bats. Paris. University of Chicago Press, Chicago, 111., 341 pp. Gill. T 1872. Arrangement of the families of mammals Krebs, B. 1991. Das Skelett von Henkelotherium gui- tables. Smithsonian Miscellaneous with analytical morolae gen. et sp. nov. (Eupantotheria, Mammalia) Collections. 230: 1-98. aus dem Oberen Jura von Portugal. Berliner geowis- senschaftliche 1-110. . 1886. Order I. Bruta or Edentata, pp. 46-67. In Abhandlungen. A133: Kingsley, J. S., ed.. The Standard Natural History. Lisshriissi (R. J., and R. Sahan. 1967. Squelette axial, Vol. V Mammals. S. E. Cassino & Co., Boston, Mass. pp. 584-708. In Grasse\ PP., ed., Trait6 de Zoologie. Giuj/ite. D. D.. and C. E. Ray. 1981. Glyptodonts of Vol. 16, Mammiferes. Masson et Cie.. Paris, 1 167 pp. North America. Smithsonian Contributions to Paleo- MacPhee, R. D. E. 1994. Morphology, adaptations and biology. 40: 1-255. relationships of Plesiorycteropus, and a diagnosis of the Glass. B. P. 1985. History of classification and nomen- a new order of eutherian mammals. Bulletin of of Natural 220: 1-214. clature in Xenarthra (Edentata), pp. 1-3. In Montgom- American Museum History, ery. G. G., ed.. The Ecology and Evolution of Ar- Marshall, L. G., C. de Muizon, and D. Sigogneau- madillos, Sloths, and Vermilinguas. Smithsonian In- Russell. 1995. Pucadelphys andinus (Marsupialia, stitution Press, Washington, D.C.. 451 pp. Mammalia) from the early Paleocene of Bolivia. M6- National d'Histoire Naturelle, Par- Grasse, P.-P 1955. Ordre des Edent6s. pp. 1182-1266. moires du Museum In Grasse\ P.P., ed.. Traits de Zoologie. Vol. 17. Mam- is, 165: 1-164. miferes. Masson et Cie, Paris. Matthew, W. D. 1918. A revision of the lower— Eocene Gunnell, G. E, and P. D. Ginoerich. 1993. Skeleton of Wasatch and Wind River faunas. Part V Insectivora Brachianodon weslorum, a new Middle Eocene me- (continued), Glires. Edentata. Bulletin of the Ameri- of Natural 38: 565-657. tacheiromyid (Mammalia. Palaeanodonta) from the can Museum History, Early Bridgerian (Bridger A) of the southern Green MATTHEW, W. D., and C. de Paula Cot to. 1959. The Rjver Basin. Contributions from the Museum of Pa- Cuban edentates. Bulletin of the American Museum leontology, the University of Michigan. 28: 365-392. of Natural History. 117: 1-56. Hi.ssio, K. 1982. Ein Edentate aus dem Oligozan Sud McDonald, H. G. 1987. A systematic review of the deutschlands. Mitteilungen Bayerische Staatssamm- Plio-Pleistocene scelidotheriine ground sloths (Mam- lung fur Palaontologie und Historische Geologic 22: malia: Xenarthra: Mylodontidae). Ph.D. thesis. Uni- 91-%. versity of Toronto, Toronto, Ontario, Canada, 499 pp. Hoiiyn her. R. 1958. Xenarthra, pp. 535-636. In Piv- M< Ki nna. M. C. 1975. Toward a phylogenetic classi- eteau. J., ed.. Traite" de Paleontologic Vol. 2. no. 6. fication of the Mammalia, pp. 21-66. In Luckett, W. Mammiferes Evolution. Masson et Cie. Paris. P., and E S. Szalay, eds., Phylogeny of Primates. Ple- num Press. New York, 483 1961. Description d'un squelette de Planops pp. (Gravigrade du Miocene de Patagonie). Mammalia. Novackk, M. J. 1986. The skull of leptictid insectivor- 25: 57-96. ans and the higher-level classification of eutherian mammals. Bulletin of the American Museum of Nat- . 1982. Les cdent£s x6narthres, un groupe sin- ural 183: 1-112. gulier de la faune Neotropical, pp. 385-443. In Gal- History. litelli, E. M.. ed.. Paleontology. Essential of Historical Novacek, M. J., and A. Wyss. 1986. Higher-level re- Geology. STEM Mocchi Modena Press, Modena. It- lationships of the recent eutherian orders: morpholog- aly. 524 pp. ical evidence. Cladistics. 2: 257-287.

34 FIELDIANA: GEOLOGY Novacek, M. J., A. R. Wyss, and M. C. McKenna. Central Wyoming. Part II. Palaeanodonta (Mamma- 1988. The major groups of eutherian mammals, pp. lia). Annals of the Carnegie Museum, 60: 63-82. 31-71. In Benton, M. J., ed.. The Phylogeny and Clas- Scott. W B. 1903-1904. Mammalia of the Santa Cruz sification of Vol. 2, Mammals. Clarendon Tetrapods. Beds. Part 1 : Edentata. Reports of the Princeton Ex- 326 Press, Oxford, pp. peditions to Patagonia, 5: 1-364.

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GAUDLN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE 35 Appendix Table 1. Summary of Morphological Data

No. Diaphrag- No. of of matic Taxon TV LV vertebra MP AP Spinal nerves

Zaedyus pichiy Appendix Table 2. Summary of Morphological Data

Taxon AMZ and ALZ PMZ and PLZ AX PX Notes

Zaedyus pichiy T3-S1 T2-L3 Ventral mp: T7- Dorsal ap: T6- Opisthocoelus cen- FMNH 104817 amz abuts alz pmz abuts plz Sl, confluent L3, confluent tra, T1-T9; rib from T3-T8 from T2-T7 with alz from with plz from head articulates T9-S1; dorsal T8-L3; lateral between anterior tp: Ll-Sl ap (for rib): centrum and pos- T8orT9- terior pedicel T10; ventral ap: Tl 1-L3

, villo- Ventral 1 Chaetophractus T4(r)/T5(l)-L5 T3(r)/T4(l)-L4 dp: T5- Dorsal ap: T4- L5 fused to S ; op- sus amz abuts alz pmz abuts plz T6; ventral L4, confluent isthocoelus cen- FMNH 60467 from T4/5-T8 from T3-T8 mp: T7-L5, with plz from tra; rib head as confluent with L2-L4; lateral Zaedyus alz from L3- ap (for rib): L5; dorsal tp: T8-T10; ven- L1-L5 tral ap: Tl 1- L4 Euphractus sexcinctus T5-L4 T4-L3 Ventral mp: T7- Dorsal ap: T6- L4 fused to SI; me- fmnh 152051 amz abuts alz pmz abuts plz L4, confluent L3, confluent dial zygapophy- from T5-T9 from T4-T8 with alz from with plz from ses enrolled as in T9-L4; dorsal T8-L3; lateral pangolins; opisth- tp: L1-L4 ap (for rib): ocoelus centra; T8-T10; ven- rib head as Zae- tral ap: Tll- dyus L3 Tolypeutes matacus T9-S1 T8-L4 Ventral mp: Dorsal ap: T6(r)/ Rib head as Zaed- fmnh 124569 (juv.) T7(r)/T8(l)- T7(l)-L4, yus Sl, confluent confluent with with alz in plz in LV; LV ventral ap (for thoracic, lumbar, and sacral ribs): T11-L4 Tamandua mexicana T14-S1 T13-L2 Ventral mp: Dorsal ap: T13- Rib head articulates fmnh 69597 T14-S1, con- L2, confluent exclusively with fluent with alz with plz in anterior centrum; in all; dorsal all; ventral ap: neural spines ro- sacral rib: SI L2 bust, uniform height; anterior zygapophyses separated by midline notch; thoracic zygapophyses wide, extending lateral to centrum Cyclopes didactylus T15-S1 T14-L2 Ventral mp: Dorsal lamina: Rib head, neural fmnh 69969 T13-S1, con- T12-T13; dor- spines, zygapoph- fluent with alz sal ap: T14- yses as Taman- in all L2 dua Bradypus variegatus L2-S1 L1-L3 Absent Absent Rib head articulates fmnh 69589 between posterior centrum and anterior pedicel; all vertebral processes reduced; neural spines uniform height; anterior zygapophyses separated by midline notch

GAUDEN: THE MORPHOLOGY OF XENARTHROUS VERTEBRAE 37 Appendix Table 2. Continued

Taxon AMZ and ALZ PMZ and PLZ AX PX Notes

Choloepus hoffmani SI (r) L3(r) Absent Absent Vertebral processes fmnh 147993 as in Bradypus; anterior zygapophyses separated by midline notch as in Bra- dypus Hapalops sp. T20-S1 T19-L3 PI 3 146 only- PI 3 146 only- Rib head, neural fmnh P13133, P13145, ventral mp: dorsal ap: T22 spines as Taman- P13146, P15318 LI (right side (right side dua\ zygapophy- only), L3 only), L2 ses of anterior thoracics very wide, extending lateral to centrum; anterior zygapophyses separated by narrow midline notch

1 From T7-T9, the lateral zygapophyseal facet is divided in two in this specimen. The "lateral" lateral zygapophyseal facet articulates with the medial surface of the anapophysis of the preceding vertebra.

38 FIELDIANA: GEOLOGY

A Selected Listing of Other Fieldiana: Geology Titles Available

A Preliminary Survey of Fossil Leaves and Weil-Preserved Reproductive Structures from the Sentinel Butte Formation (Paleocene) near Almont, North Dakota. By Peter R. Crane, Steven R. Manchester, and David L. Dilcher. Fieldiana: Geology, n.s., no. 20, 1990. 63 pages, 36 illus. Publication 1418, $13.00

Osteology of Simosaurus gaillardoti and the Relationships of Stem-Group Sauropterygia. By Olivier Rieppel. Fieldiana: Geology, n.s., no. 28, 1994. 85 pages, 71 illus. Publication 1462, $18.00

Giant Short-Faced Bear (Arctodus simus yukonensis) Remains from Fulton County, Northern Indiana. By Ronald L. Richards and William D. Turnbull. Fieldiana: Geology, n.s., no. 30, 1995. 34 pages, 20 illus. Publication 1465, $10.00

Pachypleurosaurs (Reptilia: Sauropterygia) from the Lower Muschelkalk, and a Review of the Pachypleurosauroidea. By Olivier Rieppel and Lin Kebang. Fieldiana: Geology, n.s., no. 32, 1995. 44 pages, 28 illus. Publication 1473, $12.00

The Mammalian Faunas of the Washakie Formation, Eocene Age, of Southern Wyoming. Part III. The Perissodactyls. By Steven M. McCarroll, John J. Flynn, and William D. Turnbull. Fieldiana: Geology, n.s., no. 33, 1996. 38 pages, 13 illus., 5 tables. Publication 1474, $11.00

A Revision of the Genus Nothosaurus (Reptilia: Sauropterygia) from the Germanic Triassic, with Comments on the Status of Conchiosaurus clavatus. By Olivier Rieppel and Rupert Wild. Fieldiana: Geology, n.s., no. 34, 1996. 82 pages, 66 illus. Publication 1479, $17.00

The Status of the Sauropterygian Reptile Genera Ceresiosaurus, Lariosaurus, and Silvestrosaurus from the Middle Triassic of Europe. By Olivier Rieppel. Fieldiana: Geology, n.s., no. 38, 1998. 46 pages, 21 illus. Publication 1490, $15.00

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