FOSSIL TURTLE RESEARCH

VOLUME 1

Proceedings of the Symposium on Turtle Origins, Evolution and Systematics

August 18 – 20, 2003, St. Petersburg, Russia

Edited by Igor G. Danilov and James F. Parham

St. Petersburg, 2006 FOSSIL TURTLE RESEARCH VOLUME 1

Editors: Igor G. Danilov and James F. Parham

Proceedings of the Symposium on Turtle Origins, Evolution and Systematics August 18 – 20, 2003, St. Petersburg, Russia

Published in St. Petersburg, March 2006

Papers should be cited as (e.g.): Joyce W. G. and Karl H.-V. (2006), «The world’s oldest fossil turtle: fact versus fi ction,» in: Danilov I. G. and Parham J. F. (eds.), Fossil Turtle Research, Vol. 1, Russ. J. Herpetol., 13(Suppl.), pp. 104-111.

This issue is published with the fi nancial support of Dr. Ren Hirayama, grants of the President of the Russian Federation to the Leading Scientifi c Schools (Nsh-1647.2003.4 and Nsh-4212.2006.4), grant of the Russian Foundation for Basic Research 04-05-65000-a and with the use of the offi ce and laboratory facilities of the Zoological Institute of the Russian Academy of Sciences.

Cover photo: PIN 52-1a, holotype of Yaxartemys longicauda Riabinin, 1948, Upper Jurassic of Kazakhstan, Karatau Ridge, vicinity of Mikhailovka village Photograph: Igor Danilov

ISSN 1026-2296 Fossil Turtle Research Vol. 1, 2006, pp. 93 — 103

THE PRESENCE OF CLEITHRA IN THE BASAL TURTLE Kayentachelys aprix

Walter G. Joyce1, Farish A. Jenkins, Jr.2 and Timothy Rowe3

A morphological review of all available Kayentachelys aprix material reveals the presence of cleithra, a primitive dermal component of the pectoral girdle. These structures are homologous with the equivalently placed «epiplastral processes» of other basal turtles, thus revealing the unambiguous retention of cleithra in the turtle stem lineage. The occurrence of cleithra in primitive turtles calls into question their placement within crown Sauria, as reacquisition and subsequent loss of cleithra within the turtle stem lineage is thereby implied. Fossil evidence reveals that cleithra were lost twice within the turtle crown group.

INTRODUCTION strap-like cleithrum that overlapped numerous ribs (Jarvik, 1996; Coates et al., 2002; Clack, 2002), The shoulder girdle of tetrapods is a com- resembling in general shape and anatomical posi- posite structure consisting of dermal and endo- tion the scapula of many modern tetrapods. Large chondral elements. Paired anterior and posterior cleithra are present in numerous stem representa- coracoids, scapulae, and suprascapulae represent tives of crown Tetrapoda (Carroll, 1988). Among the endochondral components of the girdle in basal tetrapods. The dermal components are com- prised of a medial interclavicle, paired clavicles, and cleithra (Fig. 1). There is a general trend throughout the phylogeny of tetrapods towards a simplifi cation of this pattern, particularly through the successive reduction of the dermal elements and the increased ossifi cation of the endochon- dral components (Romer, 1956). The history of the cleithrum is characterized by independent occurrences of reduction and loss in various groups of tetrapods. The earliest known stem tetrapods, exemplifi ed by the Devonian taxon Ichthyostega, possessed an extremely large,

1 Peabody Museum of Natural History, Yale University, 170 Whitney Avenue, New Haven, Connecticut 06520, USA. E-mail: [email protected] 2 Department of Organismic and Evolutionary Biology and Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, Massachusetts 02138, USA Fig. 1. The components of the shoulder girdle in primitive 3 The University of Texas at Austin, Department of Geo- tetrapods (modifi ed from Romer, 1956). Abbreviations: aco, logical Sciences, 1 University Station C1100, Austin, Texas anterior coracoid; cl, clavicle; cth, cleithrum; icl, interclavicle; 78712, USA pco, posterior coracoid; sc, scapula; ss, suprascapula. 94 Walter G. Joyce et al. extant amphibians with pectoral girdles, cleithra tles, including Proganochelys quenstedti, Proterochersis are absent in salamanders, but remain preserved robusta Fraas, 1913, and Palaeochersis talampayensis as a narrow ossifi cation along the anterior border Rougier et al., 1995. Reduced processes are known of the scapula in some extant frogs (Duellman for a series of others, including Meiolania platyceps and Trueb, 1986), demonstrating another loss of Owen, 1886, Mongolochelys efremovi Khosatzky, 1997, cleithra within crown Amphibia. Glyptops plicatulus (Cope, 1877), and Xinjiangchelys Relative to the condition seen in basal stem latimarginalis (Young and Chow, 1953) (see Joyce, tetrapods, the cleithrum of basal amniotes is re- In Press, for summary of distribution). duced to a spoon-shaped bar that sits along the If the presence of cleithra can be demonstrat- anterodorsal rim of the enlarged scapula and com- ed for a basal turtle taxon, then the homology of monly overlaps the ascending process of the clavi- the equivalently placed epiplastral processes of cle (Fig. 1) (Carroll, 1988). All living amniotes lack other turtles must be reconsidered as indicating cleithra. However, the presence of cleithra in early the presence of cleithra in basal turtles in gen- stem synapsids taxa (Caseidae, Ophiacodontidae, eral. As a consequence, the commonly hypoth- Edaphosauridae, Sphenacodontidae, and Gorgo- esized placement of turtles within crown Sauria nopsida; Carroll, 1988; Gauthier et al., 1988) and (e.g., deBraga and Rieppel, 1997, Kumazawa and in stem reptilian taxa (captorhinids, milleretids, Nishida, 1999, Zardoya and Meyer, 2000), a clade Macroleter, and Paleothyris; Laurin and Reisz, 1995; that demonstrably lacks cleithra, would entail the deBraga and Rieppel, 1997) indicates that cleithra unparsimonious conclusion that cleithra reap- were lost at least twice within Amniota. peared along the phylogenetic stem of turtles only The shoulder girdle of extant turtles is sub- to be subsequently lost again in crown turtles. The stantially modifi ed relative to the condition in alternative placement of turtles outside of crown ancestral amniotes. Only the coracoid, scapulae, Sauria, in contrast, only requires the primitive re- clavicles, and the interclavicle remain as distinct tention of cleithra with its subsequent loss. bones in the adult. The clavicles and the interclav- In view of the evidence that the newly de- icle are expanded, fl at elements that form the sol- scribed elements in Kayentachelys aprix are cleithra, id anterior part of the plastron of the turtle shell they are referred to as such in this manuscript. (Romer, 1956; Fig. 2). They are more commonly Institutional abbreviations – MCZ, referred to as the epiplastra and the entoplastron, Museum of Comparative Zoology, Harvard respectively. The scapulae and coracoids, in con- University, Massachusetts, USA; MNA, Museum trast, retain their original identity and function of Northern Arizona, Flagstaff, Arizona, USA; of bearing the limbs, but their structure is unique TMM, Texas Memorial Museum, The University among vertebrates by being able to move indepen- of Texas, Austin, Texas, USA; UCMP, University dently of the dermal component and by being situ- of California Museum of Paleontology, Berkeley, ated within the ribcage. Cleithra were originally California, USA. thought to be present (Jaekel, 1915) in the oldest unambiguous fossil «turtle,» Proganochelys quen- MATERIAL AND METHODS stedti Baur, 1887, but the structures in question were later reinterpreted as simple outgrowths of Remains of Kayentachelys aprix are a com- the epiplastra and renamed epiplastral processes mon occurrence in the Lower Jurassic Kayenta (Gaffney, 1990). Massive «epiplastral processes» Formation of Coconino County, Arizona. that stretch from the plastron to the carapace have Specimens are typically found as isolated, broken been reported for a number of other primitive tur- elements, but almost complete skeletons are oc- The presence of cleithra in Kayentachelys aprix 95

Fig. 2. The shoulder girdle of extant turtles as exemplifi ed by Emys orbicularis (redrawn from Bojanus, 1819). Abbreviations: co, coracoid; cl, clavicle (= epiplastron); icl, interclavicle (= entoplastron); sc, scapula. casionally found as well. Sediments in this region Numerous specimens are available with are dominated by variegated mudstones and silt- partially or fully preserved cleithra, epiplastra, stones, ranging from red to purple to blue refl ect- and entoplastra. All material is diagnosable as ing changing redox conditions during deposition Kayentachelys aprix based on the associated cranial and diagenesis. Fossils vary in their preservation or postcranial remains. Given that the visceral from being heavily cracked, crushed, and thickly side of the epiplastra and entoplastron are not ex- coated with iron oxides, to being preserved three- posed in many specimens and that others are too dimensionally and without an oxide coating (Clark heavily encrusted with iron oxides to display any and Fastovsky, 1986). detail, our analysis is restricted to the six best-pre- served specimens: MCZ 8917, an almost complete, 96 Walter G. Joyce et al. partially disarticulated skeleton, including fused the anterior plastral lobe as in Proganochelys quen- left epiplastron and cleithrum (Fig. 3A-B); MCZ stedti. The posterior process of the entoplastron is 8986, a partial skeleton lightly encrusted with not fully preserved in any specimen, but appears iron oxides, including disarticulated epiplastra to have almost reached the hyoplastral–mesoplas- and almost complete entoplastron; MNA V1563, tron contact posteriorly. a fragmentary skeleton, including articulated par- The epiplastra of K. aprix are small elements tial left hyoplastron, entoplastron, right and left that form the anterolateral margins of the plas- epiplastra, and fused partial left cleithrum; TMM tron. Their sutures with the neighboring plas- 43658-1, partial skeleton consisting of disarticu- tral elements are clearly visible in all specimens. lated shell and limb elements, including articulat- Posterolaterally the epiplastra contact the hyo- ed fragment of anterior plastron lobe consisting plastra along a simple, interdigitating suture. The of medial epiplastral parts, anterior half of ento- remaining sutures with the entoplastron, in con- plastron, and the bases of cleithra (Figs. 3C, 4); trast, are complex. Medially the epiplastra contact UCMP 150073, partial shell with almost complete, the entoplastron along oblique sutures that ren- articulated anterior plastral lobe, no cleithra pre- der the visceral exposure of the anterior plastral served; UCMP 150074, partial skeleton consisting process narrower relative to the external expo- of almost complete plastron, including entoplas- sure. Posteromedially, the epiplastra envelop the tron and left epiplastron, no cleithra preserved entoplastron by means of a short, massive ridge (Fig. 3D). extending below and an expansive, and notably Specimens were prepared and examined us- thin fl ange overlapping above. ing a binocular microscope. In addition, TMM Partial cleithra are present in several speci- 43658-1 was scanned using the University of Texas mens, but notably absent from others. When High Resolution x-ray CT Facility (Rowe et al., present, the cleithra are positioned over the pos- 1997; Ketcham and Carlson, 2001) to observe any teromedial, overlapping contact of the epiplastra sutural relationships between elements that may with the entoplastron. In the specimen in which exist within the specimen. the most complete cleithrum is preserved (MCZ 8917; Fig. 3A, B), this element is a slender rod that DESCRIPTION tapers dorsally and broadens ventrally at the artic- ulation with the epiplastron and entoplastron. The The cross-shaped entoplastron of Kayentachelys cleithrum is oriented posterodorsally (at about 50 aprix is characteristic of basal turtles (Fig. 3D). An degrees from horizontal) and is inclined slightly anterior process extends to the anterior rim of laterally. The full extent of the cleithrum is unclear the plastron, thus blocking medial contact of the even in this specimen because the dorsal end is epiplastra. Pronounced lateral processes are pres- broken. However, based on the degree of tapering ent, but they do not reach the lateral margin of and the lack of articular facets on the carapace of

Fig. 3. Stereoscopic photographs of Kayentachelys aprix specimens. A, B – MCZ 8917, an almost completely preserved left cleithrum (cth) fused to an isolated left epiplastron (epi): A – ventral view; B – left lateral view. A crack along the base of the cleithrum (arrow) does not represent a suture, but rather a break to the fossil. C – Ventral view of TMM 43658-1, isolated fragment of the anterior plastral lobe, consisting of the partial left and right epiplastra, the anterior portion of the epiplastron, and the bases of both cleithra (arrows). D – Ventral view of UCMP 150074, an almost complete anterior plastral lobe, consisting of the left epiplastron (epi), the entoplastron (ent), and both hyoplastra (hyo). The overview image below was digitally enhanced to emphasize the location of the articular scar of the missing left cleithrum (arrows) and the suture between the left epiplastron and the entoplastron. All scale bars equal 0.5 cm. The presence of cleithra in Kayentachelys aprix 97 98 Walter G. Joyce et al. all known material of Kayentachelys aprix material, it appears certain that the cleithrum did not pos- sess a bony contact with the carapace, as seen in Proganochelys quenstedti. In cross-section, the base of the cleithrum has the shape of an acute triangle, with its apex pointing anteriorly. The posterior, short side of the triangle bears a shallow groove of uncertain function. In all specimens in which the cleithra are preserved, the posterior overlapping contact of the cleithrum with the entoplastron is clearly visible, but an anterior suture with the epiplastra is absent, indicating that the cleithrum and epiplastra are fully fused with one another in this area. However, high resolution CT scans of TMM 43658-1 (Figs. 3C) reveal the persistence of a dense lamellar zone (arrows, Fig. 4), which repre- sents the original cortical contact of the cleithrum with plastral elements. Furthermore, the denser cleithra are histologically distinct from the under- lying elements. The remaining specimens of Kayentachelys aprix, particularly UCMP 150074, provide a fur- ther perspective on cleithral attachment. In these specimens, cleithra are completely missing and the plastron reveals a slightly depressed articu- Fig. 4. Computer-generated volumetric renderings of the lar surface with no signs of breakage (Fig. 3D). epiplastron and cleithra in Kayentachelys aprix (TMM 43658-1), Furthermore, those parts of underlying epiplas- based on high-resolution X-ray CT imagery consisting of tron that form much of this surface are fragile, but 135 horizontal slices gathered at interslice spacings («slice intact, indicating that the cleithra disarticulated thickness») of 0.0588 mm. A – horizontal slice, anterior toward top of image; B – parasagittal section; C – coronal easily. section. The small arrows indicate the lamellar bone that separate the cleithra from the plastron. DISCUSSION girdle of the primitive tetrapod Archegosaurus dech- The presence of cleithra in primitive turtles eni (Goldfuss, 1847). Unfortunately, the accompa- has not always been controversial. Jaekel (1915) nying description is rather short, leaving unclear was the fi rst to report well-developed cleithra for whether Jaekel (1915) was aware of the detailed the Upper Triassic turtle Stegochelys dux, a taxon morphology of these structures, and in particular currently recognized as a junior synonym of the ventral contact of the cleithra with the dorsal Proganochelys quenstedti (Gaffney, 1990). Although surface of the plastron. not specifi cally stated, Jaekel’s identifi cation ap- The presence of cleithra was not doubted in pears to have been primarily driven by the topo- Proganochelys quenstedti for most of the following logical position of these structures anterior to the century until Gaffney (1990) carried out a com- scapulae and by comparisons with the pectoral prehensive morphological review of this taxon. The presence of cleithra in Kayentachelys aprix 99

Although Gaffney originally believed that cleithra it is logically impossible for the projections to be were present in P. quenstedti, he ultimately conclud- clavicles as well. Lee’s homology scheme can thus ed that the structures were instead outgrowths of be rejected. The two primary competing hypoth- the epiplastra. An explicit rational was not provid- eses are therefore that these structures are either ed. However, given that well-developed ascending cleithra (following Jaekel, 1915) or the ascending processes of the clavicle occur in many primitive processes of the epiplastra (following Gaffney, tetrapods and that the clavicle is homologous with 1990). the epiplastron, an interpretation of these struc- The hypothesis that these elements are as- tures as processes of the epiplastron appears plau- cending clavicular processes would be supported sible. by the following interpretations or features: 1) they A third possibility was put forward by Lee are true outgrowths of the epiplastron, 2) they are (1996) who suggested that the vertical anterior inseparate from the epiplastron, 3) postmortem projections of the plastron of primitive turtles breakage would likely occur along the narrower represent the clavicles and that the epiplastra part of the process, not between the broad base are separate bones that must be considered neo- and the epiplastron, and 4) the character of the morphs (see Table 1 for comparison with the other bone of the process and epiplastron are indistin- hypotheses). Although an explicit rationale was guishable. None of these features or interpreta- not provided, it is noteworthy that Lee’s (1997) tions is confi rmed by this study. In contrast, the interpretation was formulated with explicit refer- hypothesis that these elements are cleithra is di- ence to Gaffney’s (1990) conclusion that cleithra rectly supported by the observations that 1) the were absent. structures are ontogenetically independent from An overwhelming amount of evidence indi- the epiplastra (clavicles), 2) even when fused cates that the epiplastra of extant hard-shelled with the epiplastra, sutural evidence of their own turtles derive partially or fully from embryologi- identity is present during later ontogeny, 3) the cal precursors that must be interpreted as clavicles structures separate along the ontogenetic sutural (e.g., Zangerl, 1939, 1969; Walker, 1947; Williams contacts under postmortem mechanical stress, and McDowell, 1952; Cherepanov, 1984, 1997, and 4) the bone density differs from that of the 2005; Rieppel, 1993; Gilbert et al., 2001; Sheil bordering epiplastra, and an abrupt transition and Greenbaum, 2005). A crucial point is the de- in bone density is seen at the cleithral-epiplastral velopmental relationship of the processes to the contact. Inasmuch as both the cleithrum and the epiplastra. If the processes are outgrowths of the clavicle are dermal elements and positioned ante- epiplastra (i.e., the clavicles), the interpretation of rior to the scapula, no additional information can Gaffney (1990) is validated. However, if the pro- be gained from topological arguments. cesses are proposed to be elements separate from Although not explicitly stated, Gaffney (1990) the epiplastra (Lee, 1996, 1997) and the epiplastra observed some of these features when assessing the are indeed formed by clavicular precursors, then identity of these structures in Proganochelys quenst-

TABLE 1. Comparison of primary homology assessments to the anterior plastral region of primitive turtles.

Jaekel (1915) Gaffney (1990) Lee (1996)

epiplastral process cleithrum clavicle clavicle epiplastron clavicle clavicle neomorph entoplastron interclavicle interclavicle interclavicle 100 Walter G. Joyce et al. edti (Gaffney, pers. comm. WGJ, 2003). Gaffney’s these structures should not be considered to have careful external analysis of all available P. quen- been derived from girdle elements at all, but rath- stedti material did not reveal any sutures along er are neomorphs, does not make a difference, the base of the processes and he consequently because the acquisition of neomorphs would also concluded that they were part of the epiplastra. add an additional step. Naturally, the presence of A similar observation can be made for about half cleithra (or equivalent neomorphs) in turtles is a of the available material for Kayentachelys aprix. In just a single character that may be rendered homo- addition, in the same specimens of K. aprix and plastic by a comprehensive phylogenetic analysis. all available specimens of P. quenstedti, the process The presence of cleithra in the primitive tur- tends to break well above its base, indicating a tle Kayentachelys aprix has interesting implications fi rm connecting between the process and the epi- regarding basal turtle evolution as well. When fi rst plastron. described, this taxon was assessed to be the most Although all of the material of Proganochelys basal known representative of the cryptodiran quenstedti and some specimens of Kayentachelys stem lineage (Gaffney et al., 1987). Considering aprix suggest that the processes represent out- the absence of cleithra in all living pleurodires and growths of the epiplastra, the remaining K. aprix cryptodires, this would imply the loss of cleithra in material provides multiple lines of evidence that both lineages. A recent, comprehensive analysis of the processes should be regarded as independent basal turtles relationships, however, that used 136 structures. We conclude that the structures in osteological characters with 171 derived character question are cleithra that initially are independent states for 45 fossil and 22 living species of turtles but fuse with the epiplastra during later ontogeny. (Joyce, In Press) placed K. aprix along the phylo- Unfortunately, the available K. aprix material does genetic stem of crown turtles. Despite this reas- not allow an assessment of the proposed ontoge- sessment of the placement of K. aprix, this analysis netic fusion, because all specimens in this study does not reveal the loss of cleithra to be an un- are approximately equal in size. Modest size dif- ambiguous synapomorphy of crown Testudines. ferences do exist, but these cannot be quantifi ed Instead, the presence of remnant cleithra in the rigorously because no two specimens possess com- unambiguous stem cryptodire taxa Baenidae and parable landmarks for measurement. Xinjiangchelys latimarginalis indicates that these Considering that equivalent structures structures were lost independently along the stem can be found in other primitive turtles, such as lineages of crown Pleurodira and Cryptodira. Proterochersis robusta and Palaeochersis talampayen- Cleithra were thus lost at least twice in turtles sis, our fi nding fi rmly establishes the presence (Fig. 5). of cleithra in primitive turtles and indicates that cleithra were lost in the turtle lineage indepen- CONCLUSIONS dently of the loss seen in the mammal stem lineage and in saurian reptiles (Matsuoka et al., 2005). A morphological review of all Kayentachelys The presence of cleithra in primitive tur- aprix material reveals that this taxon possesses tles has intriguing phylogenetic implications. cleithra. In addition to topological arguments, Placement of turtles within crown Sauria, a clade this conclusion is primarily supported by observa- that demonstrably lacks cleithra, requires the un- tions that demonstrate this element to be develop- parsimonious conclusion that cleithra reappeared mentally independent from the epiplastron and along the phylogenetic stem of turtles, only to be hence not an outgrowth thereof. The equivalently subsequently lost again. An ad hoc hypothesis that placed «epiplastral processes» of other primitive The presence of cleithra in Kayentachelys aprix 101

the Navajo Nation for access to conduct paleontologi- cal studies. This study was supported by the National Science Foundation (IIS-0208675).

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