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Premaxillae of the Extinct Megalonychid , Neocnus, and , and their Phylogenetic Implication....

Article in Journal of Mammalian Evolution · September 2015 DOI: 10.1007/s10914-015-9308-7

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The user has requested enhancement of the downloaded file. Premaxillae of the Extinct Megalonychid Sloths Acratocnus, Neocnus, and Megalonyx, and their Phylogenetic Implications (Mammalia, )

Lauren M. Lyon, Chelsea Powell, H. Gregory McDonald & Timothy J. Gaudin

Journal of Mammalian Evolution

ISSN 1064-7554

J Evol DOI 10.1007/s10914-015-9308-7

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J Mammal Evol DOI 10.1007/s10914-015-9308-7

ORIGINAL PAPER

Premaxillae of the Extinct Megalonychid Sloths Acratocnus, Neocnus,andMegalonyx, and their Phylogenetic Implications (Mammalia, Xenarthra)

Lauren M. Lyon1,2 & Chelsea Powell 1 & H. Gregory McDonald3 & Timothy J. Gaudin1

# Springer Science+Business Media New York 2015

Abstract In most folivorans, the premaxilla is loosely at- maxilla in two of Megalonyx from , the tached to the maxilla, so that it is often missing in otherwise North American Land Mammal Age (NALMA) very well-preserved fossil skulls. Despite its infrequent pres- M. leptostomus and Rancholabrean NALMA M. jeffersonii. ervation in sloths, the premaxilla has been shown to have These species show a progressive reorientation of the premax- phylogenetically significant variation among the taxa that do illa within Megalonyx from a primitive horizontal element preserve the element. In the family , the to a nearly vertical element, and some significant changes in premaxilla is known only in the early taxon Eucholoeops the anatomy of the incisive foramen. Morphological evidence (Santacrucian South American Land Mammal Age suggests that a broadened, plate-like premaxilla constitutes a [SALMA]), the extant two-toed Choloepus,andthe synapomorphy for the entire clade Megalonychidae. North American Neogene taxon Megalonyx, the last described Furthermore, although Eucholoeops retains a short anterior only in an unpublished Master’s thesis. We report here process of the premaxilla like that of megatherioid sloths, this the discovery of the premaxilla in two genera of extinct process is lacking in the other megalonychids, suggesting that megalonychids, Neocnus and Acratocnus. These small bodied, the loss of this process may unite late to Recent semiarboreal megalonychid sloths are endemic to the islands megalonychids. of the Greater Antilles. Though the presence of sloths in the Caribbean dates at least to the late , the best known Keywords Megalonychidae . Premaxilla . Neocnus . taxa derive from to cave deposits in Acratocnus . Megalonyx . Folivora , , and . We also describe the pre-

Introduction * Timothy J. Gaudin [email protected] The premaxilla is normally the anteriormost bone of the ven- Lauren M. Lyon tral rostrum in the mammalian skull. Among placentals it has [email protected] an extensive sutural attachment to the maxilla posteriorly, a H. Gregory McDonald dorsal attachment to the nasal and occasionally the frontal [email protected] bone, and often a small posterior contact with the vomer (Novacek 1993). It is typically tripartite, consisting of a body, a palatal process, and a dorsal process. The body, which is 1 Department of Biological & Environmental Sciences, University of Tennessee at Chattanooga, 615 McCallie Avenue, horizontal and lateral to the incisive foramen, forms the an- Chattanooga, TN 37403-2598, USA terolateral margin of the hard palate and houses the 2 Department of Geosciences and Don Sunquist Center of Excellence teeth in most , although these may be secondarily in , East Tennessee State University, 325 Treasure Lane, lost as, for example, in some artiodactyls, sirenians, and in Johnson City, TN 37614, USA most xenarthrans (Ungar 2010). The palatal process is also 3 Museum Management Program, National Park Service, 1201 horizontal but lies medial to the incisive foramen. The dorsal Oakridge Drive, Fort Collins, CO 80525, USA process extends along the lateral margin of the external nares Author's personal copy

J Mammal Evol

(Evans and Christiansen 1979). In Folivora (=Tardigrada; investigate the systematic implications of premaxillary morphol- Fariña and Vizcaíno 2003), the normally housed in ogy within Megalonychidae, and between Megalonychidae the body of the premaxilla have been lost, and the dorsal and other sloths. process is greatly reduced or absent (Engelmann 1985; Gaudin 2004). Thus, only the body and the palatal process remain, described herein as the lateral and medial rami of Material and Methods the premaxilla, respectively. This follows terminology long in use among sloth specialists, who refer to these portions of The present study was occasioned by the discovery of two iso- the bone as the medial and lateral Barms^ or Bbranches^ (see, lated premaxillary bones in museum collection drawers. Both e.g., Scott 1903–4; Stock 1925) of the premaxilla, formalized represent the first instance in which the bone has been recovered as the medial and lateral rami by De Iuliis (1994). There is for the relevant taxa, despite the fact that both taxa have been often an additional anterior process in sloths, defined as a thoroughly described in the literature, and are known from nu- distinct projection extending forward from the junction of merous well-preserved skulls (e.g., see Anthony 1926;Paula the lateral and medial rami (following De Iuliis 1994;De Couto 1967;MacPheeetal.2000; Gaudin 2011). The first pre- Iuliis et al. 2011). The xenarthran premaxilla typically forms maxilla was found among the extensive collection of sloth re- most, if not all, of the margin of the incisive foramen, which mains from Holocene cave deposits in housed at the serves as the opening into the oral cavity for the nasopalatine Museum of Natural History, and derives from the small bodied duct (Wible and Gaudin 2004). megalonychid sloth, Neocnus comes (UF 248501, right side In most folivoran species, the premaxilla is not sutured to the only; Fig. 2). The second was found at the American Museum maxilla. This means that the premaxilla is rarely found articu- of Natural History, and pertains to the mid-sized / lated with fossil skulls (indeed, the premaxilla is frequently Holocene megalonychid sloth from Puerto Rico, Acratocnus missing in museum specimens of the extant three-toed sloth odontrigonus (AMNH 17716, left side only; Fig. 3). Figures 2 Bradypus), and its morphology is poorly known. There are and 3 show the bones in dorsal and ventral view, and include exceptions, e.g., the Mio- nothrothere, reconstructions showing the positions the bones would occupy (McDonald and Muizon 2002). In the megathere in the intact skull. One of us (HGM) had also noted the preser- and the scelidothere, , the lateral vation of the premaxilla in specimens of the late Pliocene ramus is often fused to the skull and the premaxilla is (Blancan NALMA) North American species Megalonyx commonly preserved, though curiously this is not the case in leptostomus (UF 216900, right side only; Fig. 4) from the their respective close Neotropical relatives, and Florida Museum of Natural History, and the late Pleistocene . Despite its infrequent preservation, De Iuliis (1994) (Rancholabrean NALMA) North American species Megalonyx has shown that the bone exhibits significant variation among jeffersonii from the Florida Museum of Natural History living and fossil sloths that can be systematically informative. (UF 213841), the Idaho Museum of Natural History (IMNH In the present study, descriptions of the morphology of the 23034), and the Denver Museum of Nature and Science premaxilla are provided in six species of megalonychid sloths: (DMNS EVP. 65000). Descriptions of M. jeffersonii are based Choloepus didactylus, C. hoffmanni, Neocnus comes, largely on the UF specimen, which was loaned to us (Fig. 5). Acratocnus odontrigonus, Megalonyx leptostomus,and In order to evaluate the morphology of these newly dis- M. jeffersonii. Choloepus represents the only extant of covered and/or newly described megalonychid premaxillae, Megalonychidae, and its two living species are close relatives of they are compared in detail to that of the only extant the fossil taxa from the West Indies, Neocnus and Acratocnus megalonychids, the two species of two-toed sloth Choloepus (MacPhee et al. 2000; Gaudin 2004; McDonald et al. 2013), and hoffmanni and C. didactylus (Fig. 1a), as well as to the are described primarily for comparative purposes. In three of the megalonychid Eucholoeops from the Santacrucian SALMA four fossil species (all but M. jeffersonii), the premaxillae have (late early Miocene), which was illustrated in Scott (1903–4; never been described before. The premaxilla in M. jeffersonii was Fig. 1b, c). The basal megatherioid sloth from illustrated by Leidy (1855: pl. III and VI, Fig. 2) based on a single the Santacrucian SALMA (Fig. 1d), whose premaxillae are specimen, but identified in the text as the Bintermaxilla.^ The described and illustrated in Scott (1903–4), De Iuliis (1994), only other description of the premaxilla in M. jeffersonii is in and Bargo et al. (2009), will also be used to represent the an unpublished Master’sthesis(McDonald1977). These six spe- plesiomorphic condition in megatherioid sloths. cies represent the only megalonychids in which the premaxilla is known, with the sole exception of the basal form Eucholoeops Institutional Abbreviations: AMNH, American Museum from the late early Miocene Santacrucian South American Land of Natural History, New York, NY, USA; CM, Carnegie Mammal Age (SALMA) of (Fig. 1;Scott1903–4; Museum of Natural History, Pittsburgh, PA, USA; Bargo et al. 2009). The goal of this study is to describe these DMNS, Denver Museum of Nature and Science, newly discovered fossil megalonychid premaxillae, and to Denver, CO, USA; FMNH, Field Museum, Chicago, IL, Author's personal copy

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Fig. 1 Premaxillae of megatherioid and megalonychid sloths. a 1903–4: pl. 31, fig. 2). Abbreviations; ap, anterior process of premaxilla; Choloepus didactylus (UTCM 1000) shown in ventral view; b C1, alveolus for upper caniniform tooth; if, incisive foramen; iof, Eucholoeops fronto shown in ventral view (modified from Scott 1903– infraorbital foramen; j,jugal;lr, lateral ramus of premaxilla; M1,alveolus 4: pl. 56, fig. 2); c isolated premaxillae of E. ingens (MPM PV 3401) for upper first molariform tooth; mr, medial ramus of premaxilla; mx, shown in ventral view (modified from Bargo et al. 2009); d Hapalops maxilla. Scale=1 cm longiceps (YPM-PU 15523) shown in ventral view (modified from Scott

USA; IMNH, Idaho Museum of Natural History, Boise, anterolateral edge of the lateral ramus and the midsagittal ID, USA; MPM PV, Museo Regional Provincial Padre plane is approximately 50° in C. didactylus (UTCM 1000) BM.J. Molina,^ Río Gallegos, Santa Cruz Province, and 55° in C. didactylus (AMNH 133405), but closer to 60° ; UF, Florida Museum of Natural History, in C. hoffmanni (UTCM 1912; AMNH 26918). University of Florida, Gainesville, FL, USA; UTCM, The preserved right premaxilla of Neocnus (UF 248501; University of Tennessee at Chattanooga Museum of Fig. 2) is V-shaped like that of Choloepus. The medial and Natural History, University of Tennessee at Chattanooga, anterolateral edges are fairly straight, forming roughly a 55° Chattanooga, TN, USA; YPM-PU, Princeton University angle between them. The lateral ramus was likely only loosely Collection, Peabody Museum of Natural History, Yale attached to the anterior portion of the maxilla instead of being University, New Haven, CT, USA. tightly sutured to it, because the bone is often missing, even in Other abbreviations: NALMA, North American Land very well-preserved Neocnus skulls where, for example, ear Mammal Age; SALMA, South American Land ossicles are preserved (e.g., N. dousman, UF 76363 and Mammal Age. 76364; MacPhee et al. 2000; Gaudin 2011). The premaxillary contact on the maxilla is strongly concave anteriorly. It is horizontal along the posterior edge of the incisive foramen General Shape of Premaxillae (Figs. 1, 2, 3, 4 and 5) and curves sharply anteriorly along the lateral edge of the contact, running nearly straight anteriorly along the lateral Choloepus species have a V-shaped premaxilla in both dorsal margin of the premaxilla, resembling the contact surface in and ventral views (Naples 1982;Gaudin2004). The shape of the basal megalonychid Eucholoeops fronto (Scott 1903–4). the medial ramus of Choloepus hoffmanni (CM 3883) resem- The preserved left premaxilla of Acratocnus (AMNH bles that of the basal megatherioid Hapalops elongatus (Scott 17716; Fig. 3) is more BU-^ than BV^-shaped. The angle be- 1903–4; De Iuliis 1994), and the two even bear a similarly tween the midline of the lateral ramus (employed in place of shaped incisive foramen (see below). The lateral ramus of the uneven anterolateral edge) and the sagittal plane is 20°, the premaxilla is sutured posteriorly to the maxilla. In which is much smaller than that of any of the other C. didactylus (UTCM 1000), this premaxillary/maxillary megalonychids examined in this study. Like Neocnus, the pre- suture forms a shallow, anteriorly concave contact that is maxilla of Acratocnus was presumably loosely attached, be- roughly orthogonal to the long axis of the skull, whereas in cause its presence has never before been noted despite a mul- C. hoffmanni (AMNH 26918; UTCM 1912) the suture is titude of well-preserved specimens from this genus described straighter and more oblique in orientation, tilted slightly in the literature (e.g., AMNH 17722, including a specimen anterolaterally to posteromedially. The lateral ramus extends with in situ ear ossicles described in Patterson et al. 1992; posterolaterally from the anterior midline to meet the maxilla, for others see Anthony 1926; MacPhee et al. 2000; Rega whereas the medial ramus extends posteriorly along the et al. 2002). The contact between the maxilla and posterior sagittal axis of the skull. The incisive foramen separates the end of the lateral ramus is weakly concave anteromedially, and medial and lateral rami posteriorly. The angle between the thus intermediate between the condition found in Neocnus and Author's personal copy

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Fig. 2 Premaxilla of Neocnus. a isolated right premaxilla of N. comes (UF 248501) shown in dorsal view; b in ventral view; c skull of N. comes (based on UF 170527) shown in ventral view, with premaxilla reconstructed in place. Abbreviations; C1, upper caniniform tooth; cho, choana; es, epitympanic sinus; fm, foramen magnum; fo, foramen ovale; gl, glenoid fossa; if, incisive foramen; lr, lateral ramus of premaxilla; M4, upper fourth molariform tooth; mr, medial ramus of premaxilla; mx, Fig. 3 Premaxilla of Acratocnus. a isolated left premaxilla of maxilla; occ, occipital condyle; pm, premaxilla; pr,promontorium;shf, A. odontrigonus (AMNH 17716) shown in dorsal view; b in ventral stylohyal fossa. Scale=1 cm view; c skull of A. odontrigonus (based on AMNH 17722) shown in ventral view, with premaxilla reconstructed in place. Abbreviations; ap, Choloepus. The contact in Choloepus is either oriented anterior process of premaxilla; C1, alveolus for upper caniniform tooth; cho, choana; eam, external auditory meatus; ec, ectotympanic; fm, mediolaterally or slanted slightly anteromedially, whereas in foramen magnum; fo, foramen ovale; gl, glenoid fossa; if,incisive Neocnus, it is strongly tilted anteromedially. foramen; lr, lateral ramus of premaxilla; M4, upper fourth molariform The premaxilla in Megalonyx leptostomus (UF 216900; tooth; mr, medial ramus of premaxilla; mx, maxilla; occ, occipital Fig. 4) is roughly trapezoidal in ventral view. The lateral ra- condyle; pm,premaxilla;shf,stylohyalfossa.Scale=1cm mus forms a 78° angle with the midline. Along its ventral surface, a distinct groove runs from the incisive foramen to- of bone anterior to the incisive foramen formed by the lateral wards the anterior edge of the premaxilla, a feature unknown ramus and/or the confluence of the medial and lateral rami, in other megalonychids. The ventral surface is convex trans- also occurs in the extinct megalonychids Neocnus (UF versely on either side of the groove. The bulk of the ventral 248501; Fig. 2a), Acratocnus (AMNH 17716; Fig. 3a), and surface is comprised of an anteroposteriorly elongated, trans- Eucholoeops (Scott 1903–4; Bargo et al. 2009;Fig.1b, c), but versely narrow plate-like area anterior to the incisive foramen. not in the extant megalonychid Choloepus (Fig. 1a) or the A plate-like expansion of the premaxilla, i.e., a broad, flat area basal megatherioid Hapalops (Fig. 1d;Scott1903–4; De Author's personal copy

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Fig. 4 Premaxilla of Megalonyx leptostomus. a ventral view of skull of M. leptostomus (UF 216900) with preserved left premaxilla in situ; b anterior view of skull of M. leptostomus (UF 216900) with preserved left premaxilla in situ. Abbreviations; apl, anterior plate of premaxilla; C1, alveolus for upper caniniform tooth; cho, choana; ec, ectotympanic; fm, foramen magnum; fo, foramen ovale; gl, glenoid fossa; if, incisive foramen; iof, infraorbital foramen; lf, lacrimal foramen; lr, lateral ramus of premaxilla; M1, alveolus for upper first molariform tooth; M4,upperright fourth molariform tooth; mr, medial ramus of premaxilla; mt, maxilloturbinal; mx,maxilla;n, nasal; nt, nasoturbinal; occ, occipital condyle; shf, stylohyal fossa; zp, zygomatic process of squamosal; zpm, zygomatic process of maxilla. Scale=5 cm

Iuliis 1994). In M. leptostomus, this area is distinctive because orientation is particularly unusual and derived. The strongly of its great length and narrow transverse width. Interestingly, downturned premaxilla is fused laterally to the maxilla, fitting the premaxilla of M. leptostomus is even more unusual due to against the medial surfaces of the bone forming the alveoli of its strong longitudinal curvature. The posterior portion of the the greatly enlarged caniniforms. The surface that would face medial ramus is nearly horizontal and separated by a distinct dorsally in other sloths now faces anteriorly. The foramen that gap from the upper surface of the anterior hard palate, which is typically separates the rami has become a pit that is only greatly thickened dorsoventrally. The anterior portion of the evident on the dorsal/anterior surface of the bone, where it is premaxilla is nearly vertical, with its former dorsal surface separated from the functional incisive foramen not only by the now strongly downturned and facing anteriorly. In ventral wall of bone at the bottom of the pit, but also by the Choloepus, Acratocnus, Neocnus, Eucholoeops,and fusion of the medial and lateral rami immediately lateral to the Hapalops, the premaxilla is horizontal and nearly flat. In an- pit. The new functional incisive foramen is located entirely terior view, the plate-like portion of the M. leptostomus pre- behind the premaxilla, i.e., entirely between the premaxilla maxilla (=dorsal surface in other megalonychids) has a de- and maxilla. This functional incisive foramen appears deeply pression that deepens into a strong fossa near its ventral edge. recessed posterodorsally in ventral view due to the thickening A similar fossa is present on the dorsal surface of the premax- of the hard palate, as in M. leptostomus. The steep anterior illa in Acratocnus (AMNH 17716; Fig. 3a), but not in surface of the premaxilla is nearly straight, in contrast to the Choloepus (Fig. 1a)orNeocnus (UF 248501; Fig. 2a). The longitudinal curvature that occurs in M. leptostomus,and fossa in M. leptostomus is framed by a blunt, rounded ridge, forms roughly a 65° angle with the hard palate in lateral view. forming the anteroventral edge of the premaxilla itself, and a In ventral view, the angle between the anterolateral edge of the median ridge. The latter is also present in Acratocnus (AMNH premaxilla and the midline in M. jeffersonii (UF 213841) is 17716; Fig. 1a), and is elongated longitudinally in both taxa. 55°. The premaxilla is roughly triangular in this view, with a The premaxilla in M. jeffersonii (UF 213841; IMNH short transverse width and elongated anteroposterior length 23034; DMNS EPV. 65000; Fig. 5) is highly atypical for like that of M. leptostomus (UF 216900). Viewed anteriorly, sloths, and for mammals in general, in that it forms a nearly the premaxilla is clearly fused laterally to the maxilla. The vertical plate-like expanse of bone anterior to the incisive fossa on the dorsal/anterior surface described above for foramen, and its lateral and medial rami are fused together. M. leptostomus (UF 216900) varies in its development in Given that this bone represents the horizontal palatal portion M. jeffersonii. In UF 213841 this fossa is absent, replaced by of the premaxilla in other mammals, its near vertical a broad groove marking the site of the maxillary/premaxillary Author's personal copy

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Fig. 5 Isolated maxilla and premaxilla of Megalonyx jeffersonii. a ventral view of M. jeffersonii (UF 213841) with preserved left and right premaxilla in situ, and cranial end of premaxilla facing the bottom of the drawing; b anterior view of M. jeffersonii (UF 213841) with preserved left and right premaxilla in situ; c stereophotographs in dorsal view of M. jeffersonii (UF 213841) with preserved left and right premaxilla in situ. Abbreviations; apl, anterior plate of premaxilla; C1, alveolus for upper caniniform tooth; cif, closed incisive foramen; if, new incisive foramen behind premaxilla; iof, infraorbital foramen; lr,lateral ramus of premaxilla; M1, alveolus for upper first molariform tooth; M2, M3,upper right second and third molariform teeth; mr, medial ramus of premaxilla; mx, maxilla; zpm, zygomatic process of maxilla. Scale=5 cm (a, b); scale=1 cm (c)

fusion. In both IMNH 23034 and DMNS EPV. 65000, the extending far anteriorly. A much shorter version of the process fossa is present and closely resembles the aforementioned de- is evident in Eucholoeops fronto (Fig. 1b). It is very short pression in M. leptostomus (UF 216900). In ventral view, the indeed in E. ingens (Fig. 1c; MPM PV 3401), and might even anterior margin of the paired premaxillae in M. jeffersonii (UF be construed as absent. 213841) has a small notch in the midline, reminiscent of the Much like mylodontine and scelidotheriine sloths and the condition in Acratocnus (AMNH 17716; Fig. 3a)thoughless megatheriid genus Eremotherium (De Iuliis 1994), the anteri- well developed. This condition, and even the notch itself, is or process is clearly absent in Choloepus (Fig. 1a)and reduced or absent in IMNH 23034 due to weathering of the Neocnus (Fig. 2). premaxillary edge. DMNS EPV 65000 has a much deeper, The premaxilla of Acratocnus has a short anterior process narrow notch, quite unlike that of UF 213841 or the other (Fig. 3), more like that of Eucholoeops than the elongated Megalonyx specimens. process of Hapalops. The process extends anterolaterally and sits well lateral to the midline, in contrast to both Shape of Anterior Process (Figs. 1, 2, 3, 4 and 5) Eucholoeops (Scott 1903–4; Bargo et al. 2009)and Hapalops (Scott 1903–4; De Iuliis 1994), where the process As noted above, we define the anterior process of the premax- adjoins the sagittal plane along its medial edge and extends illa as a distinct, narrow projection extending forward from the straight forward anteriorly. This would have created a rather confluence of the lateral and medial rami. The process is well distinct, deep V-shaped median notch in the anterior margin of developed in the basal megatherioid Hapalops (Fig. 1d), the paired premaxillae when in articulation (Fig. 3c). The Author's personal copy

J Mammal Evol presence of such a median notch may be a unique feature margin of the medial ramus forms an unbroken, slightly con- among sloths, the closest approximation being the spatulate vex curve and, as in the extant genus, tapers posteriorly, but to anterior tip of the enlarged premaxilla in some species of a more rounded point. Like Choloepus, the dorsal surface is Thalassocnus (McDonald and Muizon 2002). concave with a raised medial edge, part of a larger depression In Megalonyx, the presence or absence of the anterior pro- surrounding the anterior and medial margins of the incisive cess is somewhat more difficult to assess, due to the anteriorly foramen. In contrast, the ventral surface is strongly convex. extended anterior plate of the premaxilla that extends forward The medial ramus in Acratocnus (AMNH 17716) is much from the incisive foramina (see above; Figs. 4 and 5). more substantial than that of Neocnus or Choloepus. Its min- However, the shape in Megalonyx does resemble that of imum width is roughly 70 % that of the lateral ramus Choloepus and Neocnus in the sense that the anterior edge (Table 1). As in Choloepus and Hapalops (Scott 1903–4; De of the premaxilla is v-shaped, with one edge becoming con- Iuliis 1994), the lateral edge bears a lateral process at roughly tinuous with the sagittal edge of the medial ramus, and the the midpoint of its length, though the process in Choloepus other continuous with the anterolateral edge of the lateral ra- and Hapalops is much smaller than that of Acratocnus.The mus. Therefore, our interpretation is that the anterior process margin posterior to this process is irregularly convex laterally, is absent, as in Choloepus and Neocnus. the ramus itself ending posteromedially in a sharp point as in Choloepus. Anterior to this lateral process, the lateral edge is Shape of Medial Rami (Figs. 1, 2, 3, 4 and 5) strongly concave to form the inner margin of the incisive foramen. In dorsal view, the ramus bears a tall, narrow ridge The medial ramus in Choloepus lies medial to the incisive along its medial edge. Lateral to the ridge is a wide, clearly foramen. Its median edge is straight, as in all megalonychids demarcated concavity that broadens anteriorly and extends except M. leptostomus (see below). The anterior half of its forward past the confluence of the lateral and medial rami, lateral edge bears a lateral curvature that represents the margin as noted above. This lateral concavity is especially pro- of the incisive foramen. This is followed by a small lateral nounced anteriorly, as in M. leptostomus (see below). Like process near the midpoint of the ramus. Behind this process, Choloepus, the posterior portion of the medial ramus con- the straight lateral margin angles posteromedially, so that the tinues much farther posteriorly and dorsally than the lateral medial ramus tapers to a sharp point posteriorly. The mini- ramus, extending into the nasal cavity. mum width of the medial ramus is less than half the minimum The medial ramus of M. leptostomus (UF 216900) has a width of the lateral ramus, as in Eucholoeops (Table 1;Scott slightly concave medial edge, so presumably a gap would 1903–4; Bargo et al. 2009), although the overall shape of the have been present between the left and right medial rami. medial ramus more closely resembles that of Hapalops (Scott The medial ramus tapers to a sharp point posteriorly like that 1903–4; De Iuliis 1994), due to the presence of the triangular of Acratocnus (AMNH 17716) and Choloepus, and at the lateral process. The left and right medial rami are sutured at midpoint the medial ramus is half the width of the lateral the midline in C. didactylus (UTCM 1000; AMNH 133405), ramus (Table 1). The lateral margin is deeply concave along but fused across the midline in C. hoffmanni (UTCM 1912; its entire length to accommodate the anteroposteriorly elon- AMNH 26918); however, this may represent age differences gated incisive foramen. The surface of the medial ramus is in the specimens rather than interspecific variation. The pos- convex ventrally and concave dorsally. It is horizontal poste- terior portion of the medial ramus extends dorsal to the lateral riorly, extending well into the anterior reaches of the nasal ramus and enters into the nasal cavity, separated in cavity dorsal to the hard palate, and is separated from the C. hoffmanni (UTCM 1912) by a small gap from the dorsal dorsal surface of the maxilla by a small gap. In dorsal and surface of the hard palate. In C. didactylus (UTCM 1000), the lateral views, a small gap is visible between the posterior tip medial rami closely approach, but do not touch, the vomer. of the medial ramus and the vomer, but this is probably due to This also appears to be the case in C. hoffmanni (UTCM postmortem damage of the latter in this specimen. The vomer 1912). In C. hoffmanni (AMNH 26918), the posterior end of and medial ramus were likely in contact in the intact skull. The the medial ramus is fused to the anterior portion of the maxilla, anterior portion of the medial ramus curves sharply ventrally whereas in the other Choloepus specimens, this is a sutural to meet the nearly vertical lateral ramus/anterior plate. The contact. dorsoventral thickening of the anterior portion of the palate In Neocnus (UF 248501), the medial ramus is significantly in Megalonyx, as noted above and a feature absent in other shorter posteriorly than that of Choloepus , yet the edges of the megalonychids, necessitates the great longitudinal curvature process appear intact, so this does not seem to be the result of of the premaxilla in M. leptostomus. postmortem breakage. The shortened medial ramus does not In anterior view, M. jeffersonii (UF 213841) has a short reach the maxilla, so the posterior portion of the left and right medial ramus with a straight median edge. The medial ramus incisive foramina are confluent across the midline, forming a has a strong lateral process and a concave lateral margin dorsal gap between the premaxilla and maxilla (Fig. 2b). The lateral (=posterior in other megalonychids) to this process. It tapers to Author's personal copy

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Table 1 Measurements of the lateral ramus, medial ramus, and incisive foramen in Hapalops, and the described megalonychids

Taxon & Specimen(s) Minimum transverse Minimum transverse Maximum transverse Ratio width of Ratio width of width of the Medial width of the Lateral width of the Incisive Medial Ramus to Incisive Foramen Ramus (mm) Ramus (mm) Foramen (mm) Lateral Ramus to Lateral Ramus

Hapalops longiceps, YPM-PU 15523 4.1 3.0 5.0 1.4 1.7 Hapalops elongatus, YPM-PU 15545 2.0 2.0 3.2 1.0 1.6 Eucholoeops fronto, (Scott 1903–4: 2.0 5.0 2.0 0.4 0.4 pl. 56, Fig. 2) Eucholoeops ingens, MPM PV 3401 3.1 8.1 3.0 0.4 0.4 Choloepus didactylus, UTCM 1000 1.2 3.0 2.5 0.4 0.8 Choloepus didactylus, AMNH 133405 1.0 3.2 2.0 0.3 0.6 Choloepus hoffmanni, UTCM 1912 1.1 3.8 2.7 0.3 0.7 Choloepus hoffmanni, AMNH 26918 1.0 3.7 3.1 0.3 0.8 Neocnus comes, UF 248501 2.0 10.1 5.0 0.2 0.5 Acratocnus odontrigonus, AMNH 17716 5.2 7.0 1.0 0.7 0.1 Megalonyx leptostomus, UF 216900 4.7 9.2 2.8 0.5 0.3 Megalonyx jeffersonii, UF 214841 4.1 17.2 2.3 0.2 0.1

a point as in Acratocnus (AMNH 17716), Choloepus, and degree to which the lateral rami are developed in both M. leptostomus (UF 216900). However, in contrast to C. hoffmanni and C. didactylus is intermediate between M. leptostomus, the medial ramus is oriented vertically, and Hapalops, where the diameter of the incisive foramen is equal does not enter the nasal cavity. It is instead separated from the to or greater than the minimum transverse width of the ramus, maxilla posteriorly by the functional incisive foramen, and is and the other megalonychids examined in this study, in which likely separated from the missing vomer as well. Unlike other the diameter of the incisive foramen is no more than half the known megalonychids, there is no incisive foramen anterior to width of the lateral rami. In Choloepus, this ratio is between the lateral process of the medial ramus, only a circular pit 0.6 and 0.8 (Table 1). As noted above, the transverse width of visible solely in anterior view. This is due to the fusion of the lateral ramus is nearly three times greater than that of the the two rami lateral and deep to the pit. The anterior/dorsal medial ramus in Choloepus, but this appears to be due to a surface of the medial ramus bears a sharp median crest. The narrowing of the latter rather than a broadening of the lateral remainder of its surface is concave dorsoventrally and convex ramus. anteroposteriorly. The posterior/ventral surface is transversely In Neocnus (UF 248501), the lateral ramus is rounded convex with a large raised median boss flanked by vascular posterolaterally where it contacts the maxilla. The width of grooves. the lateral ramus is nearly uniform, with a slight expansion near its junction with the maxilla, both mediolaterally and Shape of Lateral Rami (Figs. 1, 2, 3, 4 and 5) dorsoventrally, like Choloepus. Neocnus has the broadest lat- eral ramus, relative to the diameter of the incisive foramen, In Choloepus, the posterior end of the lateral ramus is sutured among all of the megalonychids examined in this study, with firmly or fused to the maxilla like Megalonyx, and unlike the the sole exception of M. jeffersonii (Table 1). The lateral ra- condition in Acratocnus, Neocnus, Eucholoeops,and mus is five times broader than the incisive foramen, and twice Hapalops. The medial edge of the lateral ramus is concave as broad as the medial ramus. Like Choloepus, the greatest to accommodate the incisive foramen, whereas the anterolat- width is at the premaxillary/maxillary junction. The ramus is eral edge is either straight (C. hoffmanni AMNH 26918, gently convex transversely on its ventral surface and flatter UTCM 1912; C. didactylus AMNH 133405) or slightly con- dorsally. As noted above, a vascular trough occurs on the vex (C. didactylus, UTCM 1000, CM 3883). The lateral ra- dorsal surface of the premaxilla, beginning at the lateral mar- mus flares proximally, with its widest point at the maxillary/ gin of the incisive foramen and extending approximately half- premaxillary suture. The ventral surface is weakly convex way across the lateral ramus anteriorly. The anterolateral edge transversely, whereas the dorsal surface is more strongly con- is straight as in Choloepus, and the medial edge is marked by vex. In C. hoffmanni (UTCM 1912), the proximal end appears two concavities, one for the aforementioned vascular groove to expand slightly dorsally in lateral view, reminiscent of the and one for the anterior edge of the incisive foramen. very reduced dorsal premaxillary process in the late Miocene The lateral ramus of Acratocnus (AMNH 17716) is less nothrotheriid sloth Mionothropus (De Iuliis et al. 2011). The uniform in shape compared to that of Choloepus or Author's personal copy

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Neocnus. The medial edge of the ramus is concave medially than twice the diameter of the medial ramus (ratio=0.1; for the incisive foramen as in Choloepus and Neocnus.The Table 1). posterior terminus of this edge forms a sharp angle with the contact surface for the maxilla. This posteromedial point near- Shape of Incisive Foramina (Figs. 1, 2, 3, 4 and 5) ly contacts the lateral process of the medial ramus, almost completely enveloping the incisive foramen. However, given In Choloepus, the shape of the incisive foramen can vary the dorsal inclination of the medial ramus, we believe that the among individuals, depending upon the amount of bone maxilla would participate in the posterior margin of the inci- growth where the medial ramus elevates dorsally to enter the sive foramen in an intact skull (Fig. 1b). The lateral edge of the external nares, along with the size of the lateral process of the lateral ramus is uneven, forming an irregular W-shaped curve medial ramus and its position relative to the anterior edge of from the anterior process of the premaxilla to the contact sur- the maxilla. The foramen can form an inverted teardrop shape face for the maxilla. The lateral ramus is very broad in (C. hoffmanni, AMNH 26918), a comma shape (C. didactylus, Acratocnus and nearly uniform in width, resembling the shape UTCM 1000; C. hoffmanni, CM 3883, UTCM 1912), or an found in Neocnus. Indeed, its minimum width is ten times irregularly bilobate shape (C. didactylus, AMNH 133405). greater than the maximum diameter of the incisive foramen, However, in all of these, the anteriormost portion is wider although the foramen itself looks to be narrowed relative to transversely and is ovate in shape, with the foramen tapering other megalonychids. The ratio of medial to lateral ramus posteromedially. An ovate incisive foramen is also present in width is 0.7 (Table 1), but, as noted above, this is likely due the basal megatherioid Hapalops elongatus (Scott 1903–4; De to the broadening of the medial ramus in this taxon. Like both Iuliis 1994)andEucholoeops fronto (Scott 1903–4), though it Neocnus and Choloepus, the lateral ramus in Acratocnus is is more triangular in H. longiceps (Scott 1903–4; De Iuliis widest at the premaxillary/maxillary junction. The ventral sur- 1994)andE. ingens (Bargo et al. 2009; De Iuliis et al. face of the lateral ramus is nearly flat in Acratocnus. The 2014). The margin of the foramen is formed mostly by the dorsal surface is weakly convex in the proximal half, but far- medial and lateral rami of the premaxilla, but the maxilla also ther forward there is a strongly elevated, straight longitudinal participates in the posterolateral margin in all of the ridge passing forward from the incisive foramen to the anterior Choloepus specimens examined in the present study. process, with a concavity immediately lateral to the ridge that In Neocnus (UF 248501), the tiny medial rami do not close- has been described above. ly approach the maxilla posteriorly, causing the left and right The lateral ramus in M. leptostomus (UF 216900) is con- incisive foramen to fuse into one large, roughly heart-shaped siderably shorter along its anterolateral axis than that of opening. Neocnus and M. jeffersonii are the only Choloepus, Neocnus,andAcratocnus, and much shorter than megalonychids known to have a medial ramus that does the medial ramus. The lateral ramus is separated from the not extend posteriorly past the anterior margin of the maxilla, maxilla by a deep groove on its ventral surface, and the contact so that the incisive foramen lies behind the premaxilla. with the maxilla faces almost directly laterally. The lateral However, in Neocnus,unlikeM. jeffersonii, a portion of the ramus appears to be fused to the maxilla in anterior view foramen still extends forward between the lateral and medial (=dorsal surface) along the lateral margin of the anterior rami. (=dorsal) surface, although the bone on the right is missing, The incisive foramen in Acratocnus (AMNH 17716) is suggesting the fusion may not have been bilateral. The width ovate and elongated anteroposteriorly. The medial edge has of the lateral ramus, at its most narrow point perpendicular to a somewhat stronger concave profile than the lateral edge. The its anterolateral axis, is at least twice the maximum diameter of lateral process of the medial ramus appears to almost meet the the incisive foramen, as occurs in all other megalonychids lateral edge of the foramen, nearly enclosing the opening with- except Choloepus. The width of the lateral ramus is also more in the premaxilla, although, as noted above, we believe the than three times the width of the medial ramus at its midpoint maxilla forms a small portion of the posterolateral margin. (Table 1). In M. leptostomus (UF 216900), the incisive foramen is In M. jeffersonii (UF 213841), the lateral ramus is difficult vaguely triangular and elongated in the anterolateral plane. to demarcate from the large plate-like expanse of bone that lies The incisive foramen is rounded anteriorly. It then tapers to anterior and ventral to the pit that represents the former inci- a point posteriorly, framed by the posteromedial edge of the sive foramen. A tall straight crest situated anterolateral to the medial ramus and the medial edge of the V-shaped notch that pit marks the medial edge of the lateral ramus. The lateral marks the anterior limit of the maxilla. The incisive foramen is portion of the lateral ramus is fused to the maxilla. The pitted deeply recessed due to the anterior thickness of the maxilla. and rugose anterior/dorsal surface of the lateral ramus region The incisive foramen in M. jeffersonii (UF 213841) is lo- is weakly concave transversely. Like Neocnus, the width of cated farther posterodorsally than in other megalonychids and the lateral ramus is five times the diameter of the former inci- other sloths in general. It is comparable only to sive foramen, and as in most other megalonychids, it is more M. leptostomus (UF 216900) in this respect, due to the anterior Author's personal copy

J Mammal Evol thickening of the hard palate that occurs in Megalonyx.The the incisive foramen that we have termed an Banterior plate.^ functional left and right incisive foramina are confluent be- This structure appears to be another distinguishing feature of hind the premaxilla and together form a chevron or V-shaped megalonychid premaxillae, as it is also present in Megalonyx opening in dorsal view (Fig. 5c), similar to the condition in despite its highly modified morphology, and in the Neocnus (UF 248501). However, the incisive foramen lies Santacrucian megalonychid Eucholoeops. The extant two- entirely behind the premaxilla in M. jeffersonii,incontrastto toed sloth Choloepus is the only megalonychid that lacks an Neocnus (see above), and is bordered by the premaxilla ante- anterior plate, causing it to more closely resemble the premax- riorly and the maxilla posteriorly. illary morphology of the basal megatherioid Hapalops (Scott 1903–4). The presence of a long anterior process in Hapalops and a Discussion somewhat shorter but otherwise similar process in Eucholoeops (Fig. 1) stands in sharp contrast to the condition The premaxilla of the basal megatherioid Hapalops in late Miocene to Recent megalonychids, which lack the (Santacrucian SALMA, late early Miocene) likely represents process entirely (Fig. 6).Thelossofthisprocessinlate the primitive condition for the clade Megatherioidea Miocene to Recent megalonychids for which it is known, with (Engelmann 1985;DeIuliis1994), which includes the sloth the exception of Acratocnus, is undoubtedly a derived feature. families , Nothrotheriidae, and Megalonychidae In the latter taxon, the process exhibits a highly distinctive according to Gaudin (2004). Hapalops has an elongated, nar- morphology (Fig. 3), as described above in the present study, row projection of bone called the anterior process extending suggesting it may be a neomorph. forward adjacent to the midline, as well as relatively narrow The premaxilla of the late Pliocene (Blancan NALMA) medial and lateral rami that are roughly equivalent in transverse megalonychid M. leptostomus (UF 216900) is newly de- width (Table 1). The premaxillae are also known from several scribed in this study. Also, new detailed descriptions and il- specimens of the contemporaneous early megalonychid genus lustrations are provided for the premaxilla of the late Eucholoeops, that differ from the premaxillae of Hapalops in Pleistocene (Rancholabrean NALMA) megalonychid several respects. The most notable differences are the reduction M. jeffersonii (based on UF 213841; IMNH 23034; and in length of the anterior process and the extensive broadening of DMNS EPV. 65000). The North American genus the lateral ramus. This broadening appears to be a characteristic Megalonyx exhibits a dramatic increase in the thickness of of nearly all megalonychid sloths, and may serve as a useful the anterior portion of the hard palate. This increase in thick- diagnostic feature for the family. Premaxillae are known from ness is likely due to the need to accommodate the enlarged several other Santacrucian sloths, e.g., Pelecyodon (FMNH anterior teeth (Leidy 1855;McDonald1977). As a result, the P13126) and Hyperleptus (Scott 1903–4), both of which tradi- premaxilla becomes curved longitudinally in M. leptostomus, tionally have been classified in the same subfamily with the anterior portion deflected ventrally to a degree that it (Schismotheriinae; e.g., see Patterson et al. 1992; McKenna takes on a nearly vertical orientation (Fig. 4). This strongly and Bell 1997)asHapalops and other basal megatherioids contrasts with the flat, horizontal premaxillae characteristic of (sensu Gaudin 2004). Hyperleptus is recognized as a distinct other megalonychids and of other sloths in general (De Iuliis genus at least in part by its premaxillary morphology. The an- 1994). In M. jeffersonii, the thickening of the palate and the terior process of its triangular premaxilla is either reduced or enlargement of the caniniforms causes the flat premaxilla to absent, and it has greatly expanded medial and lateral rami tilt upward so that it is nearly vertical in orientation. As a separatedbyareducedincisiveforamen(Scott1903–4). consequence of these changes, both species have deeply Hyperleptus has not been included in any of the recent cladistic recessed incisive foramina. In M. jeffersonii,thisforamenis analyses of megalonychid phylogeny (Gaudin 1995, 2004; completely posterior to the premaxilla. The portion of the White and MacPhee 2001; Carlini and Scillato-Yané 2004; foramen that normally lies between the medial and lateral rami Pujos et al. 2007; McDonald et al. 2013), in part because it is has closed, its position marked by a circular pit (Fig. 5). known only from two partial skulls and a partial mandible (Scott The premaxillary morphology in the family Megalonychidae 1903–4). Nevertheless, our results suggest that it should at least is anything but uniform, with multiple autapomorphies present provisionally be considered a megalonychid based on the re- in several taxa. In the only extant megalonychid genus, semblance of its premaxilla to that of other megalonychids. Choloepus, the lateral ramus is narrower than that of other The premaxillae of the late Pleistocene/Holocene Antillean megalonychids, and it lacks an anterior plate (Fig. 1). megalonychids, Neocnus and Acratocnus, are newly reported Acratocnus is distinctive among late Miocene to Recent in this study. Each specimen has a large lateral ramus that is megalonychids in that it possesses a small anterior process of more than twice the maximum diameter of the incisive fora- the premaxilla. Although the length of the process resembles men in minimum transverse width. This broadened lateral that of Eucholoeops,inAcratocnus this process is more trian- ramus forms a wide plate-like expanse of bone anterior to gular in shape, and is displaced laterally from the midline, Author's personal copy

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Fig. 6 Phylogenetic relationships of Megalonychidae from McDonald et al. (2013), with basal megatherioid Hapalops (in gray) as an outgroup. Gray text shows distribution of derived premaxillary features in the family Megalonychidae

creating a midline notch that is unusual if not unique among . In M. jeffersonii, the entirety of the premaxilla lies sloths as a whole. Acratocnus also exhibits an expanded medial in front of the incisive foramen, which is another unique con- ramus that is nearly 70 % as wide as its broad lateral ramus dition among megalonychids. (Fig. 3).ThepremaxillaofNeocnus is distinctive in that it has In order to examine the phylogenetic implications of a greatly shortened medial ramus that does not contact the max- premaxillary morphology within Megalonychidae, we illa posteriorly. This creates a single large incisive foramen pos- plotted the distribution of the premaxillary features on terior to the premaxilla, yet the foramen still retains a portion that the recently published megalonychid phylogeny of is inserted between the medial and lateral rami (Fig. 2). McDonald et al. (2013).AsshowninFig.6, this distri- In Megalonyx, the premaxilla is apomorphic in a number of bution suggests that expanded lateral rami (all of which respects. It is narrow transversely and elongated longitudinal- are more than twice the diameter of the incisive foramen) ly, with a very short, broad lateral ramus and elongated ante- and the presence of an anterior plate are morphological rior plate. Also, it is fused to the maxilla, at least in some characteristics that can be considered new synapomor- ontogenetically older individuals, and is separated from the phies for the entire family Megalonychidae. These fea- maxilla in ventral view by a deep groove that runs the length tures are then reduced or are absent secondarily in the of the premaxillary/maxillary boundary. Megalonyx extant genus Choloepus. The shortening of the anterior leptostomus possesses a longitudinally curved premaxilla un- process in Eucholoeops relative to that of Hapalops also like that described for any other sloth (Fig. 4). This condition optimizes as a synapomorphy of Megalonychidae. This is all the more interesting because it appears to represent an process is lost entirely in late Miocene to Recent intermediate condition between the straight, horizontal pre- megalonychids, with the exception of Acratocnus,which maxilla occurring in most sloths, and the nearly vertical pre- has a short process that differs in several aspects from maxilla that is present in the late Pleistocene M. jeffersonii those of Eucholoeops and Hapalops, and likely represents (Fig. 5). The nearly vertical (i.e., strongly downturned anteri- a secondary acquisition. The loss of the anterior process orly) orientation of the premaxilla is unique not only among would then represent a synapomorphy for the late sloths, but perhaps even mammals in general, excluding those Miocene to Recent megalonychid clade. Both the family mammals with large incisor tusks like proboscideans and Megalonychidae and its late Miocene to Recent Author's personal copy

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megalonychid subclade were strongly supported as mono- Gaudin TJ (1995) The ear region of edentates and the phylogeny of phyletic in McDonald et al.’s(2013) analysis, but the the Tardigrada (Mammalia, Xenarthra). J Vertebr Paleontol 15: 672–705 results of the present study provide additional diagnostic Gaudin TJ (2004) Phylogenetic relationships among sloths (Mammalia, features. Xenarthra, Tardigrada): the craniodental evidence. Zool J Linn Soc Given the variable nature of the premaxillary morphology 140:255–305 among megalonychids, there is likely much more to be Gaudin TJ (2011) On the osteology of the auditory region and orbital wall in the extinct West Indian genus Neocnus Arredondo, learned about the evolution of this element within the family. 1961 (Placentalia, Xenarthra, Megalonychidae). Ann Carnegie Mus This, however, awaits the recovery of additional premaxillae 80:5–28 from more fossil taxa. The demonstrated utility of this bone Leidy J (1855) A memoir on the extinct sloth tribe of North America. for contributing to our knowledge of sloth systematics makes Smithsonian Contrib Knowl 7:1–68 the search for premaxillae in new sloth taxa a worthwhile MacPhee RDE, White JL, Woods CA (2000) New megalonychid sloths (Phyllophaga, Xenarthra) from the Quaternary of Hispaniola. Am endeavor. Mus Novitates 3303:1–12 McDonald HG (1977) Description of the osteology of the extinct Acknowledgments We wish to thank the following people and institu- gravigrade edentate Megalonyx with observations on its ontogeny, tions for access to specimens during this study: Richard Hulbert and Jon phylogeny, and functional anatomy. Unpublished M.Sci. Thesis. Bloch, UF, USA; Susan Bell and John Flynn, AMNH, USA. For supply- University of Florida, Gainesville ing specimen photos we wish to thank Gerry De Iuliis of the University of McDonald HG, Muizon C de (2002) The cranial anatomy of Toronto, Canada, and John Wible of the Carnegie Museum of Natural Thalassocnus (Xenarthra, Mammalia), a derived nothrothere from History, USA. We would like to thank Julia Morgan Scott for her excep- the Neogene of the (Peru). J Vertebr Paleontol 22: tional illustrations. The skull of Megalonyx leptostomus was collected by 349–365 Fred Smith and donated to the Florida Museum of Natural History. We McDonald HG, Rincón AD, Gaudin TJ (2013) A new genus of thank François Pujos and John Wible for their insightful reviews of this megalonychid sloth (Mammalia, Xenarthra) from the late work. For funding we thank the Department of Biological & Environ- Pleistocene (Lujanian) of Sierra De Perija, Zulia State, . mental Sciences at the University of Tennessee at Chattanooga, and the J Vertebr Paleontol 33(5):1226–1238 Bramblett Gift Fund. McKenna MC, and Bell SK (1997) Classification of Mammals Above the Species Level. Columbia University Press, New York Naples VL (1982) Cranial osteology and function in the tree sloths, References Bradypus and Choloepus. Am Mus Novitates 2739:1–11 Novacek MJ (1993) Patterns of diversity in the mammalian skull. In: HankenJ,HallBK(eds)TheSkull,Volume2,Patternsof – Anthony HE (1926) Mammals of Porto Rico, living and extinct Rodentia Structural and Systematic Diversity. University of Chicago Press, and Edentata. NYAcad Sci, Scientific Survey of Porto Rico and the Chicago, pp 438–545 – Virgin Islands 9:97 243 Patterson B, Segall W, Turnbull WD, Gaudin TJ (1992) The ear region in Bargo MS, Vizcaíno SF, Kay, RF (2009) Predominance of orthal masti- xenarthrans (=Edentata: Mammalia) Part II. (sloths, ant- catory movements in the early Miocene Eucholaeops (Mammalia, eaters), palaeanodonts, and a miscellany. Fieldiana Geol new ser Xenarthra, Tardigrada, Megalonychidae) and other megatherioid 24:1–79 sloths. J Vertebr Paleontol 29:870–880 Paula Couto C de (1967) Pleistocene edentates of the West Indies. Am Carlini AA, Scillato-Yané GJ (2004) The oldest Megalonychidae Mus Novitates 2304: 1–55 (Xenarthra: Tardigrada); phylogenetic relationships and an emended Pujos F, De Iuliis G, Argot C, Werdelin L (2007) A peculiar climbing diagnosis of the family. N Jb Geol Paläontol - Abh 233:423–443 Megalonychidae from the Pleistocene of Peru and its implications De Iuliis G (1994) Relationships of the Megatheriinae, Nothrotheriinae, for sloth history. Zool J Linn Soc 149:179–235 and Planopsinae: some skeletal characteristics and their importance for phylogeny. J Vertebr Paleontol 14: 577–591 Rega E, McFarlane DA, Lundberg J, Christenson K (2002) A new De Iuliis G, Gaudin TJ, Vicars MP (2011) A new genus and species of megalonychid sloth from the late Wisconsinan of the Dominican – nothrotheriid sloth (Xenathra, Tardigrada, Nothrotheriidae) from the Republic. Caribb J Sci 38:11 19 – late Miocene () of Peru. Palaeontology 54:171–205 Scott WB (1903 4) Mammalia of the Santa Cruz beds. Part I. Edentata. De Iuliis G, Pujos F, Toledo N, Bargo MS, Vizcaíno SF (2014) Eucholoeops In: Scott WB (ed) Reports of the Princeton University Expeditions – Ameghino, 1887 (Xenarthra, Tardigrada, Megalonychidae) from the to Patagonia 1896 1899. 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