A new dermatemydid (Testudines, Kinosternoidea) from the Paleocene-Eocene Thermal Maximum, Willwood Formation, southeastern Bighorn Basin, Wyoming

ABSTRACT Gomphochelys nanus, new genus and species, is described from the earliest Wasatchian (biohorizon Wa 0; ∼55.8 Ma) of the southeastern Bighorn Basin, Washakie County, Wyoming. The new taxon represents the only known dermatemydid from the Paleocene–Eocene Thermal Maximum (PETM) interval and extends the lineage back from previous records by approximately 2 million years. Gomphochelys nanus has a thick tricarinate carapace and differs from other dermatemydids in attaining a smaller adult body size, having reduced plastral features, a posteriorly situated gular–humeral sulcus, an acarinate pygal, and thick shortened peripherals. Reexamination of previously described fossil dermatemydids suggests that the taxa Baptemys tricarinata and Kallistira costilata are junior synonyms of the middle–late Wasatchian Notomorpha garmanii, and Baptemys fluviatilis is likely a junior synonym of Baptemys wyomingensis. Gomphochelys nanus is a stem dermatemydid that is similar to N. garmanii but differs in possessing symplesiomorphies with the Late Cretaceous–Paleocene genera Agomphus and Hoplochelys. Aspects of shell morphology suggest that G. nanus was a commensurate swimmer and bottom-walker like extant Dermatemys and Staurotypus. The presence of a dermatemydid (a tropically distributed clade) in the southeastern Bighorn Basin during the PETM (when global temperatures increased by 5°C–10°C over a period of ∼60 ky) further supports the hypothesis that climate was megathermal in the region during this interval and is consistent with previously documented geographic range changes in both plants and animals. Dermatemydids disappear from the fossil record at the end of the PETM and don't reemerge until the next warming event, Eocene Thermal Maximum 2. http://zoobank.org/urn:lsid:zoobank.org:pub:19A98079-5CAD-4BC5-8C21-2810AA576D98 SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP


INTRODUCTION
Turtles of the epifamily Kinosternoidea are a clade comprising fossil and extant members of the families Dermatemydidae and Kinosternidae C Hoplochelys (Kinosternia) (Hutchison and Bramble, 1981;Gaffney and Meylan, 1988;Joyce et al., 2004). Extant members are exclusively inhabitants of the New World (North, Central, and South America), and the earliest fossil stem kinosternoids from the Late Cretaceous provide evidence for a North American origin of the group (Hutchison and Archibald, 1986). The Dermatemydidae Gray, 1870, comprises the living Central American river turtle Dermatemys mawii Gray, 1847, and species of the extinct genera Baptemys Leidy, 1870, andNotomorpha Cope, 1872, and form the less diverse part of an almost exclusively North American clade within Kinosternoidea (Gaffney and Meylan, 1988). A recent analysis concerning kinosternoids proposed that the genera Hoplochelys and Agomphus are stem dermatemydids and that dermatemydids split from the sister taxon Kinosternidae prior to the late Campanian (Knauss et al., 2010). The extant kinosternoids Staurotypus and Claudius were recently classified in the family Staurotypidae based on molecular evidence (Iverson et al., 2013); however, for the present study, these two genera are retained within the concept of Kinosternidae because we regard their phylogeny as unresolved (Hutchison and Bramble, 1981;Iverson, 1991;Bourque, 2012; see results of the cladistic analysis presented below).
Although living Dermatemys is restricted to southern Mexico, Belize, and Guatemala, fossil dermatemydids are reported from more northern latitudes during the Eocene and Miocene (Fig. 1). Fossils identified as Dermatemys are reported from the early Miocene Toledo Bend Local Fauna of Newton County, Texas (Albright, 1994). Species of Notomorpha and Baptemys are known from the latter part of the early Eocene (Wasatchian NALMA, Wa 5-Wa 7) of North Dakota, Wyoming, and New Mexico; Wasatchian-Bridgerian boundary of New Mexico (Cub Mountain Formation); middle Eocene (Bridgerian NALMA) of southwestern Wyoming; Bridgerian-Uintan of northern Colorado (Washakie Formation); late Uintan-Duchesnean of Texas; and Duchesnean of Utah (Lucas et al., 1989;Westgate, 1989Westgate, , 2012Eaton et al., 1999;Holroyd et al., 2001;Holroyd, 2002). Hutchison (1998) noted the presence of a form that might be ancestral to Baptemys from the Tiffanian and Clarkforkian of the Big Bend region of Texas. To date, no Oligocene dermatemydids have been reported.
Here we describe the oldest dermatemydid from the earliest Eocene of Wyoming and discuss its relationship to Notomorpha, Baptemys, and the extant Dermatemys mawii, and implications of its morphology for the divergence of dermatemydids. We also discuss the significance of its appearance within the Paleocene-Eocene Thermal Maximum (PETM), a period of abrupt climatic warming, and why the occurrence of a tropically distributed exotherm during this brief climatic event is important for understanding the mechanisms underlying faunal turnover at the PETM.
All specimens yet known for Gomphochelys, gen. nov., were recovered from localities within a »35 m-thick sequence of FIGURE 1. Maps of dermatemydid fossil localities in North America. A, map of North America: gray squares, late Paleocene Dermatemydidae indet.; black dots, late Wasatchian Notomorpha garmanii; white dot, Bridgerian Baptemys; gray diamond, Duchesnean Baptemys; white square, Arikareean 'Dermatemys'; highlighted area at the base of the Yucatan Peninsula (Mexico, Guatemala, and Belize) represents known range of the only extant dermatemydid, Dermatemys mawii. Scale bar equals 500 miles; B, map of Wyoming with Washakie County darkened; white dot encompasses portion of the study area where Gomphochelys nanus was collected. Scale bar equals 50 miles; C, Washakie County, Wyoming; white dots indicate localities in the southern Bighorn Basin where referred specimens of G. nanus were collected (diagonally from top to bottom): WY08025; WY08192, WY09029, and WY09031; WY04152; and WY07040. Black dot indicates type locality WY06111 and adjacent WY13064. e905481-2 JOURNAL OF VERTEBRATE PALEONTOLOGY for the genus. The type of the first species, Notomorpha testudinea, was later transferred to the genus Emys by Cope (1873Cope ( , 1884 and finally placed within the geoemydid ( D batagurid) genus Echmatemys by Hay (1908) where it has since remained. The type of the second species, Notomorpha gravis, consisted of "portions of one, and probably of other specimens" (Cope, 1872:476) and was represented by a hyoplastron and part of a costal. These bones were never figured and were apparently lost before 1908 (Hay, 1908).
The type of the third species, Notomorpha garmanii, consists of mostly carapace fragments and is now conserved in the USNM under the number USNM 4129. Between Cope's (1872) original description of N. garmanii and Hay's (1908) review, Cope (1873) synonymized N. gravis and N. garmanii and in 1884 figured the bones (none of them plastral bones) of N. garmanii under the name N. gravis. In his review, Hay (1908:269) summarized the status of the two species as follows: "As stated above, Cope made his N. garmanii a synonym of N. gravis, a proper course in case there is only a single species involved; but of this there may be grave doubts. Of his gravis he described a hyoplastron and an epiplastral of one individual and a costal of what was doubtfully another. The costal had a width of 58 mm, which indicated a very large turtle. Cope at first recognized important differences between the epiplastrals; but later he considered these of less importance. We may, then, have doubts regarding the specific identity of the two lots of bones. Besides this, the materials figured all belong to his N. garmanii. Notwithstanding all this, until future discoveries shall have thrown additional light on Notomorpha, it will be best to place the few known remains under the specific name gravis." However, it seems clear from Cope's (1872Cope's ( , 1873 description of N. gravis that the plastral bones belong to a testudinoid (wide hyoplastral lip) and do not belong to the same species as N. garmanii, which is a dermatemydid (see below). It seems likely, but unproven in the absence of the type, that N. gravis is referable to Echmatemys, possibly E. testudinea ( D Notomorpha testudinea Cope, 1872). The name N. gravis is hereby restricted to the type specimen, now lost, and is best considered a nomen dubium.
Notomorpha garmanii was described based on a number of isolated bones, mostly fragmentary, found together and thought by Cope (1884) to belong to the same individual. Hay (1908:269) states that "those (bones) which Cope described as N. garmanii are now in the U. S. National Museum, some of these bearing the number 4103; others, belonging to the same individual, have the number 4129." As it now stands, all of the figured bones from this lot are cataloged under USNM 4129 and labeled as Notomorpha gravis. USNM 4103 now encompasses the types of Echmatemys testudinea. We agree with Cope and Hay that only one individual seems to be represented (USNM 4129). The type of N. garmanii is the basis of Hay's maintenance of the genus Notomorpha and is hereby considered as the genotypic species, despite the fact that Hay considered these same specimens as the type of N. gravis while simultaneously acknowledging them as the original type of Cope's N. garmanii.
As noted by Hay (1908), the basis for the genus and species rests almost entirely on the peculiar nature of the epiplastron. Hay (1908:269) was "unable to convince himself that this is the epiplastron, altho [sic] it may be such. It may be a first peripheral." Hay was nearly correct in that the bone in question is a nuchal lacking the dorsal part and most of the left side.

Baptemys tricarinata
Although the other bones in the sample have poorly preserved surficial features, some costal fragments show the presence of a dorsal carina but are not otherwise thickened dorsally. They and the associated peripherals are in complete morphological agreement with Baptemys tricarinata of Hay (1908). Baptemys tricarinata was the last Wasatchian dermatemydid to be described. It was based upon good material from the Wind River Formation of Wyoming and includes a complete plastron and the central portion of the carapace (Hay, 1908:figs. 346-349). Although the types of Notomorpha garmanii and B. tricarinata are not completely comparable because of their fragmentary nature, they are in agreement with referred material from the type area of B. tricarinata in the University of California Museum of Paleontology Collections (see below). There is no morphological or stratigraphic reason to maintain both N. garmanii and B. tricarinata as separate species. Notomorpha garmanii Cope, 1872, thus becomes the senior synonym of B. tricarinata (Hay, 1908). Baptemys tricarinata is a subjective junior synonym of N. garmanii based upon the above comparisons.

Kallistira costilata
Dermatemys? costilatus was named by Cope (1875a:36) on the basis of fragments from six individuals from the 'Wasatch beds' (now San Jose Formation) of northwestern New Mexico. At first, Cope questionably referred them to the extant genus Dermatemys Gray. Cope (1875b, 1877 later removed the question mark; however, he noted that the reference to the genus was based upon resemblances to Dermatemys wyomingensis (Leidy, 1870) from the Bridger Formation of Wyoming. He considered Baptemys Leidy (type, B. wyomingensis Leidy, 1870) as a synonym of Dermatemys. Hay (1908:267) later noted that some of Cope's original specimens of Dermatemys costilatus had been lost but considered the combination of a thick and solid shell with three carinae and high extension of the inguinal buttress as the basis for erecting a new genus, Kallistira, using Cope's remaining specimens. Hay did not designate a type specimen for Kallistira costilata (Cope) and stated that "there appears to be no reason for doubting that all of these individuals belonged to the same genus." Examination of the material studied by Hay reveals that this statement is incorrect. At least two genera are represented in the syntype sample, Echmatemys (a geoemydid) and a dermatemydid. Thus, the designation of a lectotype is critical to the disposition of the genus. All of Cope's extant syntypes in the USNM are conserved under a single museum number, USNM 1152. There is no basis now (other than Cope's and Hay's statements that there were six and four individuals represented, respectively) for assuming that any of the isolated bones belong to the same individual. Numbered paper labels glued on the specimens refer to the figure numbers in Cope (1877:pl. 24). Of those that J.H.H. has examined, Cope's numbers 21-21a (Hay, 1908:fig. 330), 18 (Hay, 1908:fig. 332), 25 (Hay, 1908: fig. 332a), and 22 (Hay, 1908:fig. 333), and three unfigured fragments, are referable to Notomorpha, as are (judging from Cope's figures) the missing specimens numbered 17 and 20. Specimens 19, 29-29a (Hay, 1908:figs. 334-334a), 30-30a (Hay, 1908: fig. 331), and 27-28 (Hay, 1908:fig. 329) are referable to Echmatemys.
Reference of some of the specimens to Echmatemys is based upon essential identity in morphology and shell texture with comparable elements of the Echmatemys testudinea-Echmatemys megaulax (Cope, 1872) complex. The remains referred to Notomorpha are all smooth and identical to comparable elements of Baptemys tricarinata ( D Notomorpha garmanii). The costal fragment showing a carina in Cope (1877:figs. 21-21a) and Hay (1908: fig. 330) is hereby selected as the lectotype of Dermatemys? costilatus Cope, 1875a ( D Kallistria costilata Hay, 1908). There are no morphological criteria upon which K. costilata can be distinguished from N. garmanii (including B. tricarinata) at present. Therefore, K. costilata is here considered a junior synonym of N. garmanii Cope, 1872.

JOURNAL OF VERTEBRATE PALEONTOLOGY e905481-3
Baptemys fluviatilis Hay (1908) named Baptemys fluviatilis based on a shell that lacked most of the plastron, which was represented, in part, by the impression of the internal mold. The specimen lacked reliable locality data, but Hay thought (and we concur) that it probably came from the Bridger Formation. As noted by Hay, the specimen agrees well with typical Baptemys wyomingensis in size and morphology. Hay distinguished the two taxa by differences in the suprapygal region and shape of the plastral hindlobe. Hay (1908:268) stated that B. fluviatilis has only two suprapygals and B. wyomingensis has three; however, his figures of B. wyomingensis (Hay, 1908:text- fig. 337; pl. 40, fig. 1; pl. 41, fig. 1) clearly indicate the presence of only two suprapygals. Because the plastral hindlobe is represented by only an impression of the dorsal surface, the actual shape of its margins are in doubt. Moreover, the specimen has been weathered (Hay, 1908:pl. 42, fig. 1) such that the margins have been planed off, thus reducing and narrowing its outline. Therefore, we suggest that B. fluviatilis be placed in synonymy with B. wyomingensis.

BAPTEMYS WYOMINGENSIS
Age of Type Locality-Wasatchian NALMA (early Eocene). Diagnosis-Carapace tricarinate; xiphiplastra straight-sided, relatively long and narrow, and converging to a point (Hay, 1908); plastron with narrowed plastral lobes, entoplastron wider than long with gular-humeral sulcus positioned on posterior two-thirds of element. Holotype-UF 225761 (Figs. 3-5), nearly complete associated carapace and plastron, missing left P10-11, most of right P4-5, most of distal C6, right half of the entoplastron, as well as the posterior-most ends of the nuchal and xiphiplastra. The holotype was first discovered by Scott Wing, Doug Boyer, and Aaron Diefendorf while they were bed tracing in July of 2006. UF 225761 was partially weathered out near the top of a ridge as disarticulated fragments (Fig. 2). It was collected and reassembled by the first author.
Type Locality-UF locality WY06111, meter level 24.85 (refer to Secord et al. [2012] for meter level context), Cabin Fork area, southeastern Bighorn Basin, Washakie County, Wyoming, U.S.A. More specific locality data are available on file in the Division of Vertebrate Paleontology, Florida Museum of Natural History. Pieces of a second Gomphochelys nanus specimen, UF 249081, as well as a large trionychid were also collected from this locality.
Comments-Fossils of G. nanus older than Wa 0 were mentioned by Bourque et al. (2008) and subsequently cited by Knauss et al. (2010). These specimens comprise a few fragmentary, probably associated shell pieces (UF 249082-249084) that were collected from a late Clarkforkian (Cf 3) area in the southern Bighorn Basin and most likely represent temporally younger fossils from the PETM that eroded into Cf 3, because they were collected near a wash and are highly weathered in appearance. Likewise, Chester et al. (2010) report Wa 0 contaminants within a Cf 3 mammalian assemblage from the southern Bighorn Basin.

DESCRIPTION AND COMPARISONS
The shell of Gomphochelys nanus is overall smaller, thicker, and more robust than similarly sized individuals of other dermatemydid taxa. In UF 225761, the sutures of the individual bony elements are well formed throughout the shell and all of the distal costals completely contact their corresponding peripheral sets (where preserved). This level of ossification indicates that G. nanus reached maturity at a relatively small size. All other species of dermatemydid examined, such as Notomorpha garmanii (including the holotype of Baptemys tricarinata), Baptemys wyomingensis, and the Central American river turtle Dermatemys mawii, reach a much larger adult size (to 65 cm for the latter sensu Ernst and Barbour [1989]). When compared with the holotype of G. nanus, similarly sized N. garmanii from the UCMP were clearly subadults (e.g., UCMP 152739) because they exhibited excessively thin shell bones, porous bone at unformed sutures along the distal costal ends and proximal peripheral ends, and the costals and periperals were not in complete sutural contact. The shell of G. nanus is comparable in overall size to Agomphus pectoralis, Hoplochelys crassa, and Staurotypus.

Carapace
The carapace of UF 225761 is preserved almost in its entirety, with some minor distortion due to crushing and carbonate-filled fractures. It is 31.8 cm in length (Table S1) and is composed of a nuchal, eight neurals, two fully formed suprapygals, a diminutive supernumerary suprapygal, eight sets of costals, a pygal, and presumably 11 sets of peripherals despite two posterior peripherals not preserved (left P10-11). The costiform processes of UF 225761 extend through P1 and terminate in the anterior third of P2. The extent of the costiform process can also be seen in the left P2 specimens, UF 249083 and UF 249086, and right P2, UF 249063, in which the cavity that receives the costiform process is preserved. In the partial shell, UF 249085, the right P1 exhibits a deep sutural cavity that would have housed the costiform process. The sutural cavity is expansive and extends across the visceral face, suggesting that the costiform process would have terminated well into P2. The costiform process is more extensive in G. nanus than in examined N. garmanii, and compares well to that of Hoplochelys clark Knauss, Joyce, Lyson, and Pearson, 2010. Peripherals of the anterior (P1-4) and posterior (P8-11) margins of the shell are short in height and thickened (Fig. 6), almost to the degree seen in Agomphus pectoralis (e.g., YPM VP .000900). The posterior peripherals are straighter, and overall the posterior margin of the carapace is straighter and less flared than that of other dematemydids. The pygal of G. nanus is wider than long, with a length of 8-9% CL. This is in contrast to D. mawii, which has a thin elongated pygal »16% CL (e.g., UF/H 29168). P11 is likewise long and narrow in D. mawii, with very narrow sutural contact with the suprapygal. Conversely, P11 broadly contacts the suprapygal in G. nanus.
Neural 1 is ovoid and four-sided and broadly contacts the nuchal. Neurals 2-8 are hexagonal and anteriorly symmetrically short-sided. The lateral sutures of N1 and N4 are nearly parallelsided. There are two fully formed suprapygals in both UF 225761 and UF 249085, but in the former there is an extra diminutive round suprapygal between the sutural contact of the left C8, N8, and suprapygal 1.
The carapace possesses three well-developed longitudinal keels, almost intermediate in relief between Hoplochelys and Notomorpha garmanii. The medial keel extends across the entire neural series. It begins on N1 at the V1-2 intervertebral sulcus, abruptly becomes faint on N2, grows prominent on the anterior of N3, becomes most prominent on N6-7, ceases (or becomes interrupted) at the V4-5 intervertebral sulcus on N8, reemerges on suprapygal 1, and terminates on suprapygal 2. The medial keel does not extend onto the pygal in either the holotype of Gomphochelys nanus, UF 225761, or referred specimen UF 249085, which is probably plesiomorphic for dermatemydids. Dermatemys mawii also lacks a keeled pygal, but it is probably secondarily lost in that species. In UF 225761 and UF 249085, the pygal is smooth dorsally and lacks any discernible keel, although it is slightly humped medially. The medial keel is crenulated on suprapygal 1 and at its posterior terminus on the anterior-most suprapygal 2 (Figs. 4, 5). In the referred specimen, UF 249085, the medial keel terminates at the V4-5 intervertebral sulcus more anteriorly than in the holotype and lacks crenulations. The location of the posterior terminus for the medial keel in G. nanus is similar to the medial keel terminus in juvenile D. mawii, i.e., around the anterior of V5. The medial carapacial keel is most pronounced and dorsally squared along the edges on N3-7. The costal keels are more rounded along the edges and extend from the middle of C1 to the posterior-most C7. On C7, the keel curves distally and terminates. These carinae are most pronounced along sets C3-6. Characteristics seen in G. nanus, such as a thickened tricarinate carapace with some narrow and nearly parallel-sided neurals (N1 and N4), are mildly similar to features in the taxon Agomphus alabamensis Gilmore, 1919, from the Paleocene Midway Formation (Garrote et al., 2006) of Alabama. That species possesses three dorsal carapacial carinae, which are autapomorphic for the genus Agomphus. Hutchison and Weems (1998) report a taxon similar to A. alabamensis from the Paleocene Williamsburg Formation of South Carolina and question the generic designation of Agomphus for that taxon.
In Gomphochelys nanus, the axillary plastral buttress terminates in the posterior third of C1. The axillary buttress contacts C1 more anteriorly in Dermatemys. The terminus for the axillary buttress is an ovate sutural pocket that lies just proximal to the C1-P3 suture. This pocket is seen clearly in the C1 fragment UF 270845, which is very thick for its small size, particularly at the peripheral-costal suture. C1 is more strongly buttressed viscerally in Dermatemys than in G. nanus. In the holotype of G. nanus, the right axillary buttress extends through P4, well into P3, and just contacts P2 along the inner C1-P2-P3 suture. Slight contact with P2 is also clear in UF 249063, UF 249083, and UF 249086 by a pit exhibited on each of these elements. The left axillary buttress terminates on C1 in the holotype, similar to that seen in the C1 fragment UF 270845. In Dermatemys, the axillary buttress substantially contacts the posterior of P2. In G. nanus, the inguinal buttress does not extend onto the costals but extensively contacts the anterior two-thirds of P8, whereas in Dermatemys it contacts the anterior third of P8.
Gomphochelys nanus possesses five vertebral scutes and four pairs of pleural scutes. In UF 225761, the V1-PL1 sulci extend across costal set 1 and onto the nuchal, forming an hourglassshaped V1. V1 is typically more straight-sided or with rounded convex lateral borders in other dermatemydids. The V1-PL1 sulcus just contacts P1 in UF 225761, but there is more significant contact in UF 249065, a left P1. Vertebrals 2-4 are elongate in shape, being much longer than wide. Vertebral 5 is roughly triangular or 'teardrop-shaped.' The vertebral and pleural sulci jut anteriorly where they overlap the three carapacial keels: on the neurals where crossed by the vertebral scutes and on the proximal costals where crossed by the pleural scutes.
In Gomphochelys nanus, the cervical scale is thick and triangular or trapezoidal (UF 225761) to square (UF 249059) in shape. There are 12 pairs of marginal scutes. M3-9 are raised on the surface of the peripherals along the dorsal sulci (i.e., the marginals are stepped above the plane of the pleurals), similar to but not as pronounced or anteroposteriorly extensive as seen in Hoplochelys and adult Staurotypus triporcatus. None of the marginals in UF 225761 contact the costals. In Dermatemys, M5 contacts or nearly contacts C2, and M7 typically contacts C4 (e.g., UF/H 29168). M9 is elevated in height from M8 and M10, similar to but convergent with the conditions seen in the kinosternine Kinosternon flavescens and geoemydid Rhinoclemmys. M9 and M11 are nearly equally elevated, and M11 is more elevated than M12. The underlap of the posterior marginals is narrow, particularly for M12 on the pygal.

Plastron
The plastron of Gomphochelys nanus is composed of an entoplastron and paired epiplastra, hyoplastra, hypoplastra, and xiphiplastra. The forelobe (i.e., the portion of the plastron comprising the epiplastra, entoplastron, and associated dermal scutes) is short, and the epiplastra and entoplastron are more reduced than other dermatemydids, similar to Agomphus pectoralis (Fig. 7). The gular-humeral sulcus of G. nanus is posteriorly situated on the forelobe (Fig. 8). In the holotype UF 225761, the sulcus extends along and just posterior to the ento-hyoplastral suture (Fig. 8A), convergent with Staurotypus (Hutchison and Bramble, 1981). Staurotypus possesses a kinetic forelobe accompanied by pliable connective tissue and only slight sutural contact between the hyoplastra and the forelobe (Bramble et al., 1984), which is not the case for G. nanus. Instead, there is rigid sutural contact between the epi-, ento-, and hyoplastra in G. nanus. In the referred specimen UF 249077 (an isolated entoplastron), the gular-humeral sulcus is situated along the ento-hyoplastral suture (Fig. 8B). In the partial shell UF 249085, the gular-humeral sulcus is situated along but just anterior to the ento-hyoplastral suture (Fig. 8C), but it is still more posteriorly positioned than what is typical for Hoplochelys, Notomorpha garmanii, Baptemys wyomingensis, and Dermatemys mawii. A notable condition is present in Agomphus pectoralis, YPM VP .000776 (Fig. 7E), where the gular-humeral sulcus was probably situated posteriorly on the entoplastron (which is not preserved), more so than in other A. pectoralis specimens examined. In that specimen, the gular-humeral sulci abruptly curve posteriorly on the hyoplastra from the ento-hyoplastral suture, well posterior to the ento-hyoplastron sutural plane. The gularhumeral sulci abruptly recurve anteriorly to the outer margins of the epiplastra. The condition in G. nanus is somewhat intermediate between YPM VP .000776 and N. garmanii (Fig. 7). There is a morphocline within the dermatemydid lineage involving the position of the gular-humeral sulcus, where it migrates progressively anteriorly on the entoplastron through time, eventually being situated either midway or on the anterior half of the The peripherals of G. nanus are relatively thick for their small size, similar to the condition in Agomphus. The peripherals of N. garmanii are comparatively longer and thinner. Left P9 of G. nanus, UF 249081, in J, dorsal, K, ventral, and L, anterior views. This specimen was found at the same locality as the holotype UF 225761. M, partial left xiphiplastron, UF 249061, in dorsal view. N, partial right xiphiplastron, UF 249062, in dorsal view. Abbreviations: abd, abdominal scute; an, anal scute; axb, axillary bridge buttress; C1-7, costals 1-7; ent, entoplastron; fem, femoral scute; gul, gular scute; hum, humeral scute; hyo, hyoplastron; hyp, hypoplastron; ifm, inframarginal scute; ing, inguinal scute; N1-8, neurals 1-8; nuc, nuchal; P1, peripheral 1; pyg, pygal; spg1-2, suprapygals 1-2; xip, xiphiplastron. Scale bar equals 2 cm. element. This morphocline, coupled with an increase in size over time, is expressed in succession across the taxa G. nanus, N. garmanii, B. wyomingensis, and D. mawii (see also Discussion).
In UF 225761, there is a set of gulars (scale set 1 of Hutchison and Bramble, 1981), of which only the medial sulcus crosses the entoplastron. Paired humerals (scale set 3 of Hutchison and Bramble, 1981) are contained almost wholly on the hyoplastra except for the anterodistal-most extensions, which overlap onto the posterior-most ends of the epiplastra. There are paired abdominals (scale set 5 of Hutchison and Bramble, 1981), femorals (scale set 6 of Hutchison and Bramble, 1981), and anals (scale set 7 of Hutchison and Bramble, 1981), the latter contained entirely on the xiphiplastra. The forelobe lacks the intergular (scale 8 of Hutchison and Bramble, 1981), which is lacking in other dermatemydids with the exception of Dermatemys. The anterior-most portions of the epiplastra are preserved intact but do not contact one another well at the midline along the interepiplastral suture. This gives the appearance of a slightly notched anterior forelobe and could be an indication that medial skin or connective tissue was present at least anteriorly between the epiplastra. A similar condition was observed in some Staurotypus, which had medial skin overlapping onto the ventral face of anterior-most forelobe. The lack of a strong inter-epiplastral sutural connection in G. nanus could indicate very slight (probably passive) medial kinesis in the forelobe, or perhaps just poor preservation along the medial suture.
The posterior tip of the hindlobe is missing in the holotype UF 225761, but it is complete in UF 249085 (Fig. 6). The posterior tip (composed of the xiphiplastral set) is pointed, which is a primitive kinosternoid feature seen in Agomphus, Hoplochelys, and Staurotypus (Hutchison and Bramble, 1981). Within Dermatemydidae, a pointed hindlobe is shared with N. garmanii. Although missing, the weathered impression of the hindlobe for the holotype of B. fluviatilis Hay, 1908 ( D B. wyomingensis here; see Taxonomic History of Dermatemydidae, above), could be perceived as pointed. Baptemys wyomingensis has a broader more lobate hindlobe. Conversely, an undescribed species of Baptemys from the late middle Eocene (Duchesnean) of southcentral Utah (Eaton et al., 1999) and Dermatemys mawii have medially notched hindlobes. In G. nanus, the hindlobe is most similar to, but more blunt and shorter than, Agomphus, Hoplochelys, and N. garmanii. In UF 225761, there is a transverse trench-like concavity just posterior to the hypo-xiphiplastral suture in visceral aspect that likely accommodated the pubis. This concavity was either not present or not as pronounced in other xiphiplastral specimens of G. nanus (e.g., UF 249061 and UF 249062) or in Notomorpha and Baptemys specimens examined. A similar concavity was observed in D. mawii, being distinct in UF/H 29168 and more subtly expressed in UF/H 47723.
In the holotype of Gomphochelys nanus, there are four inframarginals on the right side of the plastron and three on the left. The right inframarginal series is composed of three large and a single reduced scute, which is typical for Notomorpha and Baptemys (Hutchison and Bramble, 1981). These represent (from anterior to posterior) the axillary, inframarginal 1, inframarginal 2, and inguinal scutes. The inframarginals are contained almost entirely on the plastron, with the exception of the anterior-most axillary, which extends onto the middle of P4, and the posteriormost inguinal, which just overlaps the anterior quarter of P8. The posterior of inframarginal 2 (on the right) and inframarginal 1 (on the left side), as well as the anterior-most inguinals, overlap the hyo-hypoplastral sutures.

PHYLOGENETIC ANALYSIS
Characters scored in the current analysis are from Hutchison and Bramble (1981), Hutchison (1991), Iverson (1991), and Knauss et al. (2010), with the addition of six new characters (Appendix S1). The analysis comprised 38 morphological characters among 14 kinosternoid taxa (13 ingroup, one outgroup), and the character-taxon matrix was developed in the program Mesquite 2.5 (Maddison and Maddison, 2008) (Appendix S2). Taxa utilized reflect those of Knauss et al. (2010), including synonymies within the Agomphus and Hoplochelys lineages (Hutchison and Weems, 1998;Knauss et al., 2010); however, the chelydrids Macrochelys temminckii and Chelydra serpentina were not incorporated into this analysis. Two different analyses were run: one using the taxon Agomphus pectoralis as the outgroup and one using Hoplochelys clark as the outgroup, both taxa being Late Cretaceous (Maastrichtian) in age (Knauss et al., 2010). Characters 3, 22, and 29 of Knauss et al. (2010) were omitted because the former two are regarded as autapomorphic for the taxa Hoplochelys clark and Claudius angustatus, and the latter is a chelydrid synapomorphy. Skull characters were not utilized because skulls for many of the fossil taxa including Gomphochelys nanus are unknown. A parsimony analysis was conducted in PAUP* 4.0-b10 (Swofford, 2003) using 1000 branch-and-bound search replicates with furthest addition sequence, and 1000 bootstrap replicates were performed to assess support for the topologies. Character states were unordered and equally weighted, multistate characters were regarded as polymorphisms, and the maximum number of trees was set to 100,000.

Results
A single most parsimonious tree was recovered (tree length D 111; consistency index D 0.703; retention index D 0.750; rescaled consistency index D 0.527; homoplasy index D 0.514) regardless of whether Agomphus pectoralis or Hoplochelys clark was used as the outgroup taxon (Fig. 9). Gomphochelys nanus was recovered as the most basal member of the dermatemydid lineage. In Figure 9A, the Dermatemydidae and Hoplochelys are sister taxa, which in turn are sister to the Kinosternidae. In Figure 9B, Agomphus is directly ancestral to the Dermatemydidae. Staurotypus triporcatus and Claudius angustatus do not form a monophyletic group in either result. In both cladograms, Xenochelys is recovered as paraphyletic with regard to the early Eocene species Xenochelys lostcabinensis Hutchison, 1991, and late Eocene Xenochelys formosa Hay, 1906. Results of Knauss et al. (2010) and Bourque (2013) also suggest this hypothesis. Dermatemydids are reliable indicators of a warm, relatively wet climate. The sole living survivor of this family, Dermatemys mawii, inhabits numerous aquatic habitats in the neotropics of Central America from Tabasco, northern Chiapas, southern Campeche, southern Quintana Roo, and Veracruz, Mexico; northern Guatemala; Belize; and possibly northwestern Honduras (Fig. 1A) (Ernst and Barbour, 1989). The mean annual temperature (MAT) in this part of Central America ranges from 20 C to more than 25 C (68 F to more than 79 F) (Legates and Willmot, 1990a), with mean annual precipitation (MAP) of 2000-4000 mm (Legates and Willmot, 1990b). More specifically, seasonal MATs during a 2-year study on Dermatemys in Belize were 16 C-17 C (61 F-63 F) in winter and 32 C-33 C (90 F-91 F) in summer (Polisar, 1996). Campbell (1972) reported that D. mawii requires warm water temperatures of 28 C-30 C (82 F-86 F) in order to be kept successfully in zoos. MATs in the Bighorn Basin during the Wasatchian have been proposed as subtropical, in the range of 12 C-18 C (54 F-65 F) (Gunnell, 1997). However, temperatures spiked during the Carbon Isotope Excursion (CIE) at the PETM, with MATs hypothesized to have been 20 C-27 C (68 F-81 F) and MAP of approximately 1150-1200 mm (Wolfe and Dilcher, 2000;Zachos et al., 2001;Fricke and Wing, 2004;Wing et al., 2005;Woodburne et al., 2009;Secord et al., 2012). Kraus et al. (2013) document major drying of paleosols, with MAP estimates between 900 and 1200 mm during the main body of the PETM.
The first and last known occurrences of Gomphochelys nanus in the Bighorn Basin are coincident with the span of the CIE (Fig. 10); thus, its presence in the southeastern Bighorn Basin can be attributed to increased temperatures in the region at that time. Gomphochelys nanus first occurs shortly after the onset of the CIE, during either the wettest portion (Bowen et al., 2004;Secord et al., 2012) or initial dry portion  of that interval. It seems to disappear from the southern Bighorn Basin at the end of the CIE, which was marked by decreased temperatures and either increased (Secord et al., 2012) or decreased  aridity. After its disappearance at the end of the PETM, dermatemydids are missing from the fossil record in the Bighorn Basin until the late Wasatchian (Wa 5 or Biohorizon B), when they reappear with the first occurrence of Notomorpha garmanii, concurrently with the first records of the large land tortoise Hadrianus (Hutchison, 1980;Holroyd and Hutchison, 2000;Holroyd et al., 2001) and during an interval of mammalian turnover (Chew, 2009). The reemergence of dermatemydids in the central Bighorn Basin stratigraphic section is coincident with Eocene Thermal Maximum 2 (Abels et al., 2012;Chew and Oheim, 2013) and represents the beginning of the second early Eocene interval of climatic warming, with greatly increased MATs (23 C, 73 F) and increased MAP of 1400 mm (Woodburne et al., 2009). Decreased d 18 O and d 13 C values and leaf margin analyses during Wa 5 suggest that MATs approached or nearly approached those experienced at the PETM (Zachos et al., 2001;Woodburne et al., 2009).  A taxon ancestral to Baptemys has been mentioned anecdotally from the Tiffanian of Texas (Hutchison, 1998). Other Paleocene kinosternoid records include Agomphus cf. Agomphus pectoralis and Agomphus aff. Agomphus alabamensis from the late Paleocene of South Carolina (Hutchison and Weems, 1998) and A. alabamensis from the Paleocene Midway Formation of Alabama (Gilmore, 1919). These reports of kinosternoids that are more closely related to dermatemydids than the more pan-continental Cretaceous to Paleocene Hoplochelys (Knauss et al., 2010) in the southern part of the United States, and the absence of kinosternoids outside of North America, suggest that dermatemydids immigrated into the Bighorn Basin from the south and/or southeastern United States coincident with climate warming. Whereas much of the literature discussing dispersal during the PETM has focused on the importance of high-latitude dispersal to the reorganization of the fauna (Koch et al., 1992;Clyde and Gingerich, 1998;Bowen et al., 2002;Gingerich, 2003;Smith et al., 2006;Beard, 2008;Beard and Dawson, 2009), there is increasing evidence that within-continent northward range expansion is also an important component in biotic turnover at the PETM. Northward range expansion with PETM warming has been hypothesized for selected mammals: the primate Teilhardina, the phenacodontid condylarth Ectocion parvus, and possibly the condylarth Meniscotherium Beard, 2008;Burger, 2012). Several fossil plants ('Artocarpus' lessigiana, Brosipollis, ?Lanagiopollis, cf. Tricolpites hians, and Platycarya swasticoides) also have earlier occurrences or close relatives present in the Gulf Coastal Plain (e.g., Wing et al., 2005). However, the most significant and clearest evidence for intracontinental dispersal in the PETM comes from the herpetological record, in which North American endemic lineages make their first appearance within the rocks of the PETM and persist today in the North American neotropics. Smith (2009) documented a minimum of six lineages appearing within the PETM of the Bighorn Basin: three iguanid lizards, stem representatives of the xantusiid Lepidophyma (extant tropical night lizards), the extant Anniella (California legless lizard), and diploglossine anguid lizards (galliwasps). Today, representatives of these lineages are exclusively or almost exclusively found in warm and frost-free (megathermal) parts of the New World. To this list of tropical herpetofauna dispersing north at the PETM, we can now add dermatemydids. Moreover, the sole extant dermatemydid Dermatemys shares a sympatric distribution with extant representatives of PETM-associated lizard groups (e.g., numerous iguanids, Lepidophyma, and diploglossine Celestus) at the base of the Yucat an Peninsula today (Lee, 2000).

Taxonomy within Dermatemydidae
An extensive gap in the fossil record between the last occurrence of Baptemys wyomingensis and the extant taxon Dermatemys mawii has made taxonomy within the Dermatemydidae somewhat convoluted. The Eocene taxa that we attribute to the Dermatemydidae (G. nanus, N. garmanii, B. wyomingensis, and some still undescribed species) are genuinely similar to one another morphologically, occur over a relatively short period of time from one another, appear not to co-occur with one another temporally or geographically, and are all found from the same region of North America. As such, there is a high likelihood that these taxa formed a monophyletic group in the early Eocene. The incorporation of G. nanus and N. garmanii into the genus Baptemys would result in a paraphyletic Baptemys with regard to Dermatemys mawii based on our phylogenetic results, as well as those of Knauss et al. (2010). Alternatively, the incorporation of G. nanus, N. garmanii, and B. wyomingensis into the genus Dermatemys would suggest that the Dermatemydidae represents a 55.8-million-year-old anagenetic lineage. A number of autapomorphies exist for Dermatemys that best warrant generic distinction between it and early Eocene dermatemydids. Shell-based autapomorphies of D. mawii include the following: complete loss of carapacial carination (although juveniles retain a medial keel, such as UF/H 101861); presence of an intergular scute; increased number of inframarginals (typically five sets); an expansive caudal notch (Iverson and Mittermeier, 1980;Hutchison and Bramble, 1981); seven or fewer neurals; C7 and C8 pairs contact one another medially; very faintly incised scute sulci on the shell; upper borders of M4-9 do not protrude above the plane of the adjoining pleurals; larger plastron to carapace ratio; inguinal plastral buttress terminus in posterior-most P8 or anterior-most P9; free margins of posterior peripherals with sharp edge; hourglass-shaped to triangular pygal; narrow contact between P11 and suprapygal; M9 and M10 equal in height; M11 contacts pygal; and distal terminus of C8 situated in P10. Furthermore, Eocene dermatemydids can be recognized collectively by the following shell-based characters: plastral lobes short and narrow; hindlobe pointed to rounded and lacking a caudal notch; nuchal bone with short costiform processes that extend to P1 or P2; a full set of neurals (eight); marginal scutes not encroaching on the costal bones (Hay, 1908); three to four sets of inframarginals present; humeral scutes (sensu Hutchison and Bramble, 1981) extend forward and overlap epiplastra; close alignment of the epi-hyoplastral suture with the gular-humeral sulcus; anterior tips of hyoplastra curve medially around lateral wings of entoplastron; epiplastron with strong oblique suture with hyoplastron; free margins of anterior and posterior peripherals rounded; two suprapygals present; broad contact between P11 and suprapygal; pygal square to rectangular; well-incised scute sulci on the shell; upper borders of M4-9 protrude in relation to adjoining pleurals (forming a step); C7 pair with no medial contact; C8 pair with no medial contact; distal terminus of C8 situated between P10 and P11; M9 taller than M10; M11 does not contact pygal; mid-dorsal carina present at least posteriorly; and presence or absence of costal carinae. More refined character-taxon matrices combined with discoveries of dermatemydids from the late Paleogene and early Neogene, e.g., more complete dermatemydid specimens from the Duchesnean of Utah (Eaton et al., 1999) and Arikareean Toledo Bend locality (Albright, 1994), will perhaps better clarify the taxonomy and phylogeny of dermatemydids and elucidate the degree to which this group represents an anagenetic and/or cladogenetic lineage.

Dermatemydid Evolution
There are notable morphoclines with regard to size and shell thickness within the Dermatemydidae, exhibiting Cope's rule (increasing body size through time) from the Wasatchian through Uintan (Hutchison, 1980). Early Wasatchian Gomphochelys nanus fits serially into this morphocline. Both the size and relatively thickened shell of G. nanus are similar to the Cretaceous-Paleocene genus Agomphus, with which it shares a number of characters. These include shortened and thickened anterior and posterior peripherals and a completely acarinate pygal. A keeled or crested pygal develops later in the Eocene with the Wasatchian Notomorpha garmanii, becomes highly pronounced in the Bridgerian-Uintan Baptemys wyomingensis, but is probably secondarily lost in Dermatemys mawii. Gomphochelys nanus is relatively small and thick-shelled. The lineage becomes progressively larger and more thin-shelled (primarily with regard to the carapacial margins and plastron) with the emergence of N. garmanii later in the Wasatchian (Wa 5-Wa 7), and continues with B. wyomingensis in the late early-middle Eocene. Other morphoclines within Dermatemydidae include the diminishment to loss of carapacial carination, forelobe expansion, and caudal notch development in the plastral hindlobe (Hutchison, 1980;Eaton et al., 1999). Wasatchian G. nanus and N. garmanii possess tricarinate carapaces, Bridgerian-Uintan B. wyomingensis are unicarinate, and modern D. mawii is acarinate; however, juvenile D. mawii (e.g., UF/H 101861) possess a unicarinate carapace (Iverson and Mittermeier, 1980). The forelobe in G. nanus is relatively reduced compared with confamilials and is comparable to Agomphus; this includes the small size of the entoplastron and epiplastra. The ento-and epiplastra become progressively wider and more expansive through time in dermatemydids, although the expansion in entoplastral width is most significant. In a similar morphocline, the hindlobe is acutely pointed in Wasatchian G. nanus and N. garmanii (similar to Agomphus and Hoplochelys); broadly lobate in Bridgerian-Uintan B. wyomingensis; with a weakly developed medial caudal notch in Duchesnean Baptemys sp. (sensu Eaton et al., 1999); and with a broad, well-developed caudal notch in extant D. mawii. These morphological trends within the dermatemydid lineage suggest a shift towards becoming more adapted as open-water swimmers. The youngest and largest described dermatemydid species Baptemys wyomingensis and Dermatemys mawii convergently resemble extant riverine taxa, such as the geoemydids Batagur and Orlitia (see Moll and Moll, 2004, for overview of lotic-associated turtles), which have long, smooth, highly buttressed shells. This is most notable in D. mawii, which has evolved a smoother carapace and more expansive plastron than its predecessors (Hutchison and Bramble, 1981). Conversely, the small size, relatively flattened plastron with reduced plastral lobes, solid, well-ossified tricarinate carapace, and more straightbacked (less flared) posterior marginal rim of G. nanus somewhat resemble extant Staurotypus and provide evidence that G. nanus was at least in part a bottom-walking animal. However, although Staurotypus is primarily a bottom-walker, it does spend a great deal of time swimming freely as well, typically close to the substrate (J.R.B., pers. observ.). The plastron of G. nanus is not as reduced as those of more habitual bottom-walkers such as the chelydrids Macrochelys and Chelydra, which suggests that the new species was probably a commensurate active swimmer and bottom-walker. This is also seen in D. mawii, which despite its broadly developed plastron spends a great deal of time bottom-walking while foraging along the substrate (J.R.B., pers. observ.). The species D. mawii, Staurotypus salvinii, and Staurotypus triporcatus are probably good modern analogues for G. nanus with regard to locomotion, climatic requirements, and perhaps preference for lotic and associated lentic habitats.