Published in %XOOHWLQRIWKH3HDERG\0XVHXPRI1DWXUDO+LVWRU\ ± which should be cited to refer to this work. A Review of the Fossil Record of Turtles of the Clade Pan-Kinosternoidea Walter G. Joyce1 and Jason R. Bourque2 1 Corresponding author: Department of Geosciences, University of Fribourg, 1700 Fribourg, Switzerland —email: [email protected] 2 Florida Museum of Natural History, University of Florida, Gainesville, FL 32611 USA —email: [email protected] ABSTRACT Turtles of the total clade Pan-Kinosternoidea have a relatively poor fossil record that extends back to the Late Cretaceous (Campanian). The clade is found only in North America during its early history, but dispersed to Central America no later than the Miocene and to South America no later than the Pleistocene. Ancestral pan-kinosternoids were likely aquatic, bottom-walking om- nivores or carnivores that preferred low-energy freshwater habitats. The Pan-Dermatemys lineage is often recovered in more fluvial habitats, and some are specialized to feed on aquatic vegetation. Alternatively, many representatives of Kinosternon evolved specializations (e.g., plastral lobe kine- sis) that allowed them to successfully inhabit and disperse across more terrestrial habitats such as savannas and floodplains. A taxonomic review of the group concludes that of 42 named taxa, 27 are nomina valida (including two species of the controversial taxon Planetochelys), 14 are nomina invalida and only one a nomen dubium. KEYWORDS Phylogeny, biogeography, paleoecology, Kinosternoidea, Kinosternidae, Dermatemys mawii Introduction half of the 19th century surprisingly resemble the current consensus, although notable differences The term Pan-Kinosternoidea refers to the total persist. Gray (1869) grouped chelydrids, stau- clade of Kinosternoidea, which originates from the rotypines and Sternotherus into the paraphyletic common ancestor of the mud turtle Kinosternon “Crucisterna” and all Kinosternon into “Kinos- scorpioides (Linnaeus, 1766), the Mexican musk terna” and then united these two groups into the http://doc.rero.ch turtle (or Guao) Staurotypus triporcatus (Wieg- paraphyletic “Chelydradae,” which essentially mann, 1828) and the Central American river tur- includes all then-known chelydroids, with the tle (or Hicatee) Dermatemys mawii Gray, 1847. notable exception of Dermatemys mawii. Subse- The two primary crown clades within Kinoster- quently, Boulenger (1889) provided a close varia- noidea are Kinosternidae and D. mawii, and their tion of this classification that recognizes total clades are referred to as Pan-Kinosternidae chelydrids and kinosternines as two natural and Pan-Dermatemys (Joyce et al. 2004). We groups, but furthermore grouped staurotypines herein informally name representatives of the lat- and D. mawii into an intermediate “dermatemy- ter clade “pan-dermatemydids.” did” family. Siebenrock (1909) later united D. The increasingly more inclusive sister group mawii with kinosternids into his “Kinosternidae,” relationship between kinosternines and staurotyp- but this taxon also includes the aberrant Asian ines, between kinosternids and dermatemydids big-headed turtle Platysternon megacephalum and between kinosternoids and chelydrids has Gray, 1831. The modern consensus for classifying only been recognized relatively recently, although these turtles was first recognized by Baur (1893), parts of these were recognized throughout the his- who united chelydrids, dermatemydids, stau- tory of research. The first meaningful global clas- rotypines and kinosternines into “Chelydroidea,” sifications of turtles proposed during the second but this arrangement was not widely recognized, 1 perhaps because Baur (1893) died soon after and named Chelydroidea following Baur (1893) was not able to further publish his insights in (Knauss et al. 2011), current morphological stud- additional publications. ies still fail to retrieve this clade (e.g., Joyce 2007; Hay (1908a) provided an extensive classifica- Anquetin 2012; Sterli et al. 2013; Rabi et al. 2014). tion that was the first to include all fossil and living Nevertheless, the fossil record is more consistent turtles. The arrangement that Hay (1908a) pro- with the molecular arrangement in that all pan- posed greatly resembles that of Boulenger (1889) trionychian clades can be traced back to the Early by recognizing chelydrids and kinosternines, but Cretaceous of Asia, whereas all chelydroid clades greatly expanded the concept of “Dermatemydi- only emerged during the Late Cretaceous of dae” through the inclusion of an eclectic assortment North America (Crawford et al. 2015), and of fossil turtles, in particular adocids, nanhsi- because early representatives of the chelydroid lin- ungchelyids, carettochelyids and the pleurosternid eage are difficult to distinguish from one another. Compsemys. This polyphyletic wastebasket taxon We therefore speculate that it is only a matter of was primarily united in Hay’s (1908a) diagnosis by time until new fossil finds will support the emerg- a complete row of inframarginal scutes, a charac- ing molecular consensus. ter now considered to be symplesiomorphic (Mey- For institutional abbreviations see Appendix lan and Gaffney 1989), and all other characters are 1. Named pan-kinosternoid genera are listed in noted to occur only sometimes. The polyphyletic Appendix 2. nature of “Dermatemydidae” was later further cir- cumscribed to include lindholmemydids, “sino- Skeletal Morphology chelyids” and solemydids (e.g., Romer 1956; Kuhn 1964; Mlynarski 1976; Carroll 1988). The close rela- Cranium tionships of the remaining chelydroids was never- Bienz (1895) described the cranial morphology of theless recognized in these classifications (e.g., Dermatemys mawii in detail, and McDowell Williams 1952; Kuhn 1964; Mlynarski 1976), (1964), Meylan and Gaffney (1989) and Legler and though sometimes still to the inclusion of Platys- Vogt (2013) provided additional figures for that ternon megacephalum (Romer 1956). taxon. Among kinosternids, we are aware of origi- The close relationship of Dermatemys mawii nal cranial figures of decent quality for Claudius with kinosternids was highlighted in a series of angustatus (Gaffney 1979), Kinosternon leucosto- morphological studies (e.g., Williams 1950; mum (Boulenger 1889; Legler and Vogt 2013), K. McDowell 1961; Zug 1966; Albrecht 1967), but scorpioides (Pritchard and Trebbau 1984; Legler and this did not result in a change in the prevailing clas- Vogt 2013), K. subrubrum (Gaffney 1979), Stauroty- sifications. Using cranial characters and cladistic pus salvinii (Gray 1869; Boulenger 1889; Williams http://doc.rero.ch arguments, Gaffney (1975) finally united D. mawii 1952), S. triporcatus (Legler and Vogt 2013) and Ster- and Kinosternidae to the exclusion of the broad set notherus odoratus (Gaffney 1979; Bever 2009). Of of fossil taxa listed above, but furthermore grouped these, Bienz (1895), Gaffney (1979) and Bever (2009) these with Trionychia to form the clade “Triony- provide the most meaningful insights into the cra- choidea.” The clade consisting of D. mawii and nial anatomy of the group. Among unambiguous Kinosternidae was later named “Kinosternoidae” fossil pan-kinosternoids, cranial material has been (Gaffney and Meylan 1988), but subsequently reported for the Late Cretaceous (Maastrichtian) renamed “Kinosternoidea” (Joyce et al. 2004). Emarginachelys cretacea (Whetstone 1978), the early The monophyly of Kinosternoidea was more Eocene Baptemys garmanii (Estes 1988) and Bapte- recently corroborated by a series of increasingly mys wyomingensis (Hay 1908a), the late Eocene complete studies using molecular data (e.g., Shaf- Xenochelys formosa (Williams 1952), the late fer et al. 1997; Krenz et al. 2005; Barley et al. 2010; Oligocene Xenochelys floridensis (Bourque 2013), Crawford et al. 2015), but these studies also high- and the middle Miocene Kinosternon skullridgescens lighted that kinosternoids are not closely related (Bourque 2012b), Kinosternon pojoaque (Bourque to trionychians, but rather to chelydrids, as had 2012a) and Kinosternon rincon (Bourque, in press). been proposed throughout the 19th century. The skulls of pan-kinosternoids are gener- Although numerous morphological characters ally broad and triangular in shape, the eyes are exist that could unite this highly inclusive clade, mostly oriented laterally and the interorbital 2 area is broad. With few exceptions (e.g., some separates the maxillae and palatines from paired Kinosternon such as K. flavescens), extant kinos- contact along the midline. The foramina palat- ternoids have a protruding nose and this is also inum posterius are present, but typically highly apparent in the skull in lateral view. The upper reduced in size. An ascending process of the temporal emargination is typically deep, but palatine contributes to the lateral braincase wall. the lower temporal emargination only shallow This process ranges in size from small in D. (Figure 1). mawii, to intermediate in kinosternines, to enor- The prefrontals are large, cover much of the mous in staurotypines. The pterygoids are large interorbital area, have broad midline contact with elements that contact one another along the one another and form a broad descending process midline, floor the otic area and broadly contact that contacts the vomer and palatines distally and the basioccipital posteriorly. The external defines a narrow keyhole fissura ethmoidalis (see processes are minute and lack a vertical flange. Figure 1). The frontals are typically