842
Journal of Paleontology 89(5):821–844
are Kinosternon sonoriense (comprising two subspecies) and Kinosternon hirtipes murrayi, three taxa that live in the arid southwestern United States and northern Mexico (Ernst and Lovich, 2009). Kinosternon pannekollops and Kinosternon notolophus appear to be closely allied, and in turn the former is most closely related to the subrubrum group. However, K. notolophus shares a combination of carapacial features with the Sonoran mud turtle Kinosternon sonoriense sonoriense as well. It is currently unclear whether these similarities with K. s. sonoriense are purely homoplastic or phylogenetically significant based on the limited fossils of K. notolophus. If the latter, the occurrence of a plausible progenitor of K. s. sonoriense in Florida during the late Miocene has sig- nificant paleobiogeographical implications (perhaps related to more easterly distributed savanna-like habitats along the Gulf Coast [Webb, 1977]), and could even indicate a sister group relationship between the subrubrum group and K. sonoriense. The age and phylogenetic position of Kinosternon
pannekollops suggests that the subrubrum group diverged from other Kinosternon by the late middle to early late Miocene. This estimate is concordant with molecular-based divergence dates for the subrubrum group proposed by Iverson et al. (2013), although the range given in their results includes the middle Miocene. Interestingly, Kinosternon rincon is from the middle Miocene and is recovered as a plausible basal sister taxon of the subrubrum group in the phylogenetic results presented here. It is therefore not unlikely that the subrubrum group could have descended from K. rincon or a common ancestor that originated in xeric habitats of the southwestern United States. The group then likely emigrated into the open savanna-like portion of the central United States by the early Clarendonian, which is corroborated withUNSM125577 fromthe early Clarendonian of Nebraska and represents a species similar to K. rincon, but most like or synonymous with K. wakeeniense. Kinosternon wakeeniense and K. pannekollops from Texas were possibly contemporaneous and persisted in what is today the Great Plains region during the Clarendonian. This indicates that the progeni- tors of the subrubrum group (e.g., K. pannekollops)likely evolved in the late Miocene savannas of the Great Plains, and that the group has dispersed from that region over time. Under this scenario, the most likely route of dispersal for the subrubrum group would be from west to east, probably across the Gulf Coastal Plain to the Atlantic Coastal Plain. This seems plausible given that the Gulf Coastal Plain was comprised of woodland savanna and mesic forest habitats in the late Miocene (Webb, 1977; Webb et al., 1981) and that the Gulf Coastal Plain and Atlantic Coastal Plain regions (sensu lato) encompass the modern range of the subrubrum group today (Ernst and Lovich, 2009). The basal phylogenetic position and current geographic range of the Mississippi mud turtle, Kinosternon hippocrepis, suggest that the other members of the subrubrum group arose from K. hippocrepis or a common ancestor thereof that dispersed from the western Gulf Coastal Plain, from more open habitats in the central United States tomore closed canopy forests in eastern and southeastern United States. In some regard, this scenario agrees with the oldest reports of Kinosternon ‘subrubrum’ that are from the late Hemphillian (latest Miocene, earliest Pliocene) of the central United States in Kansas (Fichter, 1969).
The oldest reports of Kinosternon are based on scant water-
worn material from the Pollack Farm locality in Delaware (Holman, 1998; Weems and George, 2013). Those vouchers (USNM 483389) more likely represent a new kinosternine genus that shares similarities with both Kinosternon and Sternotherus (Bourque and Schubert, 2015). This taxon (Fig. 10; items 1 and 4 listed as Kinosterninae gen. et sp. nov.) also occurs in the early Barstovian Willacoochee Creek Local Fauna (Bryant, 1991; Bourque, 2013) of northern Florida, but there is no record of it younger than the early Barstovian. As such, the only two records are from the Atlantic and Gulf coastal plains during an interval that spans the Middle Miocene Climatic Optimum (Zachos et al., 2001), suggesting that this taxon may have gone extinct at the end of that megathermal event. However, Kinosternon herrerai (from the Gulf Coastal Plain of eastern Mexico) possesses plastral symplesiomorphies with this taxon at the exclusion of most other Kinosternon, such as very broad contact between the axillary and inguinal scutes on the hyoplastron and in many individuals a more anteriorly situated posterior humeral–femoral sulcus on the hypoplastron. Additionally, it has been suggested that K. herrerai is the most primitive modern Kinosternon in a number of morphological attributes (Bramble et al., 1984), and K. herrerai has been recovered as the most basal Kinosternon in some phylogenetic results (Iverson, 1991; results here in Fig. 9). Perhaps K. herrerai is a direct descendant (or last surviving member) of this otherwise extinct genus that used the Gulf Coastal Plain to disperse into Mexico, and like dermatemydids, staurotypines (e.g. the stem staurotypine Baltemys sensu Iverson [1991] and Bourque [2012b]), and geoemydids was once more northerly distributed, but has endured in a southern North American refugium. More complete fossils of this early to middle Miocene kinosternid will undoubtedly better our understanding of kinosternine phylogeny, particularly concern- ing the divergence between Kinosternon and Sternotherus and in ascertaining whether K. herrerai is primarily or secondarily plesiomorphic.
Accessibility of supplemental data
Supplemental data deposited in Dryad data package: http://dx.
doi.org/10.5061/dryad.v6sm8 Acknowledgments
Many thanks to E. Gaffney and C. Mehling for allowing me to borrow, prepare, and study AMNH specimens discussed here; G. Corner, J. Head, L. Albaczinskas, K. Krysko, and M. Nickerson for specimen loans; R. Hulbert, B. MacFadden, and A. Wood for discussions and providing pertinent literature; J. Bloch for equipment use; E. Woodruff for assistance with phylogenetic analysis and for uploading matrix to the University of Florida High Performance Computing Center; C. Manz for phylogenetic discussions; P. Morse for gathering map data; S. Chester for travel to AMNH; R. Portell for identifying snail fossils associated with FAM 12778; C. May and R. Hare for modern specimen donations; J. O’Brien (Amarillo, TX) for granting me access to his ranch to visit the type area of FAM 12778 in June 2013; and W. Joyce, D. Brinkman, M. Silcox, and
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