FOSSIL TORTOISES (TESTUDINES: TESTUDINIDAE) OF THE TOADSTOOL PARKBADLANDS (WHITE RIVER GROUP: CHADRON AND BRULE FORMATIONS), SIOUX ANDDAWES COUNTIES, NEBRASKA
This study is ongoing since fall of 2000. Prior to her death in 2008,Shelley Franz was a collaborator on this study. She was a full andactive participant in the field and museum visits. She preparedspecimens and helped catalogue and identify the material. I plan tofully acknowledge her contribution to this study, either as adedication or as a coauthor.
A fossil tortoise fauna is described from 21 badlands onprivate ranches in the vicinity of Toadstool Park, Sioux and Dawescounties, in northwestern Nebraska. The study is based on more than300 newly-collected specimens. This sample is supplemented byspecimens from Nebraska and elsewhere in the northern Great Plains inmuseums and private collections. The Toadstool Park tortoise faunaincludes 8 distinctive shell types from the Chadron (Peanut Peak andBig Cottonwood Creek members) and Brule (Orella and the lower andmiddle parts of Whitney members) formations in the White River Group(late Eocene and early Oligocene). Each shell type occurs in specificlayers within the progressively stacked, time-relative section ofvolcaniclastic rocks. The entire section spans approximately 5-6million years in time, beginning in the late Eocene about 35-36 Ma.Six of the eight shell types are distributed among the generaStylemys, Gopherus, and primitive Hesperotestudo. None of them aresimilar to Hesperotestudo brontops (Marsh), known from the ChadronFormation in Pennington County, South Dakota. The affinities of ShellType 1 from the Peanut Peak member are unresolved, and it is listed asgenus and species indet. Two shell types (3 and 5) from the upper BigCottonwood Creek and Orella members are consistent with previouslyrecognized Stylemys nebrascensis Leidy and Gopherus laticuneus (Cope).Shell types 2 and 4 (S. amphithorax and Shell Type 4) belong to thegenus Stylemys; however, the relationships between these types and S.nebrascensis are unclear. Two shell types (6 and 7) are small speciesfrom the Whitney member that will be named as new species of Gopherusand Hesperotestudo. A third Whitney species Shell Type 8 consists ofa large, but poorly preserved shell, whose identity remainsundetermined at this time. Morphological diversity associated with theexpression of gender and ontogeny will be described for common shelltypes. The Toadstool area tortoises are being compared with otherWhite River-aged tortoise faunas from the northern Great Plains.Depositional and ecological interpretations will be provided whenavailable.
nuchal plate from Stylemys nebrascensis
nuchal plate from Gopherus laticuneus