UC Berkeley

A Review of New Testudine Specimens from the Oligocene and Miocene of the Great Plains of North America, Including Descriptions of Two New Taxaby Eric M Holt Doctor of Philosophy in Integrative Biology University of California, Berkeley Professor Emeritus Anthony D Barnosky, Chair

The fossil record of Testudines in North America is well known for some parts of the Tertiary period, but for many stages representing millions of years our knowledge of Testudine diversity is both poor and outdated. The gaps in knowledge are pronounced for the Oligocene and Miocene in North America, despite an abundant fossil record. This study begins to address the gap in our understanding of fossil turtles by utilizing existing museum specimens from the Great Plains and adjacent Rocky Mountain basins, the majority of which have not been previously identified beyond the ordinal level. Updated identifications based on fossil shells are provided and the morphological criteria for recognizing many taxa are clarified, despite there being a bias in museum collections towards subadult and juvenile testudine specimens. A previously unknown regional variant of Gopherus edae is described, and initial steps are taken toward documenting the range of intraspecific morphological variation found within various taxa of Oligopherus, Gopherus, Stylemys, and Hesperotestudo. Chapter One lays the groundwork for future studies on tortoise diversity through time and space, and the relationship between their evolution, distribution, and climate change.

In Chapter Two the largely complete shell of a new species of tortoise, Hesperotestudo nicholsorum n. sp., is described from Lemhi County, Idaho, USA. The tortoise genus Hesperotestudo Williams, 1950b, was widespread throughout North America from Canada to El Salvador from the earliest Miocene to the late Pleistocene but retains no extant members. The specimen was collected from Late Hemingfordian (~15-16.3 Ma) sediments in the Railroad Canyon Sequence of the Sixmile Creek Formation. Previous studies have established a late Eocene paleoelevation of 3.7 km for the area. Elevational decrease due to Basin and Range extension did not happen until the middle Miocene, indicating that the fossil locality was higher in the Hemingfordian than the roughly 2 km elevation it is today. This makes H. nicholsorum both the highest elevation specimen of Hesperotestudo currently described and one of the oldest members of its genus. The new taxon shares many morphological similarities with other Hesperotestudo but also exhibits characters more commonly found in Gopherus.

In Chapter Three I describe the shell of another new tortoise, Hesperotestudo niobrarensis n. sp., from Cherry County, Nebraska, USA. The specimen was collected in 1933 from Clarendonian sediments in the Ash Hollow Formation of the Ogallala Group. An isolated left epiplastron from the Clarendonian Ash Hollow Formation near Merrit Reservoir in Cherry County, Nebraska is referred to the new species. This new taxon shares many morphological similarities with other Hesperotestudo but also exhibits a unique combination of characters not found in other members of the genus.