First report of fossil turtle eggshell west of the Colorado Plateau

Abstract

The first fossil turtle eggshell from North America west of the Colorado Plateau (UCMP 218504) is described, adding to the growing global record of chelonian eggshell. Ten blocks of mudstone were collected from the Upper Miocene (Clarendonian) Orinda Formation, Contra Costa County, California, during the excavation of the Caldecott Tunnel Fourth Bore. They contain rigid eggshell averaging 500 μm in thickness. Closely packed shell units with a height/width ratio of 2:1 are composed of radiating, acicular crystals originating from a basal core, a set of characters that support the identification of the eggshell as chelonian, and further as testudinoid. Egg diameter is estimated at between 34.2 and 42.4 mm based on shell curvature. Adult carapace length is estimated to be 70.7 cm, a size similar to fossils of Hesperotestudo known from California and present in the Orinda Formation. UCMP 218504 greatly extends the geographic record of turtle fossil eggshell and adds to the limited number of mid-Neogene eggs. It also provides evidence for a warm climate in Miocene California and demonstrates the south-eastern contraction of conditions appropriate for tortoise reproduction during the last 10 million years.

Keywords:

• Turtle
• eggs
• Testudoolithidae
• Orinda
• mitigation
• Miocene
• California

Acknowledgements

This research was made possible by Technical Agreement 04A3888 A01 between Caltrans and the Regents of the University of California, with M.B. Goodwin and C.R. Marshal as Co-PIs. The UCMP provided fossil preparation laboratory, curatorial support and funds for SEM analysis. The author was supported by a UCMP Graduate Student Researcher position during a portion of this research. Thanks to D. Barta and M. Knell for constructive criticism which improved the quality of this study. Appreciation to P.A. Holroyd and M.B. Goodwin (UCMP) for access to specimens under their care. M.B. Goodwin was supportive of this research from the beginning and this study would not have been possible without his knowledge and assistance, as well as that of A. Carr, P.A. Holroyd, L. Chang and the inestimable F. Jackson. D. Lawver’s generous assistance must be singled out, and P. Skipwith’s knowledge of herpetology was invaluable. Thanks to David Smith for his excellent artwork adapting Figure 1, and Dave Strauss for stellar photographic help with Figure 3. This is UCMP contribution number 2076.