A NEW SEA TURTLE FROM THE MIOCENE OF CALIFORNIA AND THE EVOLUTION OF STEM-CHELONIID FEEDING MORPHOLOGIES

Crown Cheloniidae contains five extant genera of hard-shelled sea turtles that exhibit diverse diets tied directly to their feeding ecomorphology. Relationships between extant and stem cheloniids, which together comprise Pan-Cheloniidae, remain enigmatic, impeding the ability to pinpoint the timing and evolution of key ecomorphological features. 

Previously undescribed turtle fossils from the Tortonian (11.6–7.2 Ma) Santa Margarita Formation in Santa Cruz County, California, may help reveal how ecomorphology evolved in the Pan-Cheloniidae. In this thesis, the age of these fossils is estimated, and their anatomical characters are described. The turtle fossils and several pan-cheloniid taxa were examined in character-by-character assessment matrices and chronostratigraphic analysis. The matrices were analyzed by maximum parsimony and Bayesian statistical analysis. This produced revised phylogenies for the group, upon which ecomorphology was plotted for examination in an evolutionary context. 

The newly described turtle material includes an articulated skull that likely represents a new taxon and possesses a shearing ecomorphology. Postcranial elements are unassignable to the skull and exhibit a suite of derived and plesiomorphic characters. Phylogenetic analyses recover the skull taxon within or sister to crown Cheloniidae. The revised phylogeny supports the iterative evolution model for shearing in pan-cheloniids such that it evolved independently at least three times. However, the uncertainty of the position of the skull taxon obfuscates exactly how shearing evolved in crown Cheloniidae. Even so, the presence of a derived fossil sea turtle skull in the Santa Margarita Formation indicates that similarly derived postcranial material should be expected among future discoveries from this unit.