WELL-PRESERVED SPECIMEN OF FOSSIL SEA TURTLE FROM ORANGE COUNTY, CALIFORNIA, SHEDS LIGHT ON THE EVOLUTION OF PELAGIC SPECIALIZATIONS

Modern hard-shelled sea turtles originated in the Miocene. However, fossils from the late Miocene, a key transitional period between the stem and crown, are rare. This thesis describes the first well-preserved stem cheloniid from the late Miocene. LACM 21850 is from the upper part of the “Monterey” Formation of Orange County, California. It includes a nearly complete skull allowing identification as Pacifichelys urbinai, a species previously described from Peru (9.5-8.6 Ma). Diatoms from LACM 21850 date it to 8.8-8.6 Ma, approximately coeval with the Peru specimens. LACM 21850 preserves three of the four limbs, allowing for direct comparison of limb anatomy with stem and crown cheloniids (i.e., pan-cheloniids). LACM 21850 shows the forelimb of P. urbinai has a mosaic of traits: the humerus and first digit are short and robust, resembling Cretaceous stem cheloniids, the radius and ulna are more similar to those of other Cenozoic pan-cheloniids, and the metapodial elements exhibit a blend of plesiomorphic and derived characters. The proportions of the limbs, proximal placement of the lateral process of the humerus, saddle-shaped distal end of the tibia, and presence of retroarticular processes on the metapodial elements, suggest P. urbinai utilized an anteroposterior quadrupedal rowing swimming style. This differs from the dorsoventral flapping “underwater flight” used by extant sea turtles. If correct, this would make P. urbinai the stratigraphically youngest known chelonioid to use the ancestral rowing style of aquatic locomotion. LACM 21850 thus provides a unique snapshot of the evolutionary transition between the stem and crown cheloniids, bridging a critical gap in our understanding of pan-cheloniid evolution.