Our Upcoming Speaker
Note: Since October 2022, we have returned to in-person meetings at the Birmingham Zoo using a hybrid Zoom-In Person format with the in-person meeting at the Lodge at the Birmingham Zoo

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

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Speaker: Tristan Stock, MS, Graduate Paleontology Student at California State University, Fullerton

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Abstract:

    Modern hard-shelled sea turtles (crown group Cheloniidae, seven extant species) originated in the middle or late Miocene. Pan-cheloniids (including the stem) are recognized from Cretaceous to Miocene strata from all continents except Antarctica. However, fossils from the Late Miocene, a key transitional period between the stem and crown lineages, are rare. New discoveries from this time have the potential to significantly enhance our understanding of how pelagic adaptations evolved within this lineage. Here we describe the first well-preserved stem cheloniid from the Late Miocene (LACM 21850), a subadult specimen of Pacifichelys from the upper Miocene “Monterey” Formation of Orange County, California (USA). LACM 21850 includes a nearly complete skull with visible palatal characters allowing identification as Pacifichelys urbinai, a species previously described from the Pisco Formation of Peru 9.5-8.6 Ma. Diatoms recovered from the skull surface of LACM 21850 allow for accurate dating to 8.8-8.6 Ma, making it approximately coeval, or slightly younger, than the Pisco specimens.
    LACM 21850 preserves most of the postcranial material, including three of the four limbs, allowing for direct comparison of limb anatomy and pelagic adaptations among Pacifichelys, Cretaceous pan-cheloniids, and crown cheloniids for the first time. The forelimb of LACM 21850 has a mosaic of traits: the humerus and first digit are short and robust, most closely resembling Cretaceous stem cheloniids, whereas the radius and ulna proportions are more similar to those of other Cenozoic pan-cheloniids, and the metapodial elements exhibit a blend of plesiomorphic and derived characteristics. The proportions of the forelimb and hindlimb, 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, all suggest that LACM 21850 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 LACM 21850 the stratigraphically youngest known chelonioid to use the ancestral rowing style of aquatic locomotion. LACM 21850 thus provides a unique snapshot of the evolutionary transitions between the stem and crown cheloniids, bridging a critical gap in our understanding of pan-cheloniid evolution.

About the Speaker: 

Tristan J. Stock holds a Master’s of Science degree in Geological Sciences with an emphasis on Vertebrate Paleontology from California State University Fullerton. He has been passionate about natural history since a young age and has devoted thousands of hours of time over twelve years volunteering at the Los Angeles County Natural History Museum, both behind the scenes conducting research as well as educating the public in exhibition halls. Tristan’s interests involve the evolutionary history of reptiles, specifically with regards to the origins of clade-specific traits, behaviors, and ecologies, as well as the relationships between fossil Archelosaurians (the bird, crocodile, and turtle clade). He plans to continue his studies in pursuit of a PhD in Vertebrate Paleontology. He is currently working on describing fossil hard-shelled sea turtles from Southern California, and has so far documented and indexed over 300 specimens of sea turtles from within the collections of the Los Angeles County Natural History Museum for use in future research.

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