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| Главный автор: | |
|---|---|
| Формат: | Recurso digital |
| Язык: | английский |
| Опубликовано: |
Zenodo
2025
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| Предметы: | |
| Online-ссылка: | https://doi.org/10.5281/zenodo.16236521 |
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Оглавление:
- <p><span lang="EN">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 <em>Pacifichelys</em> <em>urbinai</em>, 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.</span></p> <p><span lang="EN">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 <em>P. urbinai</em> 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 <em>P. urbinai</em> 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 <em>P. urbinai</em> 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.</span></p>