Identifying shark cartilage in the fossil record: the first occurrence of calcified (Chondrichthyes) cartilage from the Upper Triassic of the western United States

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Calcified cartilage is a diagnostic feature of chondrichthyan endoskeletons but is rarely preserved in the fossil record because of its susceptibility to degradation, which limits our understanding of the history and evolution of chondrichthyan endoskeletons. Here, three fossil cartilage specimens from the Upper Triassic are described, including PEFO 52632 and PEFO 54505 from the Blue Mesa Member of the Chinle Formation (early Norian; ∼220 Ma) in Petrified Forest National Park, Arizona, USA, and TTU-P25078 from the Tecovas Formation (∼220 Ma) in Crosby County, Texas, USA. The specimens exhibit polygonal tesserae, globular and prismatic calcification, fibrous intertesseral zones, lacunae, and the absence of medullary cavities; all features characteristic for tessellated calcified cartilage. We use histological thin sections, scanning electron microscopy, and computed tomography to document variation in internal organisation, including globular calcified cartilage and prismatic calcified cartilage, as well as the presence or absence of a distinct cap and body zone. A comparative assessment with extant and extinct chondrichthyans indicates the fossil material likely came from xenacanthiform sharks; both are known from the same localities. By filling a key gap in the cartilaginous fossil record, these findings clarify the structural characteristics of chondrichthyan cartilage and provide a comparative framework for identifying fossil cartilage, including fragmentary material recovered from microvertebrate bonebeds, thereby expanding documentation of chondrichthyan endoskeletal tissues in Triassic freshwater ecosystems.

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