101-11 ‘Dermal Armor’ Represents a Major Component of the Fossil Record for Late Triassic Tetrapods: Implications for Osteoderm Use in Phylogenetic Studies

Session: Paleontology, Biogeography/Biostratigraphy & Phylogenetic/Morphological Patterns

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Osteoderms, ossified elements embedded in skin, are present in most major tetrapod lineages, including both extinct and extant forms. Commonly interpreted as “dermal armor”, osteoderms are often structurally durable, and can make up a major component of the skeleton. These two characteristics uniquely prime osteoderms for preservation in the fossil record. Previous studies have documented that different tetrapod skeletal regions are over- and underrepresented in the fossil record. In certain groups (e.g., squamates) one of the most overrepresented skeletal elements include osteoderms. However, few phylogenetic datasets include osteoderms as a significant part of their character sets, even when osteoderm morphological diversity is well documented. To assess the degree of osteoderm skeletal preservation bias, we quantify the parts of the skeleton preserved in various tetrapod clades from the extremely rich Hayden Quarries (Late Triassic, Chama Basin, NM). We binned collected specimens with verified field identifications (n=2309) by: 1) taxonomic identification, and 2) general anatomical region. When considering taxonomic diversity, phytosaurs make up the majority of fossils collected at the Hayden Quarries (38.2%), followed by unspecified archosauromorphs (36.1%). The highest percentage of fossils collected from this site are osteoderms (28.7%), followed by vertebrae (21.3%). These results demonstrate that osteoderms account for at least one quarter of the skeletal elements found in the Hayden Quarries. However, for the groups found at Hayden Quarries with osteoderms (phytosaurs, aetosaurs, rauisuchids, Vancleavea, and unspecified archosauromorphs), osteoderms make up a small component of the characters used in their phylogenetic analyses. Of seven phylogenetic datasets assessed, osteoderm characters make up only ~1-3% of all characters. A notable exception to this trend is seen with species-level aetosaur data matrices. Osteoderm characters in these datasets account for upwards of 37% of all characters. These results, from a targeted survey of a productive fossil horizon bearing several groups possessing osteoderms, illustrate a potential bottleneck in understanding the evolutionary relationships of these major vertebrate clades. The discrepancy between the available fossil data, and the underuse of these data in current phylogenetic analyses, warrants consideration of how osteoderms can be incorporated to better infer relationships among Late Triassic tetrapods.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6476

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