63 - First Evidence of Paleopathology Among the “little Dinosaurs of Ghost Ranch”: A Potential Ossified Periosteal Hematoma on a Tibia of the Triassic Theropod Dinosaur Coelophysis Bauri

Poster Board Number: 63
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Abstract Body : The study of osseus pathologies preserved in the fossil record has revealed much about the individual life conditions of extinct vertebrates and the evolutionary origins of bone growth responses to stress, illness, and injury. Fossil mass mortality assemblages are often key sources of pathology data, owing to broad cross-sectional sampling of populations. One such case study is the Ghost Ranch, New Mexico, bonebed of the Late Triassic theropod dinosaur Coelophysis bauri. Despite 75 years of research on hundreds of available specimens, external morphological evidence of pathologies remains elusive. During histological examination of an isolated Ghost Ranch C. bauri right tibia (ROM 72668), originally thin-sectioned for a broader histological survey, we discovered an unusual, externally indiscernible bony overgrowth on the medial side of the tibial midshaft. Because multiple diseases and injury types may produce similar bone growth responses, we compared the ROM 72668 exostosis to previously published osseous pathologies across a phylogenetically expansive range of extant and extinct tetrapods to determine its likely etiology.

The smooth exostosis lacks any rugose external texture, commonly associated with healed fractures, osteomyelitis, or osteosarcomas. No abnormal tissue is present in the medullary cavity, in contrast to the condition expected of a fracture callus. Internally, the exostosis is composed of a lens of woven bone with radial vasculature, indicating rapid growth. It is bordered by lines of arrested growth, which likely formed due to temporary growth stoppage induced by illness or injury. There is an abrupt transition to poorly vascularized parallel-fibered bone external to the exostosis, reflecting a shift to slower growth.

Features indicating rapid growth of the exostosis initially suggested the possibility that it formed through rapid bone deposition along the tibia opposite a fractured fibula, such as in previously described tibiae of the ornithischian dinosaur Maiasaura peeblesorum. However, the medial location of the Coelophysis exostosis in not congruent with the lateral and posterior positions in the Maiasaura tibiae. In any case, the lack of a fibula associated with ROM 72668 precludes a further test of this idea. Instead, we find strong histological similarities with ossified periosteal hematomas previously described in a varanid lizard, gorgonopsian synapsid, and humans. Similar periosteal reactions are rare, but occasionally documented in Crocodylia and Aves, the extant clades that most closely bracket non-avian dinosaurs, suggesting an occurrence in an early-diverging dinosaur taxon like Coelophysis is plausible.

Periosteal hematomas can result from localized trauma or nutritional deficiencies. We cannot distinguish between these etiologies given the current sample but note that increased intra-specific conflict and/or nutritional deficiencies might be expected under the drought-stressed conditions inferred to have contributed to the deaths of the Ghost Ranch Coelophysis individuals. The fact that this pathology was only discovered through histological analysis highlights the usefulness of osteohistology in paleopathological investigations and suggests the potential for further discoveries of pathological bone during routine thin-sectioning of fossil vertebrates.