A large, pathological skeleton of smilosuchus gregorii (Archosauriformes: Phytosauria) from the upper triassic of Arizona, U.S.A., with discussion of the paleobiological implications of paleopathology in fossil archosauromorphs

Abstract

Phytosaurs were a widespread clade of Triassic predatory archosauriforms whose skull anatomy is well known, but whose paleobiology is underexplored. Here we report on a well-preserved specimen from Adamanian (early–mid-Norian) strata in Arizona that includes not only the skull and lower jaws but much of the postcranial skeleton, which exhibits extensive evidence of pathologies. This specimen has a complex taxonomic history, and we verify its referral to Smilosuchus gregorii based on multiple cranial characters. The shafts of eight limb bones preserve extensive exostoses—more paleopatho-logical elements than in any other Triassic archosauromorph. These exostoses are often centered on cavitations reminiscent of draining tracts. Extensive, irregular, prolif-erative lesions have completely engulfed the left deltopectoral crest and thoroughly altered the architecture of both femora. The animal’s presumed low metabolic rate would have allowed several months of lesion progression before it died of either nutri-tional deficiency or systemic infection. This is the fourth, and by far the most extensive, report of pathology in a phytosaur, and only the eighth in a non-dinosaurian Triassic archosauromorph. The character and location of the lesions evokes aspects of both osteomyelitis and hypertrophic osteopathy—though neither is fully consistent with the changes pres-ent, nor are these conditions well-explored in extant reptiles. The most likely cause of the pathologies exhibited here is osteomyelitis; indeed, this specimen bears more osteomyelitis-like paleopathological elements than any other fossil archosaur.