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 paleopathological elements than in any other Triassic archosauromorph. These exostoses are often centered on cavitations reminiscent of draining tracts. Extensive, irregular, proliferative 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 nutritional 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 present, 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.

Growth and limb bone histology of aetosaurs and phytosaurs from the Late Triassic Krasiejów locality (sw Poland) reveals strong environmental influence on growth pattern

“…4G). This is also contrary to the observations made in other phytosaur and aetosaur taxa where lag s are common and an efs is documented for several individuals (de Ricqlès et al, 2003;Hoffman et al, 2019;Butler et al, 2019;Heckert et al, 2021;Ponce et al, 2022;table 1). The two sampled Stagonolepis femora are nearly completely remodelled by secondary osteons, which together with their large size (table 2) indicate a late ontogenetic stage for both.…”

“…), a clear change in tissue type from flb in the inner cortex to lzb in the outer cortex was described (table 1). This change in tissue type is usually interpreted as ontogenetic change from fast growth rates in early ontogenetic stages to slower growth rates in adults and/or sexually mature individuals (de Ricqlès et al, 2003;Hoffman et al, 2019;Heckert et al, 2021;Ponce et al, 2022;table 1). Contrary, neither the taxa from Krasiejów nor the phytosaur from Heslach show woven bone in their cortex and their vascular density is moderate to low.…”

A giant phytosaur (Diapsida, Archosauria) from the Upper Triassic of India with new insights on phytosaur migration, endemism and extinction

“…The skull constitutes 13–15% of the total body length in the later‐diverging phytosaurs such as Smilosuchus (Heckert et al . 2021; p. 8, fig. 4) and Machaeroprosopus (Hurlburt et al .…”

“…Although thin ridge‐like pathological bone lesions have been reported in the limb bones of Smilosuchus gregorii (Heckert et al . 2021), the specimens of Colossosuchus techniensis preserving these crests, in particular IITKGPR759, are pristine with no signs of deformation. Pathological signatures such as fracture callus, abnormal suturing pattern and striated muscle scarring ( sensu Wolff 2008; Straight et al .…”

A new phytosaur species (Archosauriformes) from the Upper Triassic of Jameson Land, central East Greenland

“…Contrary to the phytosaur samples from Poland (Teschner et al, 2022), the Greenland phytosaur shows LAGs, indicating regular pauses in growth, which indicates that the individual lived in a more seasonal climate. Heckert et al (2021) described the histology of a pathological humerus from a phytosaur from North America. Aside from the pathological tissue, the osteohistology and microanatomy of that specimen are similar to that of the Greenland phytosaur.…”