Incongruent ontogenetic maturity indicators in a Late Triassic archosaur (Aetosauria: Typothorax coccinarum)

Abstract

Maximum individual body size in pseudosuchian archosaurs is not well constrained in the fossil record, but it may be influenced by a variety of factors including basal metabolic rate, evolutionary relationships, and environmental conditions. Body size varies among the Aetosauria in which estimated total length ranges between 1 m (e.g., Coahomasuchus kahleorum) and 5 m (e.g., Desmatosuchus spurensis). A new, very large specimen of the aetosaurian Typothorax coccinarum from Petrified Forest National Park in northeastern Arizona is nearly twice the size of all other known specimens of Typothorax and is the largest aetosaur specimen currently known worldwide. The specimen lacks co‐ossified neurocentral sutures in the trunk vertebrae which may suggest that the individual had not yet reached skeletal maturity, yet smaller specimens of T. coccinarum exhibit partially or fully co‐ossified neurocentral sutures in the same region. If body size correlates with skeletal maturity in aetosaurs, this discrepancy warns that timing of neurocentral suture co‐ossification in aetosaurs may not be a reliable indicator of ontogenetic stage. Osteohistological observations of a trunk rib demonstrate that although PEFO 42506 shows a large body size, the specimen did not deposit an external fundamental system despite depositing as many as 19 growth lines, further indicating that it had not yet reached skeletal maturity. Thus, at least within Aetosauria, neurocentral suture co‐ossification and skeletal maturity may correlate, whereas body size can be incongruent in comparison. Furthermore, this specimen indicates that non‐desmatosuchin aetosaurs could exhibit large body sizes and suggests that some aetosaurs may have experienced indeterminate growth.

The dawn of an Era: New contributions on comparative and functional anatomy of Triassic tetrapods

“…The first of these introduces none other than the largest known aetosaur to date. According to Parker et al (2024, this issue), the new specimen, attributed to Typothorax coccinarum , though exceptionally large, had not reached skeletal maturity, indicating that these animals could attain even more impressive sizes. In a second contribution, Reyes et al (2024, this issue) enrich the bestiary of newly described species in this edition by introducing Garzapelta muelleri , a new aetosaur from the renowned deposits of the Dockum Group, Texas.…”

Garzapelta muelleri gen. et sp. nov., a new aetosaur (Archosauria: Pseudosuchia) from the Late Triassic (middle Norian) middle Cooper Canyon Formation, Dockum Group, Texas, USA, and its implications on our understanding of the morphological disparity of the aetosaurian dorsal carapace

“…The largest preserved mid-trunk rib in TTU-P 10449 exhibits a length of 55 cm, but it is missing both the proximal and distal ends. Overall, the morphology described above is best exemplified by the mid-trunk ribs of T. coccinarum (PEFO 42506;Parker et al, 2024) and S. deltatylus (PEFO 34045, Parker, 2016b, figure 18d).…”

“…Currently, only D. spurensis (UMMP 7476, Case, 1922), L. meadei (TMM 31185-84, Sawin, 1947), andT. coccinarum (PEFO 42506;Parker et al, 2024) preserve sufficient rib material that can be compared with those of TTU-P 10449; however, only those of D. spurensis are formally described and figured (Case, 1922;Parker, 2008). The disparity in morphology (described below) indicates that the preserved ribs are derived from the cervical and trunk regions.…”