Disparate Growth Strategies within Aetosauria: Novel Histologic Data from the Aetosaur Coahomasuchus chathamensis

Abstract

Aetosaurs comprise a clade of quadrupedal, armored, omnivores to herbivores that lived across much of the supercontinent of Pangea during the Late Triassic. Their relative abundance in many units, and the rarity of other Triassic herbivores, points to them as key components of Late Triassic ecosystems. Debate persists about whether they were growing more or less slowly when compared to extant crocodylians, and bone histology is sparsely sampled within the group. We undertook a histological examination of Coahomasuchus chathamensis to address its ontogenetic trajectory and characterize its histology. We sampled a paramedian osteoderm from the holotype specimen, as well as five osteoderms (two paramedian, one lateral, and two of uncertain position) and two incomplete limb bones (radius and fibula), from referred specimens discovered at the type locality. Using these we estimated specimen ages with lines of arrested growth (LAGs) to determine that the study individuals reached from 2 to 7 years old. All of the sampled elements contained woven-fibered bone with extensive vasculature within the internal cortex. In some specimens, more poorly vascularized, parallel-fibered bone is evident externally. The holotype of C. chathamensis represents a juvenile individual, and raises the possibility that the holotype of C. kahleorum is a juvenile as well, complicating aetosaur systematics and diversity. When compared to aetosaurs of similar size, it is clear that C. chathamensis was growing comparatively rapidly, about 3 times the rate of similarly sized specimens of Aetosauroides scagliai from Argentina. This discovery reveals the presence of disparate growth strategies within Aetosauria. Anat Rec, 302:1504-

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.…”

“…In former studies, growth rate was interpreted as being high in the inner cortex of most phytosaurs and aetosaurs (except for Mystriosuchus sp. : Butler et al, 2019), based on the deposition of flb (i.e., highly vascularized, low organized tissue indicating fast deposition), and as low(er) in the outer cortex when tissue type had switched to lzb (de Ricqlès et al, 2003;Hoffman et al, 2019;Ponce et al, 2022;table 1). Considering the relatively high organized tissue and the low to moderate vascular density (longitudinal and reticular orientation of vascular canals), the growth rate was rather low, when compared to the growth rate in above mentioned taxa (table 1).…”

Assessing ontogenetic maturity in extinct saurian reptiles

“…These terms commonly come with modifiers as well: individuals may be called ‘skeletally mature’ (e.g. Johnson, 1977; Griffin & Nesbitt, 2016 a , b ; Hoffman, Heckert & Zanno, 2019), ‘somatically mature’ (e.g. Clarke et al ., 2005; Peecook et al ., 2018; Schwarz et al ., 2019), ‘developmentally mature’ (Fiorillo & Tykoski, 2014), etc.…”

Skull osteology of Aetosauroides scagliai Casamiquela, 1960 (Archosauria: Aetosauria) from the Late Triassic of Brazil: New insights into the paleobiology of aetosaurs

“…The surangular process is much more weakly developed in MCN-PV 2347 than in UFSM 11505 (Brust et al, 2018) and PVL 2052. This mound is also better developed in Stenomyti huangae (DMNH 61392), which is a putatively juvenile specimen (sensu Hoffman et al, 2019), than in MCN-PV 2347. This difference between A. scagliai individuals is here interpreted as intraspecific variation.…”