Novel data on aetosaur (Archosauria, Pseudosuchia) osteoderm microanatomy and histology: palaeobiological implications

Abstract

One of the most striking features of aetosaurs is the possession of an extensive bony armour composed of dorsal, ventral and appendicular osteoderms. With the purpose of establishing the main histological changes during ontogeny and the degree of histological variation within the armour, we analysed the bone histology of dorsal (paramedian and lateral), ventral and appendicular osteoderms from different taxa from the Late Triassic of South America, including Aetosauroides scagliai, Aetobarbakinoides brasiliensis and Neoaetosauroides engaeus. Histological data support an intramembranous origin for osteoderms. Nevertheless, evidence for metaplastic ossification (i.e. structural fibres) at advanced ontogenetic stages, in at least some elements, is also present. A variant type of parallel fibred bone, which we have named ‘crossed parallel fibred bone’, is characterized for aetosaurs. In this pseudosuchian group, osteoderms exhibit very important microstructural changes during ontogeny, which can be useful for determining ontogenetic stages from isolated elements. Histological data suggest a relatively early onset of sexual maturity among aetosaurs. Microanatomical analysis from different taxa reveal that having high values of compactness is the plesiomorphic condition for Aetosauria. The notably increased compactness of the osteoderms does not appear to be related to size, ontogeny, sex or reproductive status of the individuals. Although a high degree of compactness of osteoderms and other bones has been considered as evidence for an aquatic lifestyle in vertebrates, such an inference contradicts the current concept of a fully terrestrial lifestyle in aetosaurs.

Postcranial anatomy and osteoderm histology ofRiojasuchus tenuiscepsand a phylogenetic update on Ornithosuchidae (Archosauria, Pseudosuchia)

“…Finally, bone histology reveals that osteoderms of some taxa are formed by a combination of more than one kind of ossification process. For example, although the main ossification process in aetosaurs and Vancleavea campi appears to be intramembranous, in some cases structural fibers from the dermis are incorporated to the element later during the ontogeny (Ponce et al, 2017;Cerda et al, 2018).…”

“…In a phylogenetic context, for pseudosuchians, the intramembranous ossification seen in R. tenuisceps appears to be a plesiomorphic condition. Also, this mechanism is commonly observed in early stages of ontogeny in pseudosuchian osteoderms, including aetosaurs (Cerda and Desojo, 2011;Scheyer et al, 2014;Cerda et al, 2018), Revueltosaurus callenderi (Scheyer et al, 2014), and erpetosuchids and proterochampsids (Scheyer et al, 2014;Cerda et al, 2015a;Ponce et al, 2017). The metaplasic ossification has a scarce and patchy distribution among pseudosuchians, including crocodylians, some phytosaurs, and loricatans (e.g., Vickaryous and Hall, 2008;Scheyer and Desojo, 2011;Scheyer et al, 2014).…”

“…On the other hand, and opposed to what is observed in most pseudosuchians, from a qualitative point of view, the osteoderms of Riojasuchus tenuisceps show less compact bone and the internal region of cancellous bone is more extensive. Osteoderms of pseudosuchians and basal archosauriforms (e.g., aetosaurs, erpetosuquids, proterochampsids) are, in general, highly compacted (e.g., Ponce et al, 2017;Cerda et al, 2018).…”

The multiscale hierarchical structure of Heloderma suspectum osteoderms and their mechanical properties

“…Osteoderms are generally considered to provide protection in the same way as an armour , but other functions have been proposed, including thermal regulation 3 [14][15][16][17][18][19], ion storage (calcium) [20], and locomotion [21]. As a consequence of their widespread occurrence and different proposed functions, the mechanical properties [1,4,[22][23][24], morphology [1,5,7,[9][10][11][12][24][25][26][27][28][29][30] and histology [5,7,9,[26][27][28]30]of osteoderms have been widely studied.…”

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

“…2021 Aetosauroides scagliai: Czepiński et al, p. 9, 11, 13, 14. 2021a Aetosauroides scagliai: Heckert and Lucas, 2002;Taborda et al, 2015) and by the low number (5) of lines of arrested growth in osteoderm sections when compared with other individuals (e.g., Cerda and Desojo, 2010;Taborda et al, 2013Taborda et al, , 2015Cerda et al, 2018). Diagnosis (adapted from Brust et al, 2018).…”