Palaeobiological inferences of “rauisuchians” Fasolasuchus tenax (Los Colorados Fm., Argentina) and Prestosuchus chiniquensis (Santa Maria Super sequence, Brazil) from the Middle–Upper Triassic of South America based on microstructural analyses

Abstract

Abstract“Rauisuchia” is a non‐monophyletic group of quadrupedal and carnivorous pseudosuchians that inhabited the entire world during the Middle–Upper Triassic period (Anisian/Ladinian–Rhaetian). In South America, “rauisuchians” reached the largest sizes among continental carnivores. Despite their important ecological role, some aspects of their palaeobiology have been poorly examined. Here, we study appendicular bones, dorsal ribs and osteoderms of two genera, the Argentinean Fasolasuchus tenax (PVL 3850, holotype) and the Brazilian Prestosuchus chiniquensis (SNSB‐BSPG AS XXV) respectively. The femur of F. tenax is formed by laminar fibrolamellar bone, which is composed of non‐fully monorefringent woven‐fibred matrix and primary osteons; the dorsal rib has a Haversian bone composition with an external fundamental system recorded and the osteoderm is formed by well‐organised parallel‐fibred bone. The femur, humerus and fibula of P. chiniquensis are mostly composed of strongly arranged parallel‐fibred bone and a laminar vascularisation. The minimal ages obtained correspond to 9 years for F. tenax (based on the maximum number of growth marks in the osteoderm) and 4 years for P. chiniquensis (obtained from the highest count of growth marks in the femur and in the humerus). F. tenax attained somatic and skeletal maturity, while P. chiniquensis was near to reaching skeletal and sexual maturity, but it was somatically immature. The overall rapid growth rate and the high and uniform vascularisation seems to imply that these features are common in most of “rauisuchians”, except in P. chiniquensis.

Bone histology supports gregarious behavior and an early ontogenetic stage to Decuriasuchus quartacolonia (Pseudosuchia: Loricata) from the Middle‐Late Triassic of Brazil

“…By contrast, in Decuriasuchus , FLB of reticular and sub/plexiform arrangement of vascular canals predominates in the whole cortices of the samples, indicating that the bones were growing at higher pace until the moment of death, with no traces of slowing growth. This is probably explained by ontogeny, because some species of “rauisuchians” such as Prestosuchus (Ponce et al, 2023), presented a slower growth evidenced by the presence of parallel‐fibred bone.…”

“…Until now, histovariability in “Rauisuchia” is known only between appendicular bones, ribs, and osteoderms. This was accessed in Batrachotomus (Klein et al, 2017) and Fasalosuchus (Ponce et al, 2023), which presented rapid growth in the femora, and a very slow growth in the ribs. In Decuriasuchus , unfortunately, ribs with good preservation were not available, but it possesses the lowest degree of vascularization of all elements, and it was the only element to present secondary osteons.…”

“…Most of the bone histological studies in Pseudosuchia indicate a general pattern of growth, with a small period of fast growth indicated by the presence of FLB, followed by protracted slower growth as shown by the parallelfibred/lamellar bone matrix intersected by numerous growth lines (see Ricqlès et al, 2003Ricqlès et al, , 2008. However, fewer species within Paracrocodylomorpha (sensu Parrish, 1993) are considered exceptions because of a more ornithodiran-like pattern of growth (sustained high growth rates during whole life), such as the poposauroid Effigia (Nesbitt, 2007), the basal loricatans Postosuchus (Ricqlès et al, 2003), Mandasuchus (Ricqlès et al, 2003), Batrachotomus (Klein et al, 2017), Fasalosuchus (Ponce et al, 2023), Decuriasuchus (this work), the "sphenosuchians" Terrestrisuchus (Ricqlès et al, 2003) and Saltoposuchus (Spiekman, 2023), and the notosuchian Pepesuchus (Sena et al, 2018). Ricqlès et al (2003) histologically sampled Postosuchus and found FLB in the inner cortices that was protracted for few years, changing to a slow (but still considerably high) growth in the following years.…”

“…Over the last decades, studies on the growth strategies of “rauisuchians” are changing the view of Pseudosuchia as a group of slow‐growing reptiles (e.g., Cerda et al, 2011, 2013; Klein et al, 2017; Nesbitt, 2007; Ponce et al, 2023; Ricqlès et al, 2003, 2008; Scheyer & Desojo, 2011). Samples encompassing several species of “rauisuchians” were used in the studies of Scheyer and Desojo (2011) and Cerda et al (2013), such as Rauisuchus , Tikisuchus , Batrachotomus , Fasalosuchus , Prestosuchus , and Saurosuchu s. However, such analyses focused exclusively on osteoderms, due to their abundance in paleontological collections.…”

“…Klein et al (2017) analyzed Batrachotomus , from the Middle Triassic of Germany, also with a limited sample of three elements. Recently, Ponce et al (2023) studied the femur and rib of Fasolasuchus from the Late Triassic of Argentina and some appendicular bones (femur, humerus, and fibula) of the lectotype of P. chiniquensis , emphasizing Prestosuchus as having a slower growth rate when compared to other “rauisuchians.”…”