Ecomorphology and bone microstructure of Proterochampsia from the Chañares Formation

Abstract

Proterochampsians are a South American endemic group of non-archosaurian archosauriforms with morphological characteristics recollecting Recent crocodilians, and therefore have been proposed as aquatic species. However, this has not been based on careful examination of anatomical and histological features. We provide a review of the morphological and histological evidence present in the skeleton of proterochampsids and discuss its implications for inferring the lifestyles of these organisms. Anatomical features such as a secondary palate, marginal dentition, palatine teeth, morphology of the tail, limb modification, and dermal armor are reviewed, and details of histological structures are described based on bone thin sections. Histological examination reveals a predominance of fibrolamellar bone tissue, suggesting rapid periosteal osteogenesis and therefore overall fast bone growth. The existence of discontinuities (LAGs) demonstrates that these animals responded to changes in their environment. Ecomorphological features do not provide definitive evidence for the lifestyles of proterochampsids, but allow us to propose a terrestrial/amphibious condition. The same is true of the histological features, particularly compactness of the bone.

The stem-archosaur evolutionary radiation in South America

“…As a result, the centrum length profile of Chanaresuchus bonapartei indicates that its tail was not an efficient propulsor in an aquatic environment and, thus, there is no clear evidence supporting a locomotor style associated to a semiaquatic behaviour. This is congruent with recent studies that reached similar conclusions based on palaeohistological evidence (Arcucci et al, 2019). Nevertheless, it is still striking the peculiarities of the proterochampsid body plan, mainly of cranial character states previously related to a semiaquatic behaviour.…”

“…Only recently, the supposed semiaquatic behaviour of proterochampsids has been started to be tested using other lines of evidence. Arcucci et al (2019) concluded that ecomorphological features, including histological traits (bone compactness), did not provide definitive evidence for the lifestyles of proterochampsids, but a terrestrial/amphibious condition was likely.…”

Osteology and relationships of Revueltosaurus callenderi (Archosauria: Suchia) from the Upper Triassic (Norian) Chinle Formation of Petrified Forest National Park, Arizona, United States

“…This condition is similar to what is observed in previously sampled aetosaurs and phytosaurian archosauriforms, though these larger‐bodied animals show slightly higher vascularity (de Ricqlès et al, 2003). In contrast, most sampled early archosauriforms and paracrocodylomorph archosaurs show a significantly greater degree of vascularity and tissue disorganization (e.g., woven bone tissue), suggesting relatively faster growth rates in those taxa (Arcucci et al, 2019; Botha‐Brink & Smith, 2011; de Ricqlès et al, 2003; de Ricqlès et al, 2008; Garcia Marsà et al, 2020; Klein et al, 2017; Nesbitt, 2007; Ponce et al, 2017; Schachner et al, 2020). Indeed, annual bone deposition rates at the time of fastest growth preserved in this section (the time represented in the zones between LAGs 1 and 2, and 2 and 3) were ~ 21.9 and 21.5 mm 2 /year, respectively.…”

Evolution of bone cortical compactness in slow arboreal mammals

“…Analyzing the shaft of long bones’ (diaphysis) microstructure has successfully been employed to infer paleobiological and ecological aspects of extinct tetrapods (e.g., Germain and Laurin 2005; Arcucci et al. 2019; Bazzana et al. 2020; Chinsamy et al.…”