Relationships of mass properties and body proportions to locomotor habit in terrestrial Archosauria

Abstract

Throughout their 250 Myr history, archosaurian reptiles have exhibited a wide array of body sizes, shapes, and locomotor habits, especially in regard to terrestriality. These features make Archosauria a useful clade with which to study the interplay between body size, shape, and locomotor behavior, and how this interplay may have influenced locomotor evolution. Here, digital volumetric models of 80 taxa are used to explore how mass properties and body proportions relate to each other and locomotor posture in archosaurs. One-way, nonparametric, multivariate analysis of variance, based on the results of principal components analysis, shows that bipedal and quadrupedal archosaurs are largely distinguished from each other on the basis of just four anatomical parameters (p < 0.001): mass, center of mass position, and relative forelimb and hindlimb lengths. This facilitates the development of a quantitative predictive framework that can help assess gross locomotor posture in understudied or controversial taxa, such as the crocodile-line Batrachotomus (predicted quadruped) and Postosuchus (predicted biped). Compared with quadrupedal archosaurs, bipedal species tend to have relatively longer hindlimbs and a more caudally positioned whole-body center of mass, and collectively exhibit greater variance in forelimb lengths. These patterns are interpreted to reflect differing biomechanical constraints acting on the archosaurian Bauplan in bipedal versus quadrupedal groups, which may have shaped the evolutionary histories of their respective members.

Femoral specializations to locomotor habits in early archosauriforms

“…Bishop et al (2020) obtained similar results for Riojasuchus using estimated mass properties and relative hindlimb and forelimb lengths, but noted the controversial nature of this taxon's locomotor habit, whereas Grinham et al (2019) assumed Riojasuchus to be an obligate quadruped. However, Postosuchus was described, assumed, or estimated as an obligate biped in several recent studies (Bishop et al, 2020; Grinham et al, 2019; Weinbaum, 2013).…”

Musculoskeletal modelling of the Nile crocodile (Crocodylus niloticus) hindlimb: Effects of limb posture on leverage during terrestrial locomotion

“…Brainerd et al, 2010 ) and filtered using a low‐pass Butterworth filter of 10 Hz, and then exported as 3D coordinates. Anatomical and joint coordinate systems (ACS/JCS) were established using the shape‐fitting procedure outlined by Bishop et al, ( 2020 ), following the protocol designed by Kambic et al, ( 2014 ) for birds. Spherical shapes were fitted to each acetabulum and an ellipsoid to the femoral head (c.f.…”

“…Investigating the diversification of these musculoskeletal forms and estimating locomotor performance of fossil archosaurs has been a popular scientific subject for about a century (e.g. Allen et al, 2021 ; Bates & Schachner, 2012 ; Bishop et al, 2020 a; Bonaparte, 1984 ; Gatesy, 1990 ; Gauthier et al, 2011 ; Grinham et al, 2019 ; Hutchinson, 2006 ; Meers, 2003 ; Romer, 1923 ; Tsai et al, 2020 ), but has advanced considerably in recent years with the inception and improvement of software/hardware designed to quantify animal mechanics, thus providing detailed biomechanical models that were impossible 30 years ago. In addition to the computational modelling and simulation tools noted above, new advances have permitted researchers to see beneath the skin of animals to visualise and accurately quantify musculoskeletal function during a variety of motions (X‐ray Reconstruction of Moving Morphology or XROMM; Baier & Gatesy., 2013 ; Brainerd et al, 2010 ; Gatesy et al, 2010 ; Kambic et al, 2014 ; Tsai et al, 2020 ).…”

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“…Model track phase predictions were tested against gait and footfall data readily available for Belgian Malinois dogs (Maes et al, 2008), using the body proportions specified by Polet and Bertram (2019) and the MOI specified by Polet (2021b). Batrachotomus dimensions and inertial properties were determined from a volumetric model (Bishop et al, 2020). This in turn was based on the skeletal reconstruction at the Staatliches Museum für Naturkunde Stuttgart.…”

“…Nesbitt et al (2013) interpreted plantigrady and the extended calcaneal tuber as adaptation to high power rather than high speed. Apart from a consensus that Batrachotomus used parasagittal and erect locomotion, and was quadrupedal (Bishop et al, 2020), there is as yet no analysis on the kind(s) of gait it may have employed.…”