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

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
behaviour, 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, non-parametric, 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,
centre 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 to quadrupedal archosaurs, bipedal
species tend to have relatively longer hindlimbs and a more caudally
positioned whole-body centre 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.