Abstract
AbstractThroughout 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-lineBatrachotomus(predicted quadruped) andPostosuchus(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 archosaurianBauplanin bipedal versus quadrupedal groups, which may have shaped the evolutionary histories of their respective members.
Highlights
Archosauria is a diverse, long-lived (∼250 Myr) saurian clade that dominated terrestrial, aquatic, and aerial ecosystems throughout the Mesozoic Era, and persists to the present day in the form of about 23 crocodylian and 10,000 bird species (Oaks 2011; Jetz et al 2012)
The insights afforded by these lines of evidence can be important, but they are limited by the incompleteness of the fossil record, and perhaps more critically, they often cannot be generalized across large clades of disparate body plans
Comparison of the principal component (PC) scores between the two trees using symmetric Procrustes superimposition in the vegan package showed that the score sets were not significantly different
Summary
Archosauria (the “ruling reptiles”) is a diverse, long-lived (∼250 Myr) saurian clade that dominated terrestrial, aquatic, and aerial ecosystems throughout the Mesozoic Era, and persists to the present day in the form of about 23 crocodylian and 10,000 bird species (Oaks 2011; Jetz et al 2012) Throughout their history, archosaurs have displayed disparate body plans and divergent locomotor habits, including obligate bipedal and quadrupedal species, as well as volant, semiaquatic, and marine forms (Fig. 1). Given their long evolutionary history and diverse array of body sizes and shapes, terrestrial archosaurs provide a good case study for investigating how size and gross body proportions may relate to each other and locomotor behavior. The insights afforded by these lines of evidence can be important, but they are limited by the incompleteness of the fossil record (i.e., such evidence is only preserved for some species, and even footprints can rarely be assigned to a known species), and perhaps more critically, they often cannot be generalized across large clades of disparate body plans
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