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.
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). The majority of archosaurs were land dwelling, and it is well known that many important innovations related to terrestrial locomotion evolved throughout archosaur history (Bakker 1971; Charig 1972; Parrish 1986; Hutchinson and Gatesy 2000; Hutchinson 2006; Sullivan 2015). Terrestrial locomotor evolution in archosaurs is also characterized by repeated instances of morphological and functional convergence, involving members of both the ornithodiran (bird-line) and pseudosuchian (crocodile-line) lineages (Walker 1964; Parrish 1986; Sereno 1991; Carrano 2000; Nesbitt and Norell 2006; Bates and Schachner 2012; Bates et al. 2012; Kubo and Kubo 2012; Maidment and Barrett 2012; Grinham et al. 2019).
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. Yet extant archosaur species occupy a very select subset of the total range of observed archosaur postcranial morphologies, and concomitantly exhibit disparate locomotor patterns that almost certainly do not capture the full range of historical diversity in archosaur locomotor behavior (Hutchinson and Gatesy 2000; Hutchinson 2006). There is no archosaur—indeed, any animal—alive today that is of a comparable size and shape to extinct taxa like Postosuchus or Iguanodon. Although extant species may be highly useful for the development of “lower-level inferences” such as soft tissue anatomy, their utility can become increasingly uncertain for “higher-level inferences” such as locomotor function and behavior (Witmer 1995; Hutchinson 2006; Bates and Falkingham 2018). Thus, the usefulness of extant archosaur species as a basis for deriving broad inferences (grounded in homology or analogy) of locomotion for all extinct archosaurs is uncertain. Understanding locomotor habit in extinct archosaurs has therefore frequently relied on interpreting bony anatomy or footprints preserved in the fossil record. For instance, hoof-shaped manual unguals suggest at least some use of a quadrupedal stance (Maidment and Barrett 2014), and narrow-gauge footprints left by a single pair of feet may indicate at least intermittent parasagittal bipedalism (Thulborn 1990). 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 then footprints can rarely be assigned to a known species), and perhaps more critically, they often cannot be