Testing for a facultative locomotor mode in the acquisition of archosaur bipedality

Abstract

Bipedal locomotion is a defining characteristic of humans and birds and has a profound effect on how these groups interact with their environment. Results from extensive hominin research indicate that there exists an intermediate stage in hominin evolution—facultative bipedality—between obligate quadrupedality and obligate bipedality that uses both forms of locomotion. It is assumed that archosaur locomotor evolution followed this sequence of functional and hence character-state evolution. However, this assumption has never been tested in a broad phylogenetic context. We test whether facultative bipedality is a transitionary state of locomotor mode evolution in the most recent early archosaur phylogenies using maximum-likelihood ancestral state reconstructions for the first time. Across a total of seven independent transitions from quadrupedality to a state of obligate bipedality, we find that facultative bipedality exists as an intermediary mode only once, despite being acquired a total of 14 times. We also report more independent acquisitions of obligate bipedality in archosaurs than previously hypothesized, suggesting that locomotor mode is more evolutionarily fluid than expected and more readily experimented with in these reptiles.

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

“…To assess the influence of phylogeny, the K statistic (and associated p- value) of Blomberg et al (2003) was also determined in phytools. In this study, the locomotor habit of each taxon was classified a priori based on current consensus (see also Thulborn 1990; Maidment and Barrett 2014; Grinham et al 2019 and references cited therein), as either obligate biped ( n = 33), obligate quadruped ( n = 34), or “other” ( n = 13), where the last category contained taxa hypothesized to be facultatively bipedal or taxa whose habits are controversial or previously not assessed in detail. Locomotor mode is here defined as the stance adopted during straight-line, level, quasi-steady walking.…”

“…Predicting Locomotion in Extinct Archosaurs.-A secondary aim of this study was to develop a quantitative framework that could be used to help predict locomotor habit in extinct terrestrial archosaurs, and that bipeds and quadrupeds were largely distinct in PC space has facilitated this. Although beyond the scope of the present study, this new framework has the potential to provide new rigor to assessments of locomotor evolution within Archosauria, such as the frequency and phyletic distribution of shifts from quadrupedalism to bipedalism (facultative and obligate; Kubo and Kubo 2012;McPhee et al 2018;Grinham et al 2019;Chapelle et al 2020). This would first require the generation of digital volumetric models for additional taxa, particularly those surrounding inferred transitions, which can be a nontrivial and time-intensive process.…”

“…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).

Figure 1.Simplified phylogeny of Archosauria based on current consensus of interrelationships (e.g., Nesbitt 2011; Ezcurra 2016), illustrating the diversity of body forms and locomotor habits across the group.

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The relationship between body shape, body size and locomotor mode in extant lepidosaurs

“…Our assessment of locomotor mode and body segment lengths relative to body size (in the form of SVL) is novel with respect to the existing literature surrounding reptilian facultative bipedality. Much of the existing work regarding facultative bipedality generally is based on the use of ratios which, whilst informative, does not account for the body size-related constraints of an animal (Snyder, 1962;Berman et al, 2000;Grinham, VanBuren & Norman, 2019). Updated locomotor classifications, such as for Tropicagama temporalis (Wilson & Swan, 2017) and Iguana iguana (Blob & Biewener, 2001), enable a more up-to-date assessment of the anatomy of facultative bipedality in extant squamates.…”

“…By exploring the anatomies and mechanical demands associated with this locomotor mode, we can better understand the evolution of facultative bipedality that is exhibited in multiple families throughout the clade Lepidosauria (Clemente et al, 2008). Beyond lepidosaurs, facultative bipedality is hypothesized to form an intermediate stage in the evolution of bipedality in both hominins and archosaurs (Preuschoft, 2004;Maslin, Shultz & Trauth, 2015;Grinham et al, 2019). We used this study to explore body metrics associated with facultative bipedality excluding the caudal vertebrae to increase the applicability of our results beyond the living record.…”