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.
Highlights
Bipedal locomotion is one of the defining characteristics of humans and birds—some of the most widely distributed vertebrate species alive today—as well as many ricochetal mammals
The Ezcurra tree is focused primarily upon early archosauriforms. In this tree, facultative biped (FB) is only ever recovered evolving from an obligate quadruped (OQ) ancestor
Only a single complete evolutionary transition via a facultative locomotor mode from an obligate quadrupedal one exists, this result is not recovered in our sensitivity analysis
Summary
Bipedal locomotion is one of the defining characteristics of humans and birds—some of the most widely distributed vertebrate species alive today—as well as many ricochetal mammals. For example, it has been proposed that a shift toward 2 savannah-like aridity encouraged tree-dwelling populations of hominins on to the ground, where bipedal locomotion was demonstrably more energetically efficient for moving between increasingly distant arboreal habitats [1,2]. The most obvious advantage of using only hindlimbs to locomote is the freeing of forelimbs for use in functions other than those associated with support and locomotion. These functions can be broadly categorized as social use (communication, combat), micro-mechanical use (tool and object manipulation) and macro-mechanical use (flight, or environmental manipulation such as digging). There is a limited understanding of how bipedality evolved in non-human and non-avian animals
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