Evolutionary history of quadrupedal walking gaits shows mammalian release from locomotor constraint

Vertebrates employ an impressive range of strategies for coordinating
their limb movements while walking. Although this gait variation has been
quantified and hypotheses for its origins tested in select tetrapod
lineages, a comprehensive understanding of gait evolution in a
macroevolutionary context is currently lacking. We used freely-available
internet videos to nearly double the number of species with quantitative
gait data and used phylogenetic comparative methods to test key hypotheses
about gait origin and evolution. We find strong support for an ancestral
Lateral-Sequence Diagonal-Couplet gait in gnathostomes, and this mode is
remarkably conserved throughout tetrapod phylogeny. Evolutionary rate
analyses show that mammals overcame this ancestral constraint, resulting
in a greater range of gait values than any other tetrapod lineage.
Diagonal-Sequence Diagonal-Couplet gaits are significantly associated with
arboreality in mammals, though this relationship is not recovered for
other tetrapod lineages. Notably, the Lateral-Sequence Lateral-Couplet
gait, unique to mammals among extant tetrapods, is not associated with any
traditional explanations. The complex drivers of gait diversification in
mammals remain unclear, but our analyses suggest that their success was
due, in part, to released from a locomotor constraint that has likely
persisted in other extant tetrapod lineages for over 375 million years.