Abstract
Bipedal locomotion evolved along the archosaurian lineage to birds, shifting from "hip-based" to "knee-based" mechanisms. However, the roles of individual muscles in these changes and their evolutionary timings remain obscure. Using 13 three-dimensional musculoskeletal models of the hindlimbs of bird-line archosaurs, we quantify how the moment arms (i.e., leverages) of 35 locomotor muscles evolved. Our results support two hypotheses: From early theropod dinosaurs to birds, knee flexors' moment arms decreased relative to knee extensors', and medial long-axis rotator moment arms for the hip increased (trading off with decreased hip abductor moment arms). Our results reveal how, from the Triassic Period, bipedal theropod dinosaurs gradually modified their hindlimb form and function, shifting more from hip-based to knee-based locomotion and hip-abductor to hip-rotator balancing mechanisms inherited by birds. Yet, we also discover unexpected ancestral specializations in larger Jurassic theropods, lost later in the bird-line, complicating the paradigm of gradual transformation.
Highlights
The evolution of terrestrial locomotion in “ruling reptiles” (Archosauria) across the Mesozoic era is an important macroevolutionary event because of the major innovations in form and function during divergence from the original quadrupedal, possibly semisprawling ancestor, most prominent in dinosaurs (1, 2)
All quoted values are individual musculo- tendon unit (MTU) moment arms normalized by segment length, for ancestral nodes along the phylogeny (Fig. 1), given as mean ancestral character state estimates (ACEs) across the phylogeny from fastAnc reconstructions
Ninety-five percent confidence intervals are shown in Figs. 3 to 7, and their quantitative values are in the Supplementary Materials (“data_processing_files” folder: “[hip/knee/ankle]_[Ex/Ab/Ro]_ACE_data_from_dino_moment_arms_Sun_Jun_07_14–26-54_2020-MAC_2.csv”; total of five files)
Summary
The evolution of terrestrial locomotion in “ruling reptiles” (Archosauria) across the Mesozoic era is an important macroevolutionary event because of the major innovations in form and function during divergence from the original quadrupedal, possibly semisprawling ancestor, most prominent in dinosaurs (1, 2). Trackway evidence is consistent with an early shift in Triassic theropod dinosaurs to adopting another characteristic of locomotion in living birds: a walk-to-run gait transition that is continuous (involving a “grounded run” at intermediate speeds) rather than discrete (as in humans) (8). This evidence reinforces the idea that some key aspects of avian locomotion are very ancient, but the antiquity versus novelty of many specific musculotendinous mechanisms that produced general changes in archosaurian locomotion on the bird-line remains mysterious
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