Walking—and Running and Jumping—with Dinosaurs and their Cousins, Viewed Through the Lens of Evolutionary Biomechanics

Abstract

Archosauria diversified throughout the Triassic Period before experiencing two mass extinctions near its end ∼201 Mya, leaving only the crocodile-lineage (Crocodylomorpha) and bird-lineage (Dinosauria) as survivors; along with the pterosaurian flying reptiles. About 50 years ago, the “locomotor superiority hypothesis” (LSH) proposed that dinosaurs ultimately dominated by the Early Jurassic Period because their locomotion was superior to other archosaurs’. This idea has been debated continuously since, with taxonomic and morphological analyses suggesting dinosaurs were “lucky” rather than surviving due to being biologically superior. However, the LSH has never been tested biomechanically. Here we present integration of experimental data from locomotion in extant archosaurs with inverse and predictive simulations of the same behaviours using musculoskeletal models, showing that we can reliably predict how extant archosaurs walk, run and jump. These simulations have been guiding predictive simulations of extinct archosaurs to estimate how they moved, and we show our progress in that endeavour. The musculoskeletal models used in these simulations can also be used for simpler analyses of form and function such as muscle moment arms, which inform us about more basic biomechanical similarities and differences between archosaurs. Placing all these data into an evolutionary and biomechanical context, we take a fresh look at the LSH as part of a critical review of competing hypotheses for why dinosaurs (and a few other archosaur clades) survived the Late Triassic extinctions. Early dinosaurs had some quantifiable differences in locomotor function and performance vs. some other archosaurs, but other derived dinosaurian features (e.g., metabolic or growth rates, ventilatory abilities) are not necessarily mutually exclusive from the LSH; or maybe even an opportunistic replacement hypothesis; in explaining dinosaurs’ success.

Quantitative biomechanical assessment of locomotor capabilities of the stem archosaur Euparkeria capensis

“…More specifically, our analysis has allowed for the testing of the potential locomotor behaviour of an extinct stem archosaur, pointing towards an answer to the controversy over how Euparkeria moved. Morphological changes in the pelvis and hindlimb of later pseudosuchians and ornithodirans facilitated a parasagittal gait, bipedalism and digitigrade foot orientation to independently evolve at some points [ 10 , 26 , 27 , 29 ]. Ancestral archosaurs, therefore, did not exhibit bipedalism ( figure 4 ).…”

“…Contrastingly, other studies have instead classified Euparkeria as a habitual quadruped [ 10 , 30 ] with a relatively sprawling posture based on skeletal forelimb [ 41 ], hindlimb and ankle morphology [ 33 , 42 ], or alternatively with a more erect limb posture due to the similarity of the hindlimb bones and joints with those of crocodylians [ 21 , 22 , 43 ] or quantitative and functional analyses of the hindlimbs [ 26 ]. While joint mobility of its hindlimbs [ 26 ] or its muscle leverage in a comparative context [ 27 ] has been investigated, the locomotor performance of Euparkeria remains to be quantitatively tested.

Figure 1 Euparkeria model.

…”

“…( a ) Skeletal reconstruction of Euparkeria capensis , modified from Cuff et al . [ 27 ]. ( b ) Articulated digital skeleton of Euparkeria and hull and cavity models used for centre of mass (COM) (crossed circle) and inertia calculations.…”

“…The abundant fossil record of archosaurs facilitates detailed studies of their morphological and locomotor evolution [2,21,[31][32][33]. However, the exact timing and frequency of the emergence of a more upright gait and bipedalism remain contentious [2,10,21,[25][26][27]31,34,35].…”

Anatomically grounded estimation of hindlimb muscle sizes in Archosauria

“…Archosaurian reptiles represent a lineage with a remarkable disparity of their appendicular musculoskeletal system that has long fascinated scientists (e.g. Alexander, 1985 ; Charig, 1972 ; Cuff et al, 2019 , 2022 ; Dilkes, 1999 ; Gatesy, 1990 ; Gatesy & Dial, 1996 ; Hutchinson, 2001 ; Hutchinson & Gatesy, 2000 ; Maidment & Barrett, 2011 , 2012 ; Moore et al, 2022 ; Rhodes et al, 2021 ; Romer, 1923a , 1923b , 1923c ; Walker, 1977 ; Wiseman et al, 2021 ). The ancestral condition for Archosauria was quadrupedal with enlarged pelvic bones and possibly a more erect (less sprawling posture), parasagittal gait (Charig, 1972 ; Gatesy, 1990 ; Sereno, 1991 ), powered by large caudofemoral tail muscles attaching to a fourth trochanter on the femur (Dollo, 1883 ; Gatesy, 1995 ; Persons & Currie, 2011 ; Romer, 1923a , 1923b , 1927 ).…”