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

Abstract

Birds and crocodylians are the only remaining members of Archosauria (ruling reptiles) and they exhibit major differences in posture and gait, which are polar opposites in terms of locomotor strategies. Their broader lineages (Avemetatarsalia and Pseudosuchia) evolved a multitude of locomotor modes in the Triassic and Jurassic periods, including several occurrences of bipedalism. The exact timings and frequencies of bipedal origins within archosaurs, and thus their ancestral capabilities, are contentious. It is often suggested that archosaurs ancestrally exhibited some form of bipedalism. Euparkeria capensis is a central taxon for the investigation of locomotion in archosaurs due to its phylogenetic position and intermediate skeletal morphology, and is argued to be representative of facultative bipedalism in this group. However, no studies to date have biomechanically tested if bipedality was feasible in Eupakeria. Here, we use musculoskeletal models and static simulations in its hindlimb to test the influences of body posture and muscle parameter estimation methods on locomotor potential. Our analyses show that the resulting negative pitching moments around the centre of mass were prohibitive to sustainable bipedality. We conclude that it is unlikely that Euparkeria was facultatively bipedal, and was probably quadrupedal, rendering the inference of ancestral bipedal abilities in Archosauria unlikely.

Keywords: Archosauria; bipedalism; locomotion; musculoskeletal modelling.

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. Modelled body posture and ground reaction force (GRF) vector in caudal view (c) and right lateral view (d). The body segments are shown in orange and the body cavities (lung, trachea, cranial sinus/pharynx) in blue; dots denote modelled joint centres. Note that the COM is cranial to both the hip joint and the GRF vector. r_x and r_z, respectively, represent the roll and pitch moment arms of the GRF vector F about the COM, producing the COM roll and pitch moments τ_x and τ_z. COM, centre of mass; COP, centre of pressure; BA, body angle; TA, tail angle. Angle deviations in the body and tail were measured from horizontal. All drawings to scale, scale bar = 5 cm.

Figure 2.

Pitching moment around the COM of Euparkeria during early stance and asymmetrical GRF. The black silhouettes qualitatively describe the body orientations of Euparkeria at the extreme points of the simulations. Body and tail postures of six extant lizards during bipedal locomotion are illustrated as circles and the posture space they occupy is indicated by the white transparent convex hull. Positive pitching moments (nose-up) are red, while negative pitching moments (nose-down) are blue. Note that bipedal poses similar to extant lizards resulted in a negative pitching moment around the COM for Euparkeria. Crosses denote simulation results; the values in between were interpolated at 1° intervals. The top and side plots display the range and average of pitching moments in relation to the body and tail angles, respectively.

Figure 3.

Mean activation (a) and its corresponding standard deviation (b) of the Euparkeria muscles across all models and simulations at their maximally sustainable GRF. Tail angle had no influence on muscle activation. Mean muscle activation was calculated at 0°, 25°, 50° and 75°; the values in between were interpolated at 1° intervals. The distal hindlimb muscle homologies follow Hattori & Tsuihiji [85]. See electronic supplementary material, figure S4 for the muscle activation of the individual models. IF, M. iliofemoralis; ADD 1–2, M. adductor femoris 1 + 2; PIFE1–3, M. puboischiofemoralis externus 1–3; PIT, M. puboischiotibialis; PIFI1–2, M. puboischiofemoralis internus 1–2; CFB, M. caudofemoralis brevis; CFL, M. caudofemoralis longus; ISTR, M. ischitrochantericus; AMB, M. ambiens; IT1–3, M. iliotibialis 1–3; FMTE, M. femorotibialis externus; FMTI, M. femorotibialis internus; FTE, M. flexor tibialis externus, FTI1–3, M. flexor tibialis internus 1–3; ILFB, M. iliofibularis; FDL, M. flexor digitorum longus; FHL, M. flexor hallucis longus; GL, M. gastrocnemius lateralis; GM, M. gastrocnemius medialis; PL, M. peroneus (fibularis) longus; PP, M. pronator profundus; EDL, M. extensor digitorum longus; PB, M. peroneus (fibularis) brevis; TC, M. tibialis cranialis; FDBP, M. flexor digitorum brevis profundus; FDBS, M. flexor digitorum brevis superficialis; AHD, M. adductor hallucis dorsalis; EDB, M. extensor digitorum brevis.

Figure 4.

Evolution of bipedalism within Archosauria. Ancestral state reconstruction for the time-calibrated ‘Nesbitt tree’ (a) and ‘Ezcurra tree’ (b); see methods for sources of tree topologies; see electronic supplementary material, figures S5–S8 for individual time-calibrated trees. Red circles indicate bipedalism and blue circles indicate quadrupedalism; black outlined circles in (b) represent the bold clades in (a). Clades: 1, Archosauriformes; 2, Eucrocopoda; 3, Archosauria; 4, Pseudosuchia; 5, Paracrocodylomorpha; 6, Poposauroidea; 7, Loricata; 8, Crocodylomorpha; 9, Avemetatarsalia; 10, Ornithodira; 11, Pterosauromorpha; 12, Lagerpetidae; 13, Dinosauromorpha; 14, Dinosauriformes; 15, Dracohors; 16, Dinosauria; 17, Ornithischia; 18, Neornithischia; 19, Saurischia; 20, Sauropodomorpha; 21, Massopoda; 22, Sauropodiformes; 23; Sauropoda; 24, Theropoda.