Locomotion and the early Mesozoic success of Archosauromorpha

Abstract

The Triassic was a time of ecological upheaval as life recovered from the Permian-Triassic mass extinction. Archosauromorphs were a key component of the recovery, diversifying substantially during the Triassic and encompassing the origins of dinosaurs, pterosaurs and crocodylomorphs. Here, we explore the evolution of locomotion in Archosauromorpha to test whether dinosaurs show any distinctive locomotory features that might explain their success. We implement geometric morphometrics on limb bone shapes and use limb ratios to calculate bipedality and cursoriality metrics. We find that the Avemetatarsalia (dinosaurs, pterosaurs and relatives) exhibit more variable limb form and limb ratios than any other group, indicating a wider range of locomotory modes. The earliest avemetatarsalians were bipedal and cursorial, and their range of form increased through the Triassic with notable diversification shifts following extinction events. This is especially true of dinosaurs, even though these changes cannot be discriminated from a stochastic process. By contrast, the Pseudosuchia (crocodilians and relatives) were more restricted in limb form and locomotor mode with disparity decreasing through time, suggesting more limited locomotor adaptation and vulnerability to extinction. Perhaps the greater locomotor plasticity of dinosaurs gave them a competitive advantage in the changing climates of the Late Triassic.

Keywords: archosauromorphs; archosaurs; avemetatarsalians; dinosaurs; diversity; pseudosuchians.

Figure 1.

Morphospace occupation for the shape outline of the femur for four groups of archosauromorphs, non-archosaurian archosauromorphs (Rhynchosauria, Prolacertiformes, Trilophosauridae), Avemetatarsalia (excluding Dinosauria), Dinosauria, and Pseudosuchia. Shape of femora at extremes of the PC axes are displayed as silhouettes.

Figure 2.

Morphospace occupation for the shape outline of the tibia for four groups of archosauromorphs, non-archosaurian archosauromorphs (Rhynchosauria, Prolacertiformes, Trilophosauridae), Avemetatarsalia (excluding Dinosauria), Dinosauria, and Pseudosuchia. Shape of tibia at extremes of the PC axes are displayed as silhouettes.

Figure 3.

Morphospace occupation for the shape outline of the humerus for four groups of archosauromorphs, non-archosaurian archosauromorphs (Rhynchosauria, Prolacertiformes, Trilophosauridae), Avemetatarsalia (excluding Dinosauria), Dinosauria, and Pseudosuchia. Shape of humeri at extremes of the PC axes are displayed as silhouettes.

Figure 4.

Change in morphospace occupation for each limb element (femur, tibia and humerus) and for each taxonomic group (non-archosaurian archosauromorphs, Avemetatarsalia, Dinosauria and Pseudosuchia) through the Late Permian to Early Jurassic interval. The Permian-Triassic mass extinction (PTME), Carnian Pluvial Event (CPE) and end-Triassic mass extinction (ETME) are indicated on the time-scale. Time slices are presented in standard orientation in the electronic supplementary material.

Figure 5.

Disparity of limb bone shape for Dinosauria (purple) and Pseudosuchia (blue) measured as sum of variances (SoV) for femur (a), tibia (b), and humerus (c). Owing to small sample sizes, the timespan is Middle Triassic to Early Jurassic, and time bins are large. The Carnian Pluvial Event (CPE) and end-Triassic mass extinction (ETME) are indicated.

Figure 6.

Distribution of quadrupedality index (a) and cursoriality index (b) for the four taxonomic groups, shown as violin plots of the whole samples for each group. Flying and fully aquatic taxa were removed from these analyses.

Figure 7.

Distribution of quadrupedality index (a,c) and cursoriality index (b,d) for Dinosauria (a,b) and Pseudosuchia (c,d) across time. Each point represents measurements for a single species, with the spot size scaled according to the log₁₀ femur size as a proxy for overall body size. Flying and fully aquatic taxa were removed from these analyses. The Carnian Pluvial Event (CPE) and end-Triassic mass extinction (ETME) are indicated.

Figure 8.

Rates of cursorial evolution of early archosauromorphs based on cal3 time-scaled trees, with four major taxonomic groups indicated by colour: non-archosaurian archosauromorphs (red), Avemetatarsalia (excluding Dinosauria, dark blue), Dinosauria (purple), and Pseudosuchia (green). (a) Phylogeny plotted against time, and highlighting evolutionary rate from slow (blue) to fast (red, orange). (b) Cursoriality phenogram with branches colour-coded according to evolutionary rates. Body size of external and (reconstructed) internal nodes indicated by circle areas. The Permian-Triassic mass extinction (PTME), Carnian Pluvial Event (CPE) and end-Triassic mass extinction (ETME) are indicated on the timescale.