Abstract
A number of extant and extinct archosaurs evolved an elongate, narrow rostrum. This longirostrine condition has been associated with a diet comprising a higher proportion of fish and smaller prey items compared to taxa with broader, more robust snouts. The evolution of longirostrine morphology and a bulbous anterior rosette of premaxillary teeth also occurs in the spinosaurid theropod dinosaurs, leading to suggestions that at least some members of this clade also had a diet comprising a notable proportion of fish or other small vertebrates. Here we compare the rostral biomechanics of the spinosaurs Baryonyx walkeri and Spinosaurus c.f. S. aegyptiacus to three extant crocodilians: two longistrine taxa, the African slender-snouted crocodile Mecistops cataphractus and the Indian gharial Gavialis gangeticus; and the American alligator Alligator mississippiensis.Using computed tomography (CT) data, the second moments of area and moments of inertia at successive transverse slices along the rostrum were calculated for each of the species. Size-independent results tested the biomechanical benefits of material distribution within the rostra. The two spinosaur rostra were both digitally reconstructed from CT data and compared against all three crocodilians. Results show that African slender-snouted crocodile skulls are more resistant to bending than an equivalent sized gharial. The alligator has the highest resistances to bending and torsion of the crocodiles for its size and greater than that of the spinosaurs. The spinosaur rostra possess similar resistance to bending and torsion despite their different morphologies. When size is accounted for, B. walkeri performs mechanically differently from the gharial, contradicting previous studies whereas Spinosaurus does not. Biomechanical data support known feeding ecology for both African slender-snouted crocodile and alligator, and suggest that the spinosaurs were not obligate piscivores with diet being determined by individual animal size.
Highlights
Extant crocodilian rostral morphology has often been used as an indicator of feeding ecology due to a link between head-shape and prey type or feeding behaviour [1,2,3,4,5,6,7]
The results showed that torsional stress was significantly higher than bending stress in the theropod and alligator, but there was no significant difference in the gharial and B. walkeri [33]
Crocodilians Resistance to dorsoventral bending (Ix) for raw (Table S1) and size-corrected (Table S2) data shows the same trends for all species; all taxa show a minor peak at slice 4 (16% of the rostra) and a slight increase in Ix values towards the posterior of the rostrum (Figures 6a and 6b)
Summary
Extant crocodilian rostral morphology has often been used as an indicator of feeding ecology due to a link between head-shape and prey type or feeding behaviour [1,2,3,4,5,6,7]. Large, flattened skull morphologies tend to utilise lunge/ambush methods to capture food, with ‘death roll’ inertial feeding being used to break down terrestrial prey whilst narrower rostra often using slashing behaviours to capture fish [1,2,3,4,5,6,7,8,9,10,11] Testing these correlations biomechanically has become important in attempting to understand extant crocodilians, and reptilian feeding ecology in general [12,13,14,15,16,17]. Crocodilians appear to have evolved a snout that was less tolerant to feeding related loads but potentially more suited to specialised hunting methods such as ambush [9] and hydrodynamic efficiency [5,6]
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