Abstract
BackgroundThe most primitive ornithischian dinosaurs were small bipeds, but quadrupedality evolved three times independently in the clade. The transition to quadrupedality from bipedal ancestors is rare in the history of terrestrial vertebrate evolution, and extant analogues do not exist. Constraints imposed on quadrupedal ornithischians by their ancestral bipedal bauplan remain unexplored, and consequently, debate continues about their stance and gait. For example, it has been proposed that some ornithischians could run, while others consider that none were cursorial.Methodology/Principal FindingsDrawing on biomechanical concepts of limb bone scaling and locomotor theory developed for extant taxa, we use the largest dataset of ornithischian postcranial measurements so far compiled to examine stance and gait in quadrupedal ornithischians. Differences in femoral midshaft eccentricity in hadrosaurs and ceratopsids may indicate that hadrosaurs placed their feet on the midline during locomotion, while ceratopsids placed their feet more laterally, under the hips. More robust humeri in the largest ceratopsids relative to smaller taxa may be due to positive allometry in skull size with body mass in ceratopsids, while slender humeri in the largest stegosaurs may be the result of differences in dermal armor distribution within the clade. Hadrosaurs are found to display the most cursorial morphologies of the quadrupedal ornithischian cades, indicating higher locomotor performance than in ceratopsids and thyreophorans.Conclusions/SignificanceLimb bone scaling indicates that a previously unrealised diversity of stances and gaits were employed by quadrupedal ornithischians despite apparent convergence in limb morphology. Grouping quadrupedal ornithischians together as a single functional group hides this disparity. Differences in limb proportions and scaling are likely due to the possession of display structures such as horns, frills and dermal armor that may have affected the center of mass of the animal, and differences in locomotor behaviour such as migration, predator escape or home range size.
Highlights
IntroductionArising in the Late Triassic [1], primitive ornithischians were small (around 1.5 m long; e.g., [2,3]) and bipedal, but during their 170 million year evolutionary history the clade diversified into a wide range of body shapes and sizes and quadrupedality evolved within at least three lineages independently: once in the armored stegosaurs and ankylosaurs, once in the frilled ceratopsids, and once in the duck-billed hadrosaurs (Fig. 1)
Ornithischia is a monophyletic clade of mainly herbivorous dinosaurs that dominated terrestrial ecosystems for much of the Mesozoic
Allometric coefficients and p values for both Reduced major axis (RMA) regressions and Independent Contrasts are shown in Tables 1, 2, 3, 4
Summary
Arising in the Late Triassic [1], primitive ornithischians were small (around 1.5 m long; e.g., [2,3]) and bipedal, but during their 170 million year evolutionary history the clade diversified into a wide range of body shapes and sizes and quadrupedality evolved within at least three lineages independently: once in the armored stegosaurs and ankylosaurs, once in the frilled ceratopsids, and once in the duck-billed hadrosaurs (Fig. 1). Despite over years of debate regarding the stance and gait of ornithischian dinosaurs, little consensus has been reached. The most primitive ornithischian dinosaurs were small bipeds, but quadrupedality evolved three times independently in the clade. Constraints imposed on quadrupedal ornithischians by their ancestral bipedal bauplan remain unexplored, and debate continues about their stance and gait. It has been proposed that some ornithischians could run, while others consider that none were cursorial
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