Abstract
Ornithopods were key herbivorous dinosaurs in Mesozoic terrestrial ecosystems, with a variety of tooth morphologies. Several clades, especially the ‘duck-billed’ hadrosaurids, became hugely diverse and abundant almost worldwide. Yet their evolutionary dynamics have been disputed, particularly whether they diversified in response to events in plant evolution. Here we focus on their remarkable dietary adaptations, using tooth and jaw characters to examine changes in dental disparity and evolutionary rate. Ornithopods explored different areas of dental morphospace throughout their evolution, showing a long-term expansion. There were four major evolutionary rate increases, the first among basal iguanodontians in the Middle-Late Jurassic, and the three others among the Hadrosauridae, above and below the split of their two major clades, in the middle of the Late Cretaceous. These evolutionary bursts do not correspond to times of plant diversification, including the radiation of the flowering plants, and suggest that dental innovation rather than coevolution with major plant clades was a major driver in ornithopod evolution.
Highlights
Ornithopods were key herbivorous dinosaurs in Mesozoic terrestrial ecosystems, with a variety of tooth morphologies
The most basal ornithopods, exemplified by Orodromeus mackelai[8] and jeholosaurids like Haya griva[9] and Changchunsaurus parvus[10] have relatively low tooth counts, ranging from 14–17 alveolar positions. These early ornithopods have low tooth crowns that are labiolingually compressed, basally constricted, bulbous to subtriangular in shape, and ornamented with numerous apicobasal ridges
The remarkably tight clumping of all hadrosaurids in morphospace (Fig. 2c) suggests that, once their extreme dental adaptations had emerged, little changed, and that their dietary specialisation was successful over wide geographic areas and through time
Summary
Ornithopods were key herbivorous dinosaurs in Mesozoic terrestrial ecosystems, with a variety of tooth morphologies. The most basal ornithopods, exemplified by Orodromeus mackelai[8] and jeholosaurids like Haya griva[9] and Changchunsaurus parvus[10] have relatively low tooth counts, ranging from 14–17 alveolar positions These early ornithopods have low tooth crowns that are labiolingually compressed, basally constricted, bulbous to subtriangular in shape, and ornamented with numerous apicobasal ridges. The most basal iguanodontians, such as rhabdodontids[5,11], have low tooth crowns, but these become larger and more elongate, acquiring a more lanceolate profile while preserving numerous ridges and showing a prominent median carina. Hadrosaurid teeth are characterised by the reduction of ridges to a single median carina, accompanied in the hollow-crested lambeosaurines by one or two subsidiary ridges, further elongation of lanceolate tooth crowns that were at least twice as tall as wide, absence or decrease in the size of marginal denticles, tooth miniaturisation, and a substantial increase in the number of alveolar positions, reaching the extreme of nearly 60 positions in Edmontosaurini[19], and increase in the width of the occlusal surface to a maximum of three teeth arranged labiolingually[6,20,21,22,23]
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