Dinosaurs dominated terrestrial environments for over 100 million years due in part to innovative feeding strategies. Although a range of dental adaptations was present in Late Jurassic dinosaurs, it is unclear whether dinosaur ecosystems exhibited patterns of tooth disparity and dietary correlation similar to those of modern amniotes, in which carnivores possess simple teeth and herbivores exhibit complex dentitions. To investigate these patterns, we quantified dental shape in Late Jurassic dinosaurs to test relationships between diet and dental complexity.
Here, we show that Late Jurassic dinosaurs exhibited a disparity of dental complexities on par with those of modern saurians. Theropods possess relatively simple teeth, in spite of the range of morphologies tested, and is consistent with their inferred carnivorous habits. Ornithischians, in contrast, have complex dentitions, corresponding to herbivorous habits. The dentitions of macronarian sauropods are similar to some ornithischians and living herbivorous squamates but slightly more complex than other sauropods. In particular, all diplodocoid sauropods investigated possess remarkably simple teeth. The existence of simple teeth in diplodocoids, however, contrasts with the pattern observed in nearly all known herbivores (living or extinct).
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Sauropod dinosaurs exhibit a novel approach to herbivory not yet observed in other amniotes. We demonstrate that sauropod tooth complexity is related to tooth replacement rate rather than diet, which contrasts with the results from mammals and saurians. This relationship is unique to the sauropod clade, with ornithischians and theropods displaying the patterns observed in other groups. The decoupling of herbivory and tooth complexity paired with a correlation between complexity and replacement rate demonstrates a novel evolutionary strategy for plant consumption in sauropod dinosaurs.
The Late Jurassic represents an excellent system for testing hypotheses regarding dinosaur dentition. The paleoenvironment and taxic diversity of both fauna and flora are relatively well known, in part due to an extensive history of worldwide collection, but especially in that of western North America [30]. During this epoch, dinosaurs attained a remarkable diversity, with theropods, sauropods, and ornithischians exhibiting a wide array of taxa and ecologies. In particular, the herbivorous sauropods achieved colossal sizes and an exceptional diversity, with many coexisting diplodocoids and macronarians. In spite of broad similarities in the sauropod bauplan, these two clades are characterized by marked morphological differences in both cranial and postcranial elements. Importantly, these skeletons sometimes include nearly complete cranial material with associated dental elements, which reveal a diverse range of tooth shapes, especially amongst herbivores [19, 31,32,33,34] (Fig. 1). The combination of skeletal and dental microwear differences strongly suggest a partitioning of plant resources. The abundance of dentigerous cranial material has also enabled the estimation of tooth replacement rates across the sauropod and theropod clade [19, 20, 22, 35]. These data, combined with the taxonomic diversity and range of dental morphologies, present an exceptional opportunity to test ecosystem-wide patterns during a relatively narrow time span.
Here, we seek to improve our understanding of dinosaur dietary ecology by testing two hypotheses using quantitative morphological assessments. First, we evaluate the relationship between diet and dental complexity in Late Jurassic dinosaurs using the OPCR method, specifically testing whether carnivores possessed simple teeth and herbivores exhibited complex dentitions. Second, we test the hypothesis that dental complexity values are related to tooth replacement rate. Our results shed light on key evolutionary innovations that allowed multiple herbivorous dinosaur clades to diversify and coexist.
Late Jurassic dinosaurs exhibit a wide range of dental complexities (Fig. 1). Average tooth complexity varies between 9.03 and 33.75 patches per tooth (PPT) when measured with a minimum polygon count threshold of three triangles (3 patch) and a model size of 1000 triangles (Figs. 2 and 3; Additional file 1, Additional file 1: Table S2). At a minimum polygon count threshold of five triangles (5 patch) and a model size of 1000 triangles, dental complexity patterns are consistent, but dental disparity decreases, with a range from 8.60 to 24 patches (Additional file 1: Table S2). This range is similar to those of extant saurians when measured using the same procedures despite differences in gross surface morphology [36].
Average dental complexity and phylogenetic relationships of Late Jurassic dinosaurs. Ornithischian herbivores (orange, left) typically have higher dental complexities (3 patch analyses) than both theropods (purple, centre) and sauropods (blue, right). Herbivorous diplodocoid sauropods exhibit tooth complexities similar to theropod carnivorous theropods, which contrasts with patterns observed in extant and extinct herbivores. Illustrations of dental complexity in occlusal view are shown below OPCR values. D dentary, PM premaxilla and maxilla. Silhouettes courtesy of S. Abramowicz and phylopic.org
The seven measured sauropod genera display a greater range of dental complexities than those of theropods (Figs. 2 and 4; Additional file 1, Additional file 1: Table S2). The distribution of sauropod dental complexities is somewhat bimodal, with the broad-crowned teeth of macronarians exhibiting higher complexities than the narrow-crowned teeth of diplodocoids. The lowest value measured for a Late Jurassic dinosaur occurs in Tornieria (8 patches), a diplodocid from Tanzania [37]. Other diplodocids (e.g. Apatosaurus, Diplodocus) also possess relatively simple teeth, between 9 and 11 PPT (3 patch) with dental models of 1000 triangles (Figs. 2, 4 and 5). These sauropods are characterized by narrow-crowned, peg-like teeth. The two macronarians included in this study, Brachiosaurus and Camarasaurus, exhibit the highest average dental complexities of our sauropod sample, 14.39 and 18.38 PPT (3 patch), respectively (Figs. 2 and 5). The teeth of these dinosaurs are characterized by a convex labial margin, concave lingual margin, and rugose wrinkled enamel [18]. In both groups, tooth wear, including the development of large facets or the loss of enamel wrinkles, does not significantly impact dental complexity.
Violin plot illustrating the range in dental complexity of herbivorous dinosaurs. The width of each bar represents the relative proportion of teeth with a particular complexity and the median is designated by a horizontal line. Sauropods have a much lower range in complexities compared to ornithischians, the latter of which are frequently characterized by a heterodont dentition. All analyses were performed at models with 1000 triangles and a minimum patch count of three polygons
Ornithischians possess the most complex teeth of sampled Late Jurassic dinosaurs (Figs. 1, 2 and 4; Additional file 1). Nanosaurus is characterized by labiolingually compressed teeth with up to ten cusps (Additional file 1: Fig. S1C) and exhibits an average complexity of 33.75 PPT (3 patch) at a model size of 1,000 triangles. Fruitadens, one of the smallest known non-avian dinosaurs [38], possesses an average dental complexity of 23.63 PPT at the same resolution. Unworn teeth of Fruitadens can reach up to 36.5 PPT at the same parameters, rivalling the complexities of herbivorous iguanids [36]. Other ornithischians have lower complexity values. Camptosaurus has an average complexity of 20.15 PPT (3 patch) when measured with a model of 1000 triangles. Gargoyleosaurus, an early ankylosaur, possesses simpler teeth, with a complexity value of 15.68 PPT when measured at the same resolution. Similar to Fruitadens, erupted and worn teeth are simpler than unworn dentitions. In one Camptosaurus specimen (MWC 2), noteworthy ridges and mesiodistal cusps are worn away, simplifying dental morphology and complexity values of this specimen, although wear facets of UMNH VP 16455 do not result in simplified complexities. This simplification contrasts with sauropods, in which tooth wear generally does not dramatically alter dental morphology and complexity.
Here, we demonstrate a unique relationship between phenotypic tooth complexity and dietary ecology in Late Jurassic dinosaurs. Because independent evidence of general feeding ecology (e.g. herbivory, carnivory) is known for a variety of dinosaur taxa, diets of these clades can be reconstructed with relative confidence allowing for the testing of relationships between diet and dental complexity. In measured ornithischians and theropods, the previously recovered pattern is maintained; carnivorous theropods have simple teeth, whereas herbivorous ornithischians display the most complex dentitions in sampled dinosaurs (Figs. 1, 2, 4 and 5). Ornithischians, like extant herbivorous saurians [29], express the greatest range of complexities (Fig. 4), reflecting their heterodont condition. In both living saurians and mammals, there is a positive relationship between dental complexity and the amount of plant matter consumed [27, 29, 41]. Hypercarnivores and carnivores (i.e. animals that primarily consume animal material but also small amounts of plants) possess simple teeth (i.e. low dental complexity). Herbivores exhibit the highest dental complexity values, reflecting additional cusps, crenulations, and grinding surfaces that enlarge the dental surface area and help break down plant material prior to chemical digestion. Herbivorous sauropods, however, complicate this otherwise straightforward pattern by possessing relatively simple dentitions. In fact, with their extremely simple teeth, diplodocoid sauropods appear to exhibit a novel strategy of plant consumption (Figs. 2, 5 and 6). Therefore, our hypothesis that dinosaurs that lived during the Late Jurassic exhibit modern patterns of dental complexity and diet is falsified, at least for the diplodocoids included in this study. 2ff7e9595c
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