Quantitative analysis of dental microwear in hadrosaurid dinosaurs, and the implications for hypotheses of jaw mechanics and feeding

Abstract

Understanding the feeding mechanisms and diet of nonavian dinosaurs is fundamental to understanding the paleobiology of these taxa and their role in Mesozoic terrestrial ecosystems. Various methods, including biomechanical analysis and 3D computer modeling, have been used to generate detailed functional hypotheses, but in the absence of either direct observations of dinosaur feeding behavior, or close living functional analogues, testing these hypotheses is problematic. Microscopic scratches that form on teeth in vivo during feeding are known to record the relative motion of the tooth rows to each other during feeding and to capture evidence of tooth-food interactions. Analysis of this dental microwear provides a powerful tool for testing hypotheses of jaw mechanics, diet, and trophic niche; yet, quantitative analysis of microwear in dinosaurs has not been attempted. Here, we show that analysis of tooth microwear orientation provides direct evidence for the relative motions of jaws during feeding in hadrosaurid ornithopods, the dominant terrestrial herbivores of the Late Cretaceous. Statistical testing demonstrates that Edmontosaurus teeth preserve 4 distinct sets of scratches in different orientations. In terms of jaw mechanics, these data indicate an isognathic, near-vertical posterodorsal power stroke during feeding; nearvertical jaw opening; and propalinal movements in near anterior and near posterior directions. Our analysis supports the presence of a pleurokinetic hinge, and the straightness and parallelism of scratches indicate a tightly controlled occlusion. The dominance of scratched microwear fabrics suggests that Edmontosaurus was a grazer rather than a browser.Cretaceous ͉ Ornithopoda ͉ tooth ͉ trophic ecology ͉ Vertebrata R econstructing the feeding mechanisms and details of trophic ecology of extinct animals based on functional morphology is fraught with difficulty (1). In vertebrates, tooth form provides only a general guide to diet: the same tooth form can serve more than one function, and that function can vary with specific feeding behavior. Further complications arise because functional optimization of tooth form can be constrained by the need to process fallback foods during times of resource scarcity (2), and animals with an apparently specialized feeding apparatus can have generalist diets (3). These problems are especially acute in groups like herbivorous, nonavian dinosaurs, where most species have generalized homodont dentitions and lack close living analogues.Among herbivorous dinosaurs, feeding of hadrosaurids has attracted particular attention. They were the dominant herbivorous vertebrates in many Late Cretaceous ecosystems, in terms of both species richness and abundance, and they achieved a near-global distribution (4, 5). This success is frequently attributed to the complex jaw mechanisms possessed by these taxa, which would have given them a level of masticatory prowess equal to that of many extant mammals (6). Current models of feeding mechanisms in hadrosaurid dinosaurs are bas...

Associated evolution of bipedality and cursoriality among Triassic archosaurs: a phylogenetically controlled evaluation

“…Further microwear studies on extant reptiles with simple tooth morphology are needed to confidently rule out the possibility of omnivory in Silesaurus. Scratches are often inferred to have been caused by either feeding on grasses containing silica phytoliths or by ingesting soil and grit (Williams et al 2009). Grasses had not evolved in the Late Triassic (Taylor et al 2009).…”

“…Because there are no living analogues, dietary interpretation of herbivorous archosaurs from their dental microwear is challenging and rather than for detailed diet reconstruction, it has been used to provide evidence of niche partitioning between sympatric species (Fiorillo 1998;Goswami et al 2005) or as an indicator of similarities and differences in diet between closely related species (Upchurch and Barrett 2000;Fiorillo 2011;Whitlock 2011). However, microwear analysis is a powerful tool for reconstructing the jaw movement of extinct archosaurs because it does not necessarily depend on data from modern analogues (Fiorillo 1998;Rybczynski and Vickaryous 2001;Williams et al 2009). Dental microwear can reveal jaw movements that are not detectable from tooth gross morphology.…”

“…A microwear analysis of hadrosaurid dinosaurs showed that the orientation of scratches was indistinguishable between microscopic observation sites in the same teeth, among teeth in the same jaw, and even between individuals (Williams et al 2009). This is possible for hadrosaurids and other dinosaurs that possess dental batteries because teeth of these dinosaurs form one large wear facet.…”

“…Angular dispersion of the scratches was also calculated, which represents the degree of parallelism. If scratches are aligned, then angular dispersion approaches one, and if scratches are oriented more randomly then it approaches zero (Williams et al 2009). …”

Dental macro- and microwear in Carinodens belgicus, a small mosasaur from the type Maastrichtian

“…In addition to the approaches outlined above, dental macroand microwear analysis can help provide answers on diet and feeding behaviour in non-mammalian amniotes (Rybczynski & Reisz, 2001;Goswami et al, 2005;Schubert & Ungar, 2005;Williams et al, 2009;Varriale, 2011;Whitlock, 2011;Young et al, 2012). Unlike morphology, in which virtually every adaptation is still an inherited trait (except for poorly understood epigenetic features), microwear is a direct record of a physical interaction of the animal with the environment.…”