Microwear studies background

It is widely recognized in biology that acquisition of food is a basic determinant in the behavior of an organism. Patterns of food acquisition and use provide insight into the behavior of extant and extinct organisms. The tool of choice for many students interested in this aspect of the paleobiology of vertebrates is the study of dental microwear.

George Gaylord Simpson (1933), in his landmark study, used teeth to determine variation in jaw movements among Jurassic mammals, differences thought to reflect inferred food differences. Wear features are ultimately produced by the movement between upper and lower teeth, and linear features provide a record of the pattern of chewing (Fiorillo and Weishampel 1997). Numerous workers have examined microwear on teeth, using light microscopy or scanning electron microscopy as a tool to determine patterns of food use (Kay and Covert 1983; Teaford and Walker 1984; Teaford 1985; Teaford and Byrd 1989; Walker and Teaford 1989; Walker et al. 1978; Taylor and Hannam 1987; Ungar 1990, 1994, 1998; Maas 1991; Merceron et al. 2004; Rivals and Deniaux 2003; Hotton et al. 1997; Fiorillo 1998, 2006b; King et al. 1999; Goswami et al. 2005; Schubert and Ungar 2005; Solounias and Moelleken 1992, 1993; Solounias and Hayek 1993). Ungar et al. (2008) have provided an excellent overview of the history of the study of microwear, and interested students of microwear are referred to it for further details.

Broadly speaking, striae and pits are the commonly analyzed features of microwear. Some authors have defined pits as having length to width ratios of less than 2:1 while others have used a ratio of 4:1. The presence of pits is generally associated with the consumption of hard food items. The development of striae, their length, depth, and density, can be used to infer qualitatively the food hardness consumed between animals with only scratches on their wear facets. Microwear patterns in modern animals can change with seasonal variation in diet.

Whereas most of these studies have focused on diets in mammals, either extant or extinct, relatively little attention has been directed toward non-mammalian studies. However, some studies focus on critical moments in non-mammalian evolution and use results to determine behavior in early vertebrate history. For example, Purnell (2001, 2002) used microwear patterns on a variety of conodonts and heterostracan fishes to develop feeding models in early vertebrates. Additionally, Hotton et al. (1997) used microwear in the teeth of early amniotes to discern omnivory versus high-fiber herbivory.

Other studies have focused on patterns of food use within single paleoecosystems. In a study of microwear in sympatric Jurassic sauropods from western North America, specifically Diplodocus and Camarasaurus, it was demonstrated that there was niche partitioning between these taxa, and that they had distinctively different diets (Fiorillo 1998). Similarly, Goswami et al. (2005) used microwear to make dietary inferences about two cynodonts and a prosauropod from the Triassic of Madagascar. Their study also showed aspects of difference in food use patterns. In contrast, food partitioning could not be demonstrated for Late Cretaceous small theropods, either within a paleoecosystem or between ancient ecosystems separated geographically (Fiorillo 2006b, 2008). Schubert and Ungar (2005) studied the wear facets of tyrannosaurids and determined wear facets in this group to be the result of tooth-to-tooth contact between the maxillary teeth and the dentary teeth. Similarly, Rybczynski and Vickaryous (2001) used patterns of microwear in the teeth of the ankylosaurid Euoplocephalus to propose a more complex pattern of mastication than had been previously been appreciated.