Stable isotopes, hypsodonty, and the paleodiet of Hemiauchenia (Mammalia: Camelidae): a morphological specialization creating ecological generalization

Morphological adaptations may indicate increased specialization (narrowing of ecological niche) or expansion of the suite of lifestyles available to an organism (increasing niche breadth). Hypsodonty in mammals generally has been interpreted as a specialization into a grazing niche from a browsing niche. Here I examine the feeding strategy of the extinct hypsodont camel Hemiauchenia through an analysis of stable carbon isotope values from its tooth enamel, which was used to clarify its feeding strategy and to resolve conflicting interpretations of dental versus muzzle attributes. The paleodiet of Hemiauchenia is then used to test whether hypsodonty correlates to grazing within fossil Lamini. This study focuses on fossils from Florida, which is geographically ideal because unlike other regions of the country almost all extant plants on which animals browse use the C3 photosynthetic pathway. In contrast, most of the grasses and sedges utilized by grazers use the C4 photosynthetic pathway. If Hemiauchenia was an obligate grazer, the stable carbon isotope values of tooth enamel should reflect primarily a diet of C4 grass and sedge (>−1.3%). If Hemiauchenia was mainly a browser, the isotopic value should be considerably more negative reflecting ingestion primarily of C3 browse (<−7.9%). The mean δ13C values for Hemiauchenia during each time interval average more negative than −8.0%, indicating a dominantly C3 browse diet, and there is no evidence for abandonment of the browsing niche from the Hemphillian through the Rancholabrean North American Land Mammal Ages. However, an increase in the range of isotopic values indicates a diet with a higher proportion of C4 grasses and sedges through time. This study therefore suggests that Hemiauchenia was a hypsodont intermediate feeder with preference for browse during the past 5 million years. Hypsodonty is not strictly associated with obligate grazing; instead it may, in this case, represent an adaptation to widen niche breadth that allowed grazing as well as browsing.