Abstract
Palaeoecological interpretations are based on our understanding of dietary and habitat preferences of fossil taxa. While morphology provides approximations of diets, stable isotope proxies provide insights into the realized diets of animals. We present a synthesis of the isotopic ecologies (δ13C from tooth enamel) of North American mammalian herbivores since approximately 7 Ma. We ask: (i) do morphological interpretations of dietary behaviour agree with stable isotope proxy data? (ii) are grazing taxa specialists, or is grazing a means to broaden the dietary niche? and (iii) how is dietary niche breadth attained in taxa at the local level? We demonstrate that while brachydont taxa are specialized as browsers, hypsodont taxa often have broader diets that included more browse consumption than previously anticipated. It has long been accepted that morphology imposes limits on the diet; this synthesis supports prior work that herbivores with ‘grazing’ adaptions, such as hypsodont teeth, have the ability to consume grass but are also able to eat other foods. Notably, localized dietary breadth of even generalist taxa can be narrow (approx. 30 to 60% of a taxon's overall breadth). This synthesis demonstrates that ‘grazing-adapted’ taxa are varied in their diets across space and time, and this flexibility may reduce competition among ancient herbivores.
Highlights
The appearance and expansion of grassland biomes in North America during the Neogene coincided with suites of morphological adaptations and niche diversification among terrestrial mammalian herbivores [1,2,3,4,5]
: (i) do morphological interpretations of dietary behaviour agree with stable isotope proxy data? (ii) are hypsodont or grazing-adapted taxa specialists, or is hypsodonty a means to broaden the dietary niche? and (iii) how is dietary niche breadth attained at the local level within a taxon?
This paper provides a synthesis of the dietary behaviour and specialization of herbivorous mammals since the expansion of C4 grasslands in North America
Summary
The appearance and expansion of grassland biomes in North America during the Neogene coincided with suites of morphological adaptations and niche diversification among terrestrial mammalian herbivores [1,2,3,4,5]. Most notable is the evolution of horses, from small forms with multiple digits during the Palaeogene, to large taxa with broader muzzles, high-crowned teeth and increased cursoriality [6,7,8,9,10] This transition, in horses, is cited as one of evolution’s classic examples of the ability of animals to adapt to their local environment—with direct selection of features that affect an individual’s ability to consume grass. We test the following hypotheses of notable relevance to the evolution and palaeobiology of mammalian herbivores through time, focusing on hypsodonty: (i) morphological dietary interpretations largely agree with isotopic proxy data; (ii) the ability to graze does not necessitate a specialized diet of grass; and (iii) the localized isotopic breadth of hypsodont taxa is more variable and flexible than in brachydont taxa
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