Modelling equid/ruminant competition in the fossil record

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Abstract
The pattern of change in body size distribution of browsing equids and ruminants in North American faunas during the Miocene suggest that immigrant ruminants were creating a “size wedge”;, with the exclusion of endemic equid species from the middle range of body sizes. This size distribution may represent competition between equids and ruminants for suitable vegetation. Yet a simulation model of digestive kinetics (derived from living ungulates) predicts that equids should be superior to ruminants at energy maintenance, at least under conditions of unlimited quantities of food. We suggest that the observed fossil record patterns imply limited availability of browse, giving ruminants a competitive advantage. However, ungulates with more fibrous diets show the converse pattern of body sizes, with equids creating a “size wedge”; in the distribution of ruminants. The taxonomic and size diversity of intermediate‐feeding and grazing equids and ruminants during the Miocene of North America contrasts with that seen today in Africa, and we suggest that grass was less limited in year‐round availability in the Miocene savannas than in the present day savannas of East Africa.