Premaxillary shape as an indicator of the diet of seven extinct late Cenozoic new world camels

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Abstract
Premaxillary shape is a good indicator of the diets of extant artiodactyls, and probably of extinct ones as well. Grazing mammals have squarer premaxillae than browsers, and animals which exhibit dietary strategies between those of grazers and browsers (“intermediate feeders”) have premaxillae that are squarer than browsers but more pointed than grazers. A simple geometric model (Premaxillary Shape Index or PSI) is derived here, which describes premaxillary shape in a standardized and repeatable fashion. Premaxillary shapes are shown to correlate with diet in extant herbivores. The average PSI for 12 species of extant browsers is 54.9%; that of extant intermediates (20 species) 66.6%; and of grazers (10 species) 79%. The PSIs for each group are significantly different (P < 0.01). Our study indicates that, like modern camelids, Tertiary/Quaternary camels were not grazers, but rather browsers (Hemiauchenia macrocephala; Megatylopus gigas; Palaeolama weddelli; Procamelus occidentalis;, Alforjas sp. and Hemiauchenia ?vera) with PSI values between 46% and 54%, or intermediate feeders (Camelops hesternus) with a PSI of 67.2%. If these widespread and common extinct camelids were not grazers, then their central North American environment might have included a larger component of trees and shrubs than is currently thought.