Allometry and reorganization in horse skull proportions.

Allometric analysis of skull proportions in 25 species of fossil equids indicates that both scaling effects (allometry) and reorganization were factors in the evolutionary transformation of horse skulls. A relatively longer preorbital portion of the skull resulted from the ventral and forward displacement of the tooth row relative to the jaw joint and the orbit when high-crowned teeth evolved. Correlated with the increased distance between jaw joint and tooth row is an increase in the relative size of the attachment areas of masseter and internal pterygoid muscles.