THE RELATIONSHIP BETWEEN THE LENGTH OF THE JAW AND THE LENGTH OF THE MOLAR SERIES IN SOME EUTHERIAN MAMMALS

Abstract

SUMMARYThe relationship between the length of the molar row and the length of the lower jaw in mammals is expressed by the allometry equation y=bxk.A single formula cannot be applied to all the Eutheria because the value of k, the constant differential growth ratio, varies from group to group.Variation in k would appear to be adaptive, for it increases from a value of less than unity in carnivores, through insectivores and omnivores, and exceeds unity in herbivores.The value of the constant b decreases as k increases, with the result that most mammalian skulls have the same length of molar row when the jaw length lies between 90 and 110 mm.When the full complement of molars is not present, there is a reduction in the extent of the molar row. In groups with a common ancestry, this reduction is proportional to the number of teeth which are lost, but a history of tooth enlargement may partly compensate for the decrease.The zalambdodont Insectivora are anomalous in that the molar row occupies proportionally less of the jaw than in most mammalian orders. The genera may be arranged in a graded sequence, the first members of which overlap the remaining Eutheria while the final members depart widely.In Tenrec and Hemicentetes the length of the molar row is independent of jaw size.In view of the variation in the value of the constants b and k for both the zalambdodonts and the other Eutheria the paurodont/zalambdodont line cannot be set in contrast to a marsupial/eutherian line.These conclusions apply with equal force whether the molar row is related to the total jaw length or only to that part of the jaw lying posterior to the last pre‐molar.