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Early chewing mechanisms in mammalian herbivores

Published online by Cambridge University Press:  08 April 2016

John M. Rensberger
John M. Rensberger*
Affiliation:
Department of Geological Sciences and Burke Memorial Washington State Museum DB-10, University of Washington, Seattle, Washington 98195
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Abstract

A comparison of the morphology and wear of the chewing surfaces among archaic Paleocene herbivores suggests that a compressive type of interaction between opposing chewing surfaces was becoming a dominant mode of chewing, unlike the translative (grinding or shearing) mechanisms common in Eocene and later herbivores. In part, the evidence for this conclusion is that most of the wear occurs on horizontal surfaces centered at the tips of the cusps and pronounced unidirectional striae tend to be lacking in these areas. In the diverse early Paleocene periptychid condylarths, multidirectional striae occur on the wear surfaces and there is a measured loss of directionality of the crests and enamel edges from the condition in the earliest condylarth, the Late Cretaceous Protungulatum. The patterns are even more random in the later Paleocene phenacodontids, suggesting that this was a broad characteristic for the larger Paleocene condylarths.

Efficiency (that is, the volume of food processed per stroke) in compressional chewing systems is theoretically proportional to the area of the opposing surfaces, with large, relatively flat surfaces optimal, whereas in translational systems linear structures are dominant. The reduction in alignment of striae and in linearity of topographic features of the chewing surfaces is consistent with an increasing dominance of compressive chewing from the condition characteristic of palaeoryctid and probably other Cretaceous insectivores in which both compression and shearing occurred between opposing surfaces. The change was brought about by expanding surface areas at the expense of edge length, which was accomplished independently and differently by the phenacodontids and the periptychids, one by increasing both the cylindrical shape and size of the cusps, the other by increasing the cylindrical shape and converging the cusps. The retention of large paraconules and metaconules in the upper molars, cusps essentially lost in the periptychids, preadapted the phenacodontid pattern for later development of the oblique lophodonty and translatory chewing component characteristic of perissodactyls (horses and their relatives), the dominant herbivores of the Eocene.

Type
Articles
Information
Paleobiology , Volume 12 , Issue 4 , Fall 1986 , pp. 474 - 494
Copyright
Copyright © The Paleontological Society 

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References

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