Characters Useful in Determining the Position of Individual Teeth in the Permanent Cheek-Tooth Series of Merychippine Horses

Abstract

Cheek-teeth of fossil horses display structural characters which have long been recognized as of considerable value in establishing the relationships of these forms and in furnishing suggestions as to the age of the deposits in which the fossils occur. In later Cenozoic formations of western North America where complete skulls and skeletal materials are not always available, identification of members of the Equidae is often based on single teeth. In the study of an individual hypsodont equine tooth difficulty is frequently encountered in determining with certainty its position in the cheek-tooth series. Variation in the enamel pattern as exhibited by the molars and premolars makes it particularly desirable to know where an individual tooth belongs in the series. The problem of identifying teeth when detached from jaws arose in the study of a large number of merychippine teeth secured by the California Institute in the Merychippus Zone of the north Coalinga region, California. The collections from this Miocene horizon include approximately two thousand separate horse teeth, representing principally the species Merychippus californicus Merriam. While characters which assist in determining the position of a tooth apply particularly to this species, the criteria on which identification is made are applicable to individual teeth in a number of generic types of horses from the later Cenozoic. The problem was suggested by Dr. Chester Stock under whose supervision it was investigated. The writer wishes to express his thanks to Dr. Stock for his kindly guidance during the course of the study. The illustrations have been prepared by Mr. John L. Ridgway.