Date of this Version
Published in Transactions of the Nebraska Academy of Sciences, Volume 3 (1976).
The University of Nebraska State Museum collections include a large sample of dentitions and skeletal elements of a late Pliocene antilocaprid which Kent (1963) has referred to the species Texoceros cf. guymonensis. At least 95 individuals are represented in this sample which was collected from the University of Nebraska State Museum locality Gd-10, located in the SE¼ NE¼ sec. 29, T. 16 N., R. 44 W. Lower jaws and dentitions are the most common elements preserved in the sample.
Individuals of two other species of pronghorns have previously been aged on the basis of characters of the lower dentition. Dow and Wright (1962) studied various stages of tooth replacement and wear presented by individuals of the modern pronghorn species, Antilocapra americana, whose ages were known. Voorhies (1969) studied a sample of the early Pliocene pronghorn, Merycodus furcatus, from Verdigre, Nebraska and was able to demonstrate the presence of discrete yearly age classes. Voorhies was able to suggest absolute ages for these classes by comparing wear stages with those exhibited by the living African duiker, Sylvicapra grimmia, which had been studied by Riney and Child (1960). The duiker was used in this comparison because it is similar in size to Merycodus.
Texoceros is intermediate between Antilocapra and Mervcodus in geologic age and size. Because of its size, Texoceros would be expected to have matured more slowly than the smaller Merycodus and more rapidly than Antilocapra, a larger animal. Individuals of each genus which show the same stage of tooth replacement or wear would have three different ages with Merycodus being the oldest and Antilocapra, the youngest.
Texoceros was aged on the basis of the mandibular dentitions. The criteria used were the degree of replacement of the deciduous cheek teeth and eruption of the permanent cheek teeth and the wear patterns on the occlusal surfaces of these teeth. Fossettes on the molars and premolars of Antilocapra disappear with age in a fairly rigid sequence (Dow and Wright, 1962). The same appears to be true for the fossettes on the molars of Texoceros. Crown height, which Voorhies (1969) used as a criteria for aging Merycodus, was not a realistic tool to use in this study for several reasons: the inadvisability of cutting into a large number of rami to expose the base of the crown, the difficulty of establishing a precise point from which to measure crown height and the presence of secondary cement which obscures the base of the crown.
A study of the mandibular dentitions of the immature individuals in the sample of Texoceros showed that distinct age groups were present. The entire sample, therefore, was divided into groups based on the criteria mentioned previously and the minimum number of individuals in each group was determined using the method described by Voorhies (1969). Approximate absolute ages were assigned to each group by comparing the replacement and wear patterns with similar patterns exhibited by known-age individuals of Antilocapra and estimated-age individuals of Merycodus.