ONTOGENETIC AGE AND SEX DISTRIBUTIONS OF EQUUS OCCIDENTALIS FROM RANCHO LA BREA, CALIFORNIA, BASED UPON DENTAL AND PELVIC DATA

The large horse species Equus occidentalis Leidy, 1865 is the second most common large mammal herbivore from the Rancho La Brea (RLB) asphalt deposits in Los Angeles, California. Thousands of equid skeletal elements represent hundreds of individuals at the site. The teeth and pelvic bones of E. occidentalis from RLB provide indications of the minimum number of individuals, the distribution of ontogenetic age classes, and the distribution of males and females within the sample.

At RLB, more than 250 individuals of Equus occidentalis are represented by teeth. Almost 50% of this sample consists of individuals less than 1.2 years in age. Of 42 crania, 29 (69%) had large canines and are interpreted to be male. Similarly, 22 of 34 adult dentaries (65%) exhibited large canines. Thus upper and lower adult dental samples indicate a proportion of 29 adult males (71%, upper teeth) to 13 adult females (29%, lower teeth). However, of 14 subadults (2.5 - 4 years), 8 specimens (57%) had large unerupted or erupting canines and are presumed male, a proportion not significantly different from a 1:1 sex ratio.

The sample size of pelvic elements from individual localities ("pits") with published radiometric dates is smaller (N = 120) but the ontogenetic distribution is similar. Juveniles comprise 45% of the sample, subadults 39%, and adults 16%. Adult (> 5 years) pelves show a male to female ratio of 1:1.3 (N=19) but the combined adult and subadult pelvic sample yielded a male-female ratio of 1.8:1 (N=66). The differences in male-female ratios between teeth and pelves are due to 1) differences in the nature of examined samples (entire sample for dentitions, only radiometrically-dated pits for pelves), 2) different age classifications for teeth versus pelves (e.g., a dental adult is not the same as a pelvic adult); and 3) differences in the number of age classes examined for sex (i.e., pelvic data enabled interpretation of younger age classes than was possible in the dental sample).

Results suggest that bands of horses inhabited the region during the late Pleistocene. Family units consisted of a male (whether territorial or not) and females with young, the latter being especially vulnerable to asphalt entrapment. Male bachelor groups were also likely present in the area.