AGE-MORTALITY PROFILES IN LA BREA BISON: INSIGHTS INTO POPULATION DYNAMICS AND TAPHONOMY

In population dynamics, age-mortality profiles (a histogram showing the mortality in each age class) are powerful tools for interpreting ecology. A typical living population has its highest mortality rates among juveniles, and mortality declines among the older age classes. In a fossil assemblage, this is interpreted as a catastrophic death assemblage. Another common pattern is an attritional death assemblage, with high mortality rate among the youngest and oldest age classes, but the healthy adults have a very low mortality rate. Jefferson and Goldin (1989) reconstructed the age-mortality profiles for Ice Age Bison antiquus from Rancho La Brea, using the wear stages in lower jaws. They found a unimodal distribution, with a very high number of older juveniles, but few in the other age classes. This seems to suggest taphonomic or presevational distortion of the original biological population sample. For comparison, we measured all the juvenile and adult limb bones (humerus, radius, femur, tibia) using the length of the diaphysis to see what kind of profile they produced. Surprisingly, they gave a bimodal pattern, with a large number of older juveniles (as seen by Jefferson and Goldin), but also a large mode among adults in their prime. This pattern bears no resemblance to the expected patterns for known processes, and must reflect some sort of taphonomic bias in the tar pits. Apparently, individuals of these age classes are more likely to be trapped, or their bones are less likely to be destroyed during the pit wear and churning within the pit.