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Ontogenetic change in distal and proximal limb bones of juvenile Pleistocene coyotes (Canis latrans) and dire wolves (Canis dirus) from the Rancho La Brea Tar Pits, California
Large sample sizes of juvenile animal fossils are rare compared to their adult counterparts. The preponderance of adult specimens in the fossil record overshadows the entire ontogenetic growth series of an organism from the earliest stages of life and onward. This is partially because the fragile parts of younger individuals are typically poorly preserved. However, the natural asphalt seeps of the Rancho La Brea Tar Pits have yielded spectacular quantities of specimens young and old, allowing for a more complete investigation of ontogenetic growth series. We collected long bone length, thickness, and circumference data from nearly 800 separate appendicular skeleton elements across three canid species; Pleistocene coyotes (Canis latrans) and dire wolves (Canis dirus), as well as modern gray wolves (Canis lupus). Standardized major axis bivariate regressions were used to determine the ontogenetic change in limb bones and the deviation from the line of isometry (“same growth”). Using regression slopes as a proxy for long bone allometry, we were able to compare the growth patterns of the extinct canids to other cursorial animals and their modern counterparts. We found that C. latrans, C. dirus, and C. lupus long bone growth series are positively allometric, with bones growing longer faster than they do thicker. The degree of positive allometry is typically more pronounced in the distal elements than the proximal elements. This suggests an increasing degree of gracility in the distal elements compared to the relatively robust proximal elements. As expected of animals adapted to a running lifestyle, the increasing gracility of long bones would allow for a much more efficient running locomotion behavior. This would be beneficial while hunting, much like the modern gray wolf when in pursuit of smaller and faster prey. These statistical results show that coyote and dire wolf growth series are typical of other cursorial animals during ontogeny, regardless of climatic influences on body size changes during the glacial and interglacial periods of the Pleistocene.