HOW DID BISON CALVES GROW UP? POSTNATAL LIMB ALLOMETRY IN BISONANTIQUUS FROM LA BREA TAR PITS

The La Brea tar pits preserve a huge sample of juvenile bones of many mammals, one of the few fossil localities known that allows us to study how limb bones changed shape as they grew up. The extinct Pleistocene species Bison antiquus is very common at La Brea, so were able to study at least 50-60 specimens of the limb bones of juveniles, from the smallest calves to full-grown adults. We measured the diaphysis length, circumference, and cross sectional area of humeri, radii, femora and tibiae, using dial calipers and a flexible metric tape measure. Previous studies on the growth in the living species Bison bison give us a basis for comparison. The expectation is that the proximal limbs in cursorial mammals show display isometric growth in their proximal limb segments (humerus, femur), but that the distal limb segments (radius, tibia) grow long faster than they grow thick, making them more gracile. The expected isometric slopes are around 1.0 for length vs. circumference. In the radius, the growth trends in B. antiquus were indeed more gracile (slope = 2.29), even more than in B. bison (slope = 0.87). The tibia showed the same growth trends, with highly gracile (slope = 2.02) growth in B. antiquus, while B. bison had a slope of 0.75. Even some of the femora show the same trend, with the slope of B. antiquus = 2.16 (gracile) vs. 0.92 (isometric) for B. bison. This discrepancy is surprising, because previous authors have not commented that adult B. antiquus limbs are remarkably more gracile than those of living B. bison—but that is what their allometric growth trends suggest.