HOW DID LA BREA COLTS GROW UP? A STUDY OF POSTNATAL ALLOMETRIC GROWTH IN THE LIMB BONES OF EQUUS OCCIDENTALIS

The amazing fossil collections at the La Brea Tar Pits Museum includes huge numbers of juvenile limb bone in every stage of growth, allowing us to examine how limb bones changed shape as colts of Equus occidentalis grew from the smallest juvenile to adult size. We measured the diaphysis length, circumference, and cross-sectional area of a minimum of 50 juvenile specimens of humeri, radii, femora, and tibiae. Previous studies of the growth series of Equus burchelli, the common zebra, allows for comparison. The expectation is that in cursorial animals like horses, the distal limb segments (radius, tibia) show grow longer faster than they grow thicker, so they become more gracile as they grow up. In the radius of E. occidentalis (n = 60), the growth is significantly more gracile (expected slope of radius vs. circumference = 1, actual slope = 1.68) as the colts grew, even more gracile than E. burchelli (slope = 0.76, which is robust). Likewise, tibia is highly gracile in its growth trends (slope = 2.15), while E. burchelli has more robust tibial growth (slope = 0.88). Even the femora of E. occidentalis are gracile (slope = 2.11), while the femora of E. burchelli show the expected isometric slope (slope = 1.09). This is surprising, because the adult limb proportions of E. occidentalis have been considered robust or normal for horses like zebras, and they are not members of the stilt-legged lineage of horses at all—yet their growth series is highly gracile.