HOW DID MASTODONS GROW? ONTOGENETIC LONG BONE GROWTH IN THE AMERICAN MASTODONS

Neogene proboscideans, similar to other terrestrial megafauna, are supported by thick robust limbs, which are necessary to accommodate their larger body mass. How does the ontogeny of limb growth differ between these species? Having already studied the growth pattern in mammoths and living elephants, we investigated the growth curves of two different species of proboscideans (the newly named Western mastodon, Mammut pacificus, and the American mastodon, Mammut americanum) to determine their ontogenetic patterns, and compared their growth to data from extant African elephants. Contrary to expectations of increasing robustness, analysis of these measurements by several authors suggests that most elephant and mammoth limbs grew isometrically, with slopes not significantly different from 1.0. Only the ulna in the African savanna elephant (Loxodonta africana) shows slightly more robust growth, possibly because the huge head of elephantoids might require more robust support in the lower front limbs. In the case of the American mastodons, the slopes of front limbs (humerus, ulna) are all isometric (except for the ulna of M. americanum), while the slopes of the femora are significantly more gracile than expected. The slopes of the tibia were mixed, with the genus Mammut showing negative allometry, while M. pacificus was more gracile and M. americanum was isometric, possibly due to smaller samples and greater scatter of data points. Except for the tibia, most of these slopes are tightly constrained, since there is the full range of sizes, from huge adults to baby mastodons in the data set (over 24 specimens of each limb were measured), and the correlation coefficients are excellent (r2 = 0.85 or higher for nearly every sample). This pattern contrasts strongly with that seen in other proboscideans, but the presence of some of the negative growth slopes may be due to the absence of neonate specimens in our data set. In studies of living elephants, when neonates are included, the trend in the hindlimbs shifted from negatively allometric to isometric.