HOW DID MAMMOTHS AND MASTODONTS GROW AND BECOME DWARFED? ONTOGENETIC LONG BONE GROWTH COMPARED TO ISLAND DWARFING IN PLEISTOCENE PROBOSCIDEA

Elephants, much like other terrestrial megafauna, are supported by thick robust limbs, which are necessary to accommodate their larger body mass. How do these limbs grow between different species? Do baby mammoth and mastodon limbs get more robust as they grow, as might be expected? How do the trends of growth from small juvenile to large adult compare to the reduction in size seen in Pleistocene insular dwarf mammoths? Thanks to the large sample of juvenile limb bones at Rancho La Brea and many other museums, we were able to investigate the growth curves of three different species of proboscideans (the extinct American Mastodon, Mammut americanum, the Channel Islands pygmy mammoth Mammuthus exilis, and the Columbian Mammoth, Mammuthus columbi) to determine their ontogenetic patterns, and compared their growth to data from extant African elephants. Limbs were measured for total length, length from the diaphyseal growth plate, and also circumference at the midshaft, and plotted on bivariate graphs. Reduced major axis (RMA) slopes and their confidence intervals were calculated. The first surprise is that neither the pygmy mammoths nor the living African elephants show growth slopes that are as robust as expected for a huge graviportal mammal. Instead, the humerus, femur, and tibia change isometrically as they grow, and only the ulna tends to grow more robust as the mass increases rapidly. Previous studies have shown that dwarfed hippos and rhinos tended to develop more robust limbs as they shrank in size. However, the pygmy mammoths only showed more robustness in the shrinking proportions of the humerus and ulna, but the femur and tibia became reduced in size isometrically. Thus, these expectations of much greater robustness as proboscideans grow huge, or as they reduce in size, is not matched by most of the limbs we examined.