ALLOMETRIC TRENDS IN LIMB REDUCTION OF THE PLEISTOCENE MOUNTAIN GOAT OREAMNOS HARRINGTONI (ARTIODACTYLA: BOVIDAE)

Phyletic size reduction in the fossil record is quite common, and numerous studies document allometric changes in limbs after size reduction. Some instances of dwarfing (especially in rhinos, hippos, deer, goats, camels, and some proboscideans) show allometric changes to more robust limbs, while others (most proboscideans, pronghorns, and some others) show isometric size reduction of limbs. We measured the major limb elements (humerus, femur, tibia, metacarpal, metatarsal) of the Pleistocene mountain goat, Oreamnos harringtoni, and compared their mean values to the living species O. americanus to determine the interspecific slope of size reduction. Oreamnos harringtoni decreased by about 18-28% in size compared to the living mountain goat, but almost all the slopes of the major forelimb bones (humerus, metacarpal) were isometric, with a slope very close to 1.0. Only the hindlimb bones (femur, tibia, and metatarsals) were more robust than expected, with a slope of 0.90 for the femur, 0.89 for the tibia, and 0.75 for the metacarpals. The smaller mountain goats apparently had slightly more robust and stumpy hind limbs compared to the living species, possibly as an adaptation to “low-gear locomotion” on steep slopes. This is seen in some other extinct organisms, such as Pleistocene deer and goats in the Mediterranean islands, and the goatlike camel from Pliocene near Death Valley.