ONTOGENETIC GROWTH IN THE PLEISTOCENE PRONGHORN STOCKOCEROS FROM SAN JOSECITO CAVE, MEXICO

Furlong (1943) first described large samples of adult and juvenile limbs of the Pleistocene pronghorn Stockoceros conklingi from San Josecito Cave, Nuevo Leon Province, Mexico (now in the collections of the Natural History Museum of Los Angeles County). The abundance of juvenile specimens allows the determination of the growth trends in ontogeny, and comparison of the growth trends in other pronghorns. We measured the four main limb bones (humerus, radius-ulna, femur, tibia), taking both the length of the bone (minus epiphyses in juveniles) and the midshaft circumference (following the conventions of Kilbourne and Makovicky, 2012). Sample sizes were as follows: 105 humeri, 122 radius-ulnae, 64 femora, 60 tibiae. The Reduced Major Axis (RMA) fit was then calculated for all four limbs using both R and PAST software (RMA is more appropriate than a Least Squares Correlation, because there is no dependent or independent variable). All four limb bones showed slopes that were less than 1.0, suggesting that their growth is negatively allometric, growing thicker as they mature, rather than more gracile or growing isometrically (humerus slope = 0.82; radius-ulna = 0.73; femur = 0.88; tibia = 0.89). The RMA run in R software determined that all four limbs were significantly more robust, but in the PAST software, only the radius-ulna was significantly distinct from the isometric slope of 1.0, using 95% bootstrapped confidence intervals. This might be because the r² value for some of these slopes was low (0.5), although others were quite high (0.8 to 0.9 for most of them). This is very similar to the modern Antilocapra americana, which was significantly more robust in three limbs, and only isometric in the radius (Prothero et al., 2020). Similar studies should be undertaken on extinct pronghorns with abundant juvenile limb elements to see if there is an overall pattern in antilocaprids.