MASTODONS OF UNUSUAL SIZE? FOUR DIMINUTIVE FEMALE MASTODONS (MAMMUT PACIFICUS) FROM THE LATE PLEISTOCENE OF SOUTHERN CALIFORNIA, USA

The fossil record of North American mastodons is biased toward males partially due to the difficulty of identifying female mastodons. Sexual dimorphism is pronounced in mastodon tusk and body size, but there is considerable overlap in morphology for adult female and young adult male mastodons, making it difficult to interpret sex for smaller individuals. Sexual dimorphism in North American mastodons is primarily based on material from American mastodons (Mammut americanum), leaving much to be discovered about the contemporary Pacific mastodon (M. pacificus), which was recognized as a distinct species in 2019. The goal of this project is to provide insight the character of sexual dimorphism for female Pacific mastodons by describing four diminutive yet mature specimens from southern California.

One of the most complete female Pacific mastodons is the “Oceanside Mastodon” (SDNHM 86541). Discovered in 2002 in Oceanside, CA, USA, this specimen includes the complete left tusk, fragments of the right tusk, left m2, right forelimb including the humerus, radius, ulna, carpals, and metacarpals, right femur, and numerous vertebrae, ribs, and tarsals. SDNHM 86541 is identified as female by a heavily worn m2 (indicating a Laws Age Group of XXII, ~39 years) and a very small tusk (maximum diameter of 78 mm) that in the absence of skeletal indicators of age could otherwise be attributed to a juvenile. The remaining putative females (based on small size and evidence of age and/or maturation) are LACM 1266 (right femur), LACM 4100 (cranium with LM2, LM3, RM1, RM2, and RM3), and WSC 8917 (partial cranium with left and right M3). Although our sample size is small, full anatomical descriptions of these specimens will substantially increase the body of knowledge on female Pacific mastodons, laying groundwork for future studies into the paleobiology of mastodons, especially those in the western United States.