New Mexico Geological Society Annual Spring Meeting — Abstracts

Stegomastodon is among three mastodon-like elephant cousins of the family Gomphotheriidae native to the Plio-Pleistocene of New Mexico. We report what is to the best of our knowledge the first associated cranium and mandible of Stegomastodon (NMSU 15723) from New Mexico. It was excavated from the Camp Rice Formation in Las Cruces, Doña Ana County, NM. The Camp Rice in this general area is said to have a temporal range of 3.6-0.8 Ma (early Blancan to early Irvingtonian or Gauss to Matuyama chrons). Stegomastodon is believed to have become extinct at 1.2 Ma. The nearby Stegomastodon-bearing Tortugas site was biostratigraphically dated to 1.6-1.2 Ma, but the two sites have not yet been stratigraphically correlated. NMSU 15723 was excavated from unconsolidated mostly well-sorted multi-story cross-bedded fluvial channel sands. Large rip-up clasts of presumably allochthonous brown and gray clays/paleosols as well as a few intercalated beds of clays/paleosols and of gravels suggest proximity to flood plains and alluvium.

NMSU 15723 includes a complete mandible and a cranium with all four third molars (M3/m3) and the left tusk (I1), but lacking the right tusk. We refer it to Stegomastodon primitivus because it is brevirostrine, the tusk (I1) is up-curved, and upper M3 is tetralophid and lower m3 is pentalophid, or tetralophid if the two distal rows of worn conules correspond to the talonid or ‘heel’.

Stegomastodon skulls are extremely rare, so every new specimen has the potential to significantly broaden knowledge of these little-known polymorphic behemoths. NMSU 15723 is most notable for three reasons. 1) A previously undocumented weak enamel band is present on the medial side of the tusk. A lateral enamel band is variously present in the South American Notiomastodon, with which Stegomastodon has historically been confused, as well as in some other more primitive proboscideans. A spiral enamel band is also characteristic of the two other New Mexican genera of Plio-Pleistocene gomphotheres, Cuvieronius and Rhynchotherium. 2) NMSU 15723 combines S. primitivus-like molar and S. mirificus-like molar and mandibular morphologies. Specifically, its molars are tetra/pentalophid as in S. primitivus, but they exhibit ptychodonty (i.e., plication or crenulation of enamel) and the mandibular ramus is recumbent as in S. mirificus. 3) NMSU 15723 exhibits extreme antemortem tooth wear. This could be evidence of great age or that the animal consumed a more abrasive diet than that for which it was well adapted or both. The latter interpretation is consistent with the hypothesis that S. mirificus is a chronospecies of S. primitivus that was somewhat better adapted for grazing in that tooth wear could be a strong evolutionary selective pressure. Gomphotheres were ultimately entirely replaced by obligate grazers, mammoths, as New Mexico became increasingly arid.