Mammut pacificussp. nov., a newly recognized species of mastodon from the Pleistocene of western North America

Abstract

A new species of mastodon from the Pleistocene of western North America, Mammut pacificus sp. nov. is herein recognized, with specimens identified throughout California and from two localities in southern Idaho. This new taxon differs from the contemporaneous M. americanum in having narrower teeth, most prominently in M3/m3, as well as six sacral vertebrae, femur with a proportionally greater mid-shaft diameter, and no mandibular tusks at any growth stage. All known Pleistocene Mammut remains from California are consistent with our diagnosis of M. pacificus, which indicates that M. americanum was not present in California.

The easternmost occurrence ofMammut pacificus(Proboscidea: Mammutidae), based on a partial skull from eastern Montana, USA

“…There is a prominent step between the right tusk alveolus and the right maxillary tooth row of MOR 605 ( Fig. 1A), similar to WSC 18743 but in contrast to the ontogenetically older adult male M. pacificus WSC 8817 (LG XXVIII, 55 ± 4 AEY (Dooley Jr et al, 2019)). Distinct steps have been reported in female and juvenile male specimens of M. americanum (Osborn, 1936;Dooley Jr et al, 2019) hypothesized that the persistence of this feature in adult male M. pacificus such as WSC 18743 might indicate that maximum tusk size was reached later in ontogeny in M. pacificus versus M. americanum.…”

“…1). A weakly developed cingulum is present along the anterior and anterolabial margins of the right and left M3, as in all specimens of M. pacificus and most examples of M. americanum (Dooley Jr et al, 2019). Both the right and left M3 exhibit smooth enamel, moderate wear on the first loph, lighter wear on the second loph, and almost no wear on the third, fourth, and fifth lophs.…”

“…The right and left M3 of MOR 605 are pentalophodont, as in a majority of M. pacificus specimens (24 pentalophodont versus 15 tetralophodont) (Dooley Jr et al, 2019) (Fig. 1).…”

“…1). With M2 missing, it is difficult to place the specimen in a Laws Group; however, the wear state of the undamaged right M3 suggests that MOR 605 is probably in Laws Group XIX (32 ± 2 AEY) or XX (34 ± 2 AEY) (Laws, 1966), several years younger than WSC 18743, the adult male holotype of Mammut pacificus (LG XXII, 39 ± 2 AEY (Dooley Jr et al, 2019)). To estimate the basal tusk circumference of MOR 605, a digital 3-D model of the complete right tusk of WSC 18743 was resized to fit the partially preserved right alveolus of MOR 605 (Fig.…”

“…The recent recognition of the Pacific mastodon (Mammut pacificus (Dooley Jr et al, 2019)) as a new species distinct from and contemporaneous with the American mastodon (M. americanum) revealed an unrealized complexity in North American mammutid evolution during the Pleistocene. Dooley Jr et al (2019) distinguished M. pacificus from M. americanum by a suite of dental and skeletal features: (1) upper third molars (M3) and lower third molars (m3) much narrower relative to length in M. pacificus; (2) basal upper tusk diameter for males of a given age is smaller in M. pacificus; (3) mandibular tusks and alveoli are always absent in M. pacificus, while variably present in M. americanum; (4) six fused sacral vertebrae in later ontogenetic stages in M. pacificus, while M. americanum exhibits four to six and usually five; and (5) femur with larger midshaft diameter relative to length in M. pacificus . Dooley Jr et al (2019) further determined that, while M. americanum was widespread, occurring from the Eastern Seaboard to the Rocky Mountain region, south into Mexico, and north into Yukon and Alaska, M. pacificus was geographically restricted to the Irvingtonian-Rancholabrean North American Land Mammal Age (NALMA) of California (58 specimens) and southern Idaho (three specimens).…”