''Stegomastodon'' ('roof breast tooth') is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of gomphotheres, a family of proboscideans. It ranged throughout North America from the early Blancan ~4 Ma, to the early Irvingtonian (~1.2 Ma). The South American species have been synonymized with '' Notiomastodon platensis''.

Species

The following definite species have been described:''Stegomastodon''
at Fossilworks.org * ''S. aftoniae'' * ''S. mirificus'' * ''S. nebrascensis'' * ''S. primitivus'' ''S. waringi'' is considered synonymous with ''S. platensis''. A dispute now exists over whether ''S. platensis'' should remain within the North and Central American genus ''Stegomastodon'' or should be moved over to the exclusively South American genus ''

Notiomastodon ''Notiomastodon'' is an extinct proboscidean genus of gomphotheres (a distant relative to modern elephants) endemic to South America from the Pleistocene to the beginning of the Holocene. ''Notiomastodon'' specimens reached a size similar to that ...

''. The genus ''Haplomastodon'' is regarded as synonymous with the South American ''Stegomastodon'' species.

Origin and evolution

''Stegomastodon'' is considered by some to be derived from New World populations of ''

Gomphotherium ''Gomphotherium'' (; "welded beast") is an extinct genus of proboscids from the Neogene and early Pleistocene of Eurasia, Africa, North America and Asia. As of 2021, two species, ''G. annectens'' and possibly ''G. subtapiroideum'', are also k ...

''. The number of species within the genus has varied between ''S. mirificus'' being the only valid species, to Osborn's seven species of “ ascending mutations” (''S. primitivus, S. successor, S. mirificus, S. chapmani, S. texanus, S. arizonae'' and ''S. aftoniae'') Lucas et al., 2013 accepted 3 overlapping

chronospecies A chronospecies is a species derived from a sequential development pattern that involves continual and uniform changes from an extinct ancestral form on an evolutionary scale. The sequence of alterations eventually produces a population that is p ...

''S. primitivus, S. mirificus S. aftoniae.''

Description

''Stegomastodon mirificus'' is known from NMNH 10707, a roughly 30-year-old male, of which most of the skeleton has been found. Alive, it stood about tall, with a weight around . Like modern elephants, but unlike most of its closer relatives, it had just two tusks, which curved upward and were about long. The tall, head and robust lower jaw suggest a strong vertical bite. ''Stegomastodons

molars The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...

were covered in enamel and had a complex pattern of ridges and knobbly protrusions on them, giving the creature a large chewing surface that suggests it was a grazer. During the early Irvingtonian, ''Stegomastodon'' was replaced by ''

Mammuthus A mammoth is any species of the extinct elephantid genus ''Mammuthus'', one of the many genera that make up the order of trunked mammals called proboscideans. The various species of mammoth were commonly equipped with long, curved tusks and, i ...

'', which was presumably a more efficient grazer. Its brain weighed about . The genus lived in North America from the

Zanclean The Zanclean is the lowest stage or earliest age on the geologic time scale of the Pliocene. It spans the time between 5.332 ± 0.005 Ma (million years ago) and 3.6 ± 0.005 Ma. It is preceded by the Messinian Age of the Miocene Epoch, and ...

of the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Late Pleistocene. The most recent specimens in the fossil record originate from Jalisco,

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

and date to 28,000 BP, but this late date is over 1 million years after the last confirmed specimens.

Taxonomy

The South American ''Stegomastodon'' fossils were reassigned to ''

'', though in literature the outdated genus and species names continued to be used until 2016. Revised taxonomy of ''Stegomastodon'' and other trilophodont gomphotheres according to Mothé et al., 2017:

References

{{Taxonbar, from=Q136453 Gomphotheres Prehistoric placental genera Cenozoic mammals of North America Pliocene proboscideans Pleistocene proboscideans Zanclean first appearances Pleistocene genus extinctions Fossil taxa described in 1912