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

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

gomphothere Gomphotheres are an extinct group of proboscideans related to modern elephants. First appearing in Africa during the Oligocene, they dispersed into Eurasia and North America during the Miocene and arrived in South America during the Pleistocene a ...

s. It ranged throughout North America from the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Blancan The Blancan North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 4,750,000 to 1,806,000 years BP, a period of .Early Pleistocene The Early Pleistocene is an unofficial epoch (geology), sub-epoch in the international geologic timescale in chronostratigraphy, representing the earliest division of the Pleistocene Epoch within the ongoing Quaternary Period. It is currently esti ...

(early

Irvingtonian The Irvingtonian North American Land Mammal Age on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), spanning from 1.8 million – 250,000 years Before Present, BP.

, ~1.2 Ma). The former South American species have been synonymized with '' Notiomastodon platensis''.

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 . Unlike more primitive gomphotheres like ''

Gomphotherium ''Gomphotherium'' (; "nail beast" for its double set of straight tusks) is an extinct genus of gomphothere proboscidean from the Neogene of Eurasia, Africa and North America. It is the most diverse genus of gompothere, with over a dozen valid sp ...

'', it lacked lower tusks and had a shortened lower jaw. The upper tusks curved upward and were about long, and lacked enamel bands. The tall head and robust lower jaw suggest a strong vertical bite. ''Stegomastodons third molars

molars The molars or molar teeth are large, flat tooth, teeth at the back of the mouth. They are more developed in mammal, mammals. They are used primarily to comminution, grind food during mastication, chewing. The name ''molar'' derives from Latin, '' ...

had at least 5 lophs (ridge-like structures), greater than previous gomphotheres. The strong bite and increased tooth complexity are suggested to be adaptations to a grazing diet.

Origin and evolution

''Stegomastodon'' is suggested to have ultimately originated from New World populations of ''Gomphotherium,'' possibly via the intermediate genus '' Rhynchotherium''.Spencer LG 2022
The last North American gomphotheres
N Mex Mus Nat Hist Sci. 88:45–58. The earliest species of the genus appeared during the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Blancan The Blancan North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 4,750,000 to 1,806,000 years BP, a period of .Irvingtonian The Irvingtonian North American Land Mammal Age on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), spanning from 1.8 million – 250,000 years Before Present, BP.

, around 1.2 million years ago, ''Stegomastodon'' became extinct, which is suggested to be due to competition with the recently arrived

mammoths A mammoth is any species of the extinct elephantid genus ''Mammuthus.'' They lived from the late Miocene epoch (from around 6.2 million years ago) into the Holocene until about 4,000 years ago, with mammoth species at various times inhabi ...

, which were more efficient grazers. Originally, some specimens from

Jalisco Jalisco, officially the Free and Sovereign State of Jalisco, is one of the 31 states which, along with Mexico City, comprise the 32 Political divisions of Mexico, Federal Entities of Mexico. It is located in western Mexico and is bordered by s ...

Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...

were estimated to date to 28,000 years

Before Present Before Present (BP) or "years before present (YBP)" is a time scale used mainly in archaeology, geology, and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Because ...

, but this age was rejected in 2011 by Spencer G. Lucas et. al, who stated that the date was far too young to be viable and that it actually dates to the Blancan. Stegomastodon

Taxonomy

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 three 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.'' The South American ''Stegomastodon'' fossils have been reassigned to ''

Notiomastodon ''Notiomastodon'' is an extinct genus of gomphothere proboscidean (related to modern elephants), endemic to South America from the Pleistocene to the early Holocene. ''Notiomastodon'' specimens reached a size similar to that of the modern Asian ...

'' by the majority of recent authors''.'' ''Stegomastodon'' and ''Notiomastodon'' differ in numerous characters, including of the skull and limbs. 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