Stegomastodon

{{Automatic taxobox

| taxon = Stegomastodon

| fossil_range = Pliocene-Early Pleistocene
~{{fossil range|4|1.2}}

| image = Stegomastodon mirificus - Smithsonian.JPG

| image_caption = S. mirificus fossil at the Smithsonian Natural History Museum

| authority = Pohlig, 1912

| subdivision_ranks = Species

| subdivision = * †S. aftoniae

†S. mirificus
†S. nebrascensis
†S. primitivus

| type_species =

| synonyms =

Morrillia

}}

Stegomastodon ('roof breast tooth') is an extinct genus of gomphotheres. It ranged throughout North America from the Pliocene (early Blancan ~4 Ma), to the Early Pleistocene (early Irvingtonian, ~1.2 Ma). The former South American species have been synonymized with Notiomastodon platensis.

Description

File:Stegomastodon skeletal.pngStegomastodon 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 {{convert|2.6|m|ft|abbr=on}} tall, with a weight around {{convert|4.7|t|LT ST}}.{{Cite journal |last1=Larramendi |first1=A. |year=2016 |title=Shoulder height, body mass and shape of proboscideans |url=https://www.app.pan.pl/archive/published/app61/app001362014.pdf |journal=Acta Palaeontologica Polonica |volume=61 |doi=10.4202/app.00136.2014 |s2cid=2092950}} Unlike more primitive gomphotheres like Gomphotherium, it lacked lower tusks and had a shortened lower jaw. The upper tusks curved upward and were about {{convert|3.5|m|ft|sigfig=3|abbr=on}} long, and lacked enamel bands. The tall head and robust lower jaw suggest a strong vertical bite. Stegomastodon's third molars molars 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.{{cite journal |last1=Lucas |first1=Spencer G. |last2=Aguilar |first2=Ricardo H. |last3=Speilmann |first3=Justin A. |year=2011 |title=Stegomastodon (Mammalia, Proboscidea) from the Pliocene of Jalisco, Mexico and the species-level taxonomy of Stegomastodon |url=https://www.researchgate.net/publication/283052552 |journal=New Mexico Museum of Natural History and Science Bulletin |volume=53 |pages=517–553}}

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. [https://www.researchgate.net/publication/360672390_THE_LAST_NORTH_AMERICAN_GOMPHOTHERES The last North American gomphotheres]. N Mex Mus Nat Hist Sci. 88:45–58. The earliest species of the genus appeared during the Pliocene (Blancan) around 3-4 million years ago. During the early Irvingtonian, around 1.2 million years ago, Stegomastodon became extinct, which is suggested to be due to competition with the recently arrived mammoths, which were more efficient grazers. Originally, some specimens from Jalisco, Mexico were estimated to date to 28,000 years Before Present,{{Cite journal |last1=Alberdi |first1=María Teresa |last2=Juárez-Woo |first2=Javier |last3=Polaco |first3=Oscar J. |last4=Arroyo-Cabrales |first4=Joaquín |date=2009-02-01 |title=Description of the most complete skeleton of Stegomastodon (Mammalia, Gomphotheriidae) recorded for the Mexican Late Pleistocene |journal=Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen |volume=251 |issue=2 |pages=239–255 |doi=10.1127/0077-7749/2009/0251-0239}} 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.File:Stegomastodon.jpg

Taxonomy

File:Stegomastodon arizonae.jpg

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: S. primitivus, S. mirificus S. aftoniae.{{Cite book |last=Lucas, Spencer G. |title=Taxonomy and evolution of the Plio-Pleistocene proboscidean Stegomastodon in North America. |oclc=798103439}} The South American Stegomastodon fossils have been reassigned to Notiomastodon by the majority of recent authors.{{Cite journal |last1=Mothé |first1=Dimila |last2=Ferretti |first2=Marco P. |last3=Avilla |first3=Leonardo S. |date=June 2019 |title=Running Over the Same Old Ground: Stegomastodon Never Roamed South America |url=http://link.springer.com/10.1007/s10914-017-9392-y |journal=Journal of Mammalian Evolution |language=en |volume=26 |issue=2 |pages=165–177 |doi=10.1007/s10914-017-9392-y |issn=1064-7554|url-access=subscription }}{{Cite journal |last1=Alberdi |first1=María T. |last2=Prado |first2=José L. |date=2022-08-03 |title=Diversity of the fossil gomphotheres from South America |url=https://www.tandfonline.com/doi/full/10.1080/08912963.2022.2067754 |journal=Historical Biology |language=en |volume=34 |issue=8 |pages=1685–1691 |doi=10.1080/08912963.2022.2067754 |bibcode=2022HBio...34.1685A |issn=0891-2963|hdl=10261/271195 |hdl-access=free }} 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:{{cite journal |last1=Mothé |first1=Dimila |last2=dos Santos Avilla |first2=Leonardo |last3=Asevedo |first3=Lidiane |last4=Borges-Silva |first4=Leon |last5=Rosas |first5=Mariane |last6=Labarca-Encina |first6=Rafael |last7=Souberlich |first7=Ricardo |last8=Soibelzon |first8=Esteban |last9=Roman-Carrion |first9=José Luis |last10=Ríos |first10=Sergio D. |last11=Rincon |first11=Ascanio D. |last12=de Oliveira |first12=Gina Cardoso |last13=Lopes |first13=Renato Pereira |title=Sixty years after 'The mastodonts of Brazil': The state of the art of South American proboscideans (Proboscidea, Gomphotheriidae) |url=https://www.sciencedirect.com/science/article/abs/pii/S1040618216302993 |journal=Quaternary International |pages=52–64 |language=en |doi=10.1016/j.quaint.2016.08.028 |date=2 July 2017|volume=443 |bibcode=2017QuInt.443...52M |hdl=11336/48585 |hdl-access=free }}{{clade