Stirtonia (mammal)

Stirtonia is an extinct genus of New World monkeys from the Middle Miocene (Laventan in the South American land mammal ages; 13.8 to 11.8 Ma). Its remains have been found at the Konzentrat-Lagerstätte of La Venta in the Honda Group of Colombia. Two species have been described, "S. victoriae" and the type species "S. tatacoensis". Synonyms are "Homunculus tatacoensis", described by Ruben Arthur Stirton in 1951 and "Kondous laventicus" by Setoguchi in 1985. The genus is classified in Alouattini as an ancestor to the modern howler monkeys.
Etymology.
The genus "Stirtonia" is named after Ruben Arthur Stirton, who has studied the La Venta primates in the 1950s and the species epithets "tatacoensis" and "victoriae" refer to the Tatacoa Desert and La Victoria Formation respectively, where the fossils have been found.
Description.
The genus is the largest primate found at La Venta, with estimated body masses of "S. tatacoensis" at and of "S. victoriae" at . "Stirtonia tatacoensis" and "S. victoriae" are known by several teeth, a mandible and a maxilla that closely resemble, and are almost indistinguishable from, the living "Alouatta".
Fossil teeth found in the Solimões Formation at the Acre River in the border region of Brazil and Peru may belong to "Stirtonia".
Fossil record.
A lower mandible fossil of "S. tatacoensis" was discovered during fieldwork between 1944 and 1949, in the Honda Group, that has been dated to the Laventan, about 13 Ma.
Upper jaws and other cranial material of the large primate "Stirtonia victoriae" from the Perico Member of the La Dorada Formation, Honda Group were discovered in 1985 and 1986. Based on stratigraphic position, more than below the "Stirtonia tatacoensis" type locality, this was the oldest primate material known until 1987 from Colombia.
Evolution.
The evolutionary split between Atelidae, of which "Stirtonia", and Pitheciidae plus "Callicebus", has been placed at 17.0 million years ago.
Habitat.
The Honda Group, and more precisely the "Monkey Beds", are the richest site for fossil primates in South America. It has been argued that the monkeys of the Honda Group were living in habitat that was in contact with the Amazon and Orinoco Basins, and that La Venta itself was probably seasonally dry forest. From the same level as where "Stirtonia tatacoensis" has been found, also fossils of "Aotus dindensis", "Micodon", "Mohanamico", "Saimiri annectens", "Saimiri fieldsi" and "Cebupithecia" have been uncovered. "Stirtonia" reinforced the notion that leaf-eating was an
enduring and essential aspect of the howler monkey's ecophylogenetic biology.