Temporal range: Late Paleocene–Early Eocene
Ameghino 1901, p. 357
Henricosborniidae is a group of primitive notoungulates assigned to the suborder Notioprogonia together with Notostylopidae, not because these two families share any derived features, but because they do not clearly belong to any other clade. The henricosborniids are only known from the late Paleocene and early Eocene (Itaboraian-Casamayoran SALMA, ), making them slightly older and more primitive than the notostylopids. The henricosborniid dentition is the most generalized and primitive of all notoungulates, and they are believed to be near the source of all notoungulates. They have low teeth crowns and the dental formula 18.104.22.168.
George Gaylord Simpson noted that many of the mammals that Ameghino had, and Simpson himself did, describe from the Río Chico Formation, on the Atlantic coast of Patagonia, are very difficult to distinguish from each other, both on genus and family levels. Often these mammalian groups, including the henricosborniids, are only known from one or a few isolated teeth found only at a single location. Each one of these teeth most likely represents a separate species but these species can not be properly described unless some comparable material is found. McKenna & Bell 1997 synonymized ("= or including") many of the species named by Ameghino and others. Among other things, McKenna & Bell included Postpithecus in Othneilmarshia.
Simpsonotus, with two species found in the Mealla Formation in the Jujuy Province in north-western Argentina, are the most complete henricosborniids found. Based on features in their auditory region, they might be the most primitive notoungulates discovered. On the other hand, if these primitive features are shared by the other members of the family, Pascual, Vucetich & Fernández 1978 argued that the family should perhaps be recognized as a distinct suborder.
- Ameghino, Florentino (1901). "Notices préliminaires sur des ongulés nouveaux des terrains crétacés de Patagonie". Boletin de la Academia Nacional de Ciencias en Córdoba 16: 349–426. Retrieved January 2014.
- Ameghino, Florentino (1902). "Notices préliminaires sur des mammifères nouveaux des terrains crétacés de Patagonie". Boletin de la Academia Nacional de Ciencias en Córdoba 17: 5–70. Retrieved January 2014.
- Ameghino, Florentino (1904). "Nuevas especies de mamiferos Cretaceos y Terciarios de la Republica Argentina". Anales de la Sociedad Científica Argentina. 58–59: 35–41. Retrieved January 2014.
- McKenna, Malcolm C.; Bell, Susan K. (1997). Classification of Mammals Above the Species Level. New York: Columbia University Press. ISBN 0-231-11013-8. Retrieved January 2014.
- Pascual, Rosendo; Vucetich, María Guiomar; Fernández, Jorge (1978). "Los primeros mamiferos (notoungulata, henricosborniidae) de la Formacion Mealla (Grupo Salta, subgrupo Santa Barbara) : sus implicancias filogeneticas, taxonomicas y cronologicas". Ameghiniana revista de la Asociación Paleontólogica Argentina 15 (3–4): 366–390. OCLC 718305590. Retrieved January 2014.
- Rose, Kenneth David (2006). The beginning of the age of mammals. Baltimore: JHU Press. ISBN 0801884721.
- Simpson, George Gaylord (1935a). "Descriptions of the oldest known South American mammals, from the Río Chico Formation". American Museum Novitates 793: 1–21. Retrieved January 2014. Lay summary (January 2014).
- Simpson, George Gaylord (1935b). "Occurrence and Relationships of the Río Chico fauna of Patagonia". American Museum Novitates 818: 1–21. Retrieved January 2014.