Interatherium rodens (Ameghino 1887, 1894; Middle Miocene; 50cm in length) the Interatheridae and Interatherium were long considered members of the Notoungulata, a clade that has broken up in the large reptile tree. Here Interatherium nests at the base of the kangaroos and Toxodon, derived from the more basal marsupials like Didelphis. Interatherium retains several small incisors, but apparently has lost its thumb, unlike kangaroos.

Miocochilius anomopodus (Stirton 1953; Croft 2007; Late Miocene; 30cm in length) originally considered a notoungulate, this cat-sized interathere had long legs and ran on just two digits per manus and pes. A third vestige on the manus did not touch the ground.

Protypotherium antiquum (Ameghino 1882; Miocene, 25mya; 40cm in length) was originally considered a small, sheep-like notoungulate and an interethere. A full set of teeth (11 per side) lined the jaws and all had deep roots. The canine was tiny on this herbivore. The upper rostrum has several fenestrations.

Thomashuxleya externa (Ameghino 1901; Carrillo and Asher 2017; Eocene, 48-40 mya; 0.33m skull length, 1.3 m overall length, Middle Eocene) was named for the famous 19th century paleontologist. Thomashuxleya had four fingers and four toes. Considered a notoungulate, this taxon nests between Interatherium and Hegetotherium. The large premaxillary fangs and flat teeth indicate this was a large herbivore convergent with hippos.

Achaenodon robustus (Cope 1873, Eocene) was tradionally considered an artiodactyl, but here nests with the notoungulate, Thomashuxleya, only larger. The skull was extremely wide with strong jaw muscles filling the space around the narrow cranium and crests.

Periphragnis exauctus (Roth 1899 (genus), Ameghino 1902 (species), Middle Eocene to Early Oligocene; 1.5m long), is traditionally considered very close to Thomashuxleya. It has five hoof-like unguals on all four extremities, distinct from similar marsupials with four digits.

Hegetotherium mirabile (Ameghino 1887; Mid Miocene, 17mya; skull length 10cm) was originally considered a notoungulate, but here nests as a sister to Mesotherium between Interatherium and the toxodons. Tiny third incisors, canines and first premolars distinguish this genus, completing the dental arcade. Note the elevated orbits.

Mesotherium cristatum (originally 'Typotherium'; Serres 1867; Middle Pleistocene; 1.2m long) was originally considered a typotherid, mesotheriid notoungulate, with rodent-like incisors. Here it nests with Hegetotherium in the LRT. Probably fossorial. Note the deep jugals.

Notostylops murinis (Ameghino 1897, Riggs and Patterson 1935; 75cm in estimated length; Eocene; FMNH-P13319) is widely considered a 'notoungulate' a clade that has been split into several parts in the large reptile tree. Here it nests with Mesotherium, an herbivorous marsupial. The jaws narrowed anteriorly.

Ameghino F 1882. Ungulata, Typotheria, Interatheridae. Catalog de la Prov. de Buenos Aires en la Expedicion Cont. Sud-America. March 1882. Boletino Instituto Geologia Argentino, June 1882. Cont. Conocimenient Mamif. Fosil. Repub. Argentina, in Accd. Nac. Cien., Cordoba 6:474-480. 1889.

Ameghino F 1887. Observaciones generales sobre el orden de mamíferos estinguidos sud-americanos llamados toxodontes (Toxodontia) y sinopsis de los géneros y especies hasta ahora conocidos. Anales del Museo de La Plata 1:1-66.

Ameghino F 1894. Enumeration synoptique des especes de mammifères fossiles des formations éocènes de Patagonie. Boletin de la Academia Nacional de Ciencias en Cordoba (Republica Argentina) 13:259-452.

Ameghino F 1897. Les mamiferes crétacés de l´Argentine. Boletín Instituto Geográfico Argentino 18: 405–521.

Ameghino F 1901. Notices préliminaires sur des ongulés nouveaux des terrains crétacés de Patagonie [Preliminary notes on new ungulates from the Cretaceous terrains of Patagonia]. Boletin de la Academia Nacional de Ciencias de Córdoba 16:349-429.

Ameghino F 1902. Cuadro sinoptico de las formaciones sedimentarias terciarias y cretacicas de la Argentina, en relacion con el desarrollo y descendencia de los mamiferos. An. Mus. Nac. Hist. Nat., vol. 8, pp. 1-12.

Carrillo JD and Asher RJ 2017. An exceptionally well-preserved skeleton of Thomashuxleya externa (Mammalia, Notoungulata) from the Eocene of Patagonia, Argentina. Palaentologia Electronica 20.2.34A online PDF

Cope ED 1873. Fourth notice of extinct Vertebrata from the Bridger and the Green River Tertiaries. Paleontological Bulletin 17:1–4.

Croft DA 2007. The Middle Miocene (Laventan) Quebrada Honda wildlife, southern Bolivia and a description of ITS Notoungulates. Palaeontology 50(1):277-303.

Riggs ES and Patterson B 1935. Description of some notoungulates from the Casamayor ("Notostylops) beds of Patagonia. Proceedings of the American Philosophical Society 75(2):163-215.

Roth S 1899. Aviso preliminar sobre mamíferos mesozoicos encontrados en Patagonia. Revista del Museo de La Plata 9: 381–388.

Seoane FD, Cerdeño E 2019. Systematic revision of Hegetotherium and Pachyrukhos (Hegetotheriidae, Notoungulata) and a new phylogenetic analysis of Hegetotheriidae. Journal of Systematic Palaeontology. in press. doi:10.1080/14772019.2018.1545146.

Serres AERA 1867. De l’ostéographie du Mesotherium et de ses affinitiés zoologiques. Comptes Rendus de la Académie des Sciences, Paris 65:6-17, 140-148, 273-278, 429-437, 593-599, 740-748, 841-848.

Stirton RA 1953. A new genus of interatheres from the Miocene of Colombia. University of California Publications in Geological Sciences 29: 265-348.

Serres E 1867. De l’ostéographie du Mesotherium et ses affinités zoologiques. Comptes Rendus de l'Académie des Sciences de Paris: 140-148, t. 65.

Spencer WB 1896. Report on the Work of the Horn Scientific Expedition to Central Australia. Four Volumes. London: Dulau and Co.

wiki/Interatherium
wiki/Mesotherium
wiki/Hegetotherium
wiki/Thomashuxleya
wiki/Protypotherium
wiki/Miocochilius
wiki/Periphragnis
wiki/Notostylops
wiki/Achaenodon