Mesotheriids, more commonly known as mesotheres, have been known to science longer than almost any other group of notoungulates. The first mesotheriid, Mesotherium, was named by Serres in 1867. At that time, only the toxodontids Toxodon and Nesodon had been named and described. Mesotherium was named “middle beast” in reference to Serres’ belief that it represented an evolutionary intermediate between rodents – due to the animal’s large upper incisors – and “pachyderms” (i.e., ungulates) – due to its size and proportions.
Despite being one of the first named notoungulates, Mesotherium was known by the name Typotherium for much of the late 1800s and early 1900s, and it was this latter name that was made the basis for the family Typotheriidae and the notoungulate suborder Typotheria. During this interval, “typotheres” only referred to what we now think of as mesotheres, as opposed to also including hegetotheres and interatheres. Subsequent research determined that the name Mesotherium was published slightly earlier than Typotherium (though in the same calendar year) and therefore is the valid name for these mesotheres. Additionally, due to the rules of zoological nomenclature, the family must also be recognized as Mesotheriidae. Since these rules do not apply to names of groups above of the rank of family, the subordinal name Typotheria is still valid.
Mesotheres include the largest of the typothere notoungulates, and Mesotherium cristatum (the geologically youngest species) was the largest of the group; it was about the size of a sheep, well over a meter long (about 4′) and probably weighing 50-60 kg (110-130 lbs) (pers. observ.; see also Bond 1999). Mesotheres are also one of the longest-ranging families of notoungulates; they are first recorded in the earliest Oligocene (Croft et al. 2008) and they survived into the Pleistocene, an interval of nearly 35 million years. Like some other typotheres, mesotheres have greatly enlarged first incisors that are obliquely implanted and meet at their tips. In this respect, they superficially resemble rodents. Unlike rodents, however, mesotheres have enamel on both the labial and lingual surfaces of the incisors; in rodents, enamel is restricted to the labial surface. The lower incisors of mesotheres are more similar to those of rabbits than rodents, as they are straight and inclined forwards rather than curved. Mesotheres are somewhat unusual among “ungulates” in that digging was likely an important behavior, perhaps as one way of securing food (Shockey et al. 2007).
Two subfamilies have traditionally been recognized within the Mesotheriidae: Trachytheriinae and Mesotheriinae. The Trachytheriinae include the earliest representatives of the family and are generally characterized by the lack of derived character states found in later mesotheriids (e.g., no big gap between front and back teeth, no ever-growing cheek teeth). They are characteristic of Deseadan SALMA faunas, having been collected from Patagonia, Uruguay, Bolivia, and Perú, and are one of the most common mammals in the Deseadan fauna of Salla, Bolivia. They are less common in older Tinguirirican SALMA faunas. Trachytheriines may or may not be a monophyletic group (see Billet et al. 2008 for a recent analysis of Trachytherus).
Mesotheres of the subfamily Mesotheriinae are much more diverse and long-ranging than trachytheriines; nine genera and about twice as many species of mesotheriines are currently recognized. Mesotheriines form a monophyletic group (Croft et al. 2004) and are easily identified by their very large upper incisors, the large gaps (diastemata) in their upper and lower tooth rows (resulting from the loss of I2-P2 and i3-p3), and their characteristic trilobed cheek teeth (see below).
The earliest well-characterized mesotheriines come from the late early Miocene of northern Chile (Chucal) and early middle Miocene of Bolivia (Cerdas) (see Townsend and Croft 2010). However, fragmentary remains have recently been found in slightly older (Colhuehuapian) strata in Argentina (Paz et al. 2011). The geologically youngest species are found in Pleistocene (Ensenadan) faunas. Based on the appearance of several early species northern Chile and Bolivia, it has been proposed that the middle latitudes was the area of origin and/or early diversification of this successful group. Although mesotheriids are found in many Miocene and younger faunas, they have never been found further north than southern Peru. Their apparent absence from most of Brazil, Colombia, and Venezuela suggests they were excluded from these areas for ecological reasons.
Classification: Order Notoungulata; Suborder Typotheria
Recognized Genera: Altitypotherium, Caraguatypotherium, Eotypotherium, Eutypotherium, Mesotherium, Microtypotherium, Plesiotypotherium, Pseudotypotherium, Trachytherus, Typotheriopsis
- Billet, G., C. de Muizon, and B. Mamani Quispe. 2008. Late Oligocene mesotheriids (Mammalia, Notoungulata) from Salla and Lacayani (Bolivia): implications for basal mesotheriid phylogeny and distribution. Zoological Journal of the Linnean Society 152:153-200.
- Bond, M. 1999. Quaternary native ungulates of southern South America. A synthesis. Quaternary of South America and Antarctic Peninsula 12:177-206.
- Croft, D.A., J.J. Flynn, and A.R. Wyss. 2004. Notoungulata and Litopterna of the early Miocene Chucal Fauna, northern Chile. Fieldiana: Geology (New Series) 50:1-49.
- Croft, D. A., J. J. Flynn, and A. R. Wyss. 2008. The Tinguiririca Fauna of Chile and the early stages of “modernization” of South American mammal faunas. Arquivos do Museu Nacional, Rio de Janeiro 66:191-211.
- Paz, E. R., A. Kramarz, and M. Bond. 2011. Mesotheriid (Mammalia, Notoungulata) remains from the Colhuehuapian beds (early Miocene) of Chichinales Formation, Río Negro Province, Argentina. Ameghiniana 48:264-269.
- Serres, A. E. R. A. 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.
- Shockey, B.J., D.A. Croft, and F. Anaya. 2007. Analysis of function in the absence of extant functional homologues: a case study using mesotheriid notoungulates (Mammalia). Paleobiology 33:227-247.
- Townsend, B. and D.A. Croft. 2010. Middle Miocene mesotheriine diversity at Cerdas, Bolivia, and a reconsideration of Plesiotypotherium minus. Palaeontologia Electronica 13(1); 1A:1-36.
- Villarroel, C. 1974. Les Mésothérinés (Notoungulata, Mammalia) du Pliocène de Bolivie. Leurs rapports avec ceux D’Argentine. Annales de Paléontologie 60:245-281.