Mesotheriids, also 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; only the toxodontid notoungulates Toxodon and Nesodon had been described earlier. Mesotherium means "middle beast" and was named in reference to Serres' belief that it represented an intermediate between rodents (due to the animal's enlarged upper incisors) and "pachyderms" (i.e., ungulates, due to its size and proportions). Needless to say, it is now known that the closest relatives of mesotheriids are other notoungulates and not rodents or various other "ungulate" groups.
     For much of its early history, however, Mesotherium was known instead by the name Typotherium, and this genus formed the basis for the family Typotheriidae and the order (or sub-order) Typotheria. During this interval (late 1800s and early 1900s) "typotheres" referred to what we now think of as mesotheres. Subsequent research has determined that Mesotherium is the valid name - having been published slightly earlier in the same year - and due to the rules of nomenclature, Mesotheriidae is therefore the valid name of the family. Since these rules do not apply to names of groups above of the rank of family, Typotheria is still valid (and in use), but refers to a broader group of rodent-like notoungulates.
MesotheriumEutypotherium superans 
Left: Reconstruction of the head of Mesotherium (Savage and Long, 1986), one of the last-occurring members of the family. For some reason, the upper teeth are drawn unusually long and would not have appeared this way in life. Right: lateral view of the skull of Eutypotherium superans, a middle Miocene mesotheriid.
   Mesotheriids include the largest of the typothere notoungulates; some of the earliest (e.g., Trachytherus) and latest forms (e.g., Mesotherium ) likely weighed more than 120 lbs, the size of a large sheep. Mesotheriids are also one of the longest-ranging families of notoungulates; they are first recorded in the earliest Oligocene and persist through to the Pleistocene, an interval of some 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, mesotheriids 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. Mesotheres are somewhat unusual among "ungulates" in that they were probably fossorial; digging - possibly for food - was likely an important behavior.
    Two subfamilies have traditionally been recognized within the Mesotheriidae: Trachytheriinae (the branch in blue in the cladogram to the right) and Mesotheriinae (the branches in green).
Mesotheriid PhylogenyThe 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 present but less common in older Tinguirirican SALMA faunas. A mesotheriid from the Divisadero Largo Fauna of Argentina represented by fragmentary material has long been considered the earliest trachytheriine, but recent investigations in this region of Mendoza, Argentina have raised the possibility that it is actually a mesotheriine from younger (Miocene) strata. Trachytheriines may or may not be a monophyletic group.
    Mesotheriines are much more diverse and long-ranging than trachytheriines; at last count, nine genera and about 20 species were currently recognized. Mesotheriines form a monophyletic group (see cladogram from 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; see photo above), and their characteristic trilobed cheek teeth (see below).
Mesotheriine upper teethMesothere Musculoskeletal Reconstruction
Left: Illustrations of right upper first and second molars of late Miocene (Eutypotherium), Pliocene (Pseudotypotherium), and Pleistocene (Mesotherium) mesotheriines (modified from Villarroel, 1974). Note that each tooth is characterized by three lobes, giving it the appearance of an "E" (or a "3" in the case of left upper molars). Note also that the degree of overlap between the molars (i.e., imbrication) increases in progressively younger mesotheriines. Right: Reconstruction of the musculature of the Oligocene mesotheriid Trachytherus based on material from Salla, Bolivia.
   The earliest mesotheriines occur in Santacrucian and slightly younger faunas in northern Chile and Bolivia and the latest occurring species occur in Pleistocene Ensenadan faunas. (Unpublished mesotheriines may occur in the Colhuehuapian of Argentina.) 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 Perú. Their apparent absence from Brazil, Colombia, and Venezuela suggests they were excluded from these areas for paleoecological reasons.
Stratigraphic Range: Tinguirirican (early Oligocene) to Ensenadan (Pleistocene)
Geographic Distribution: most of Argentina, central and northern Chile, western Bolivia, southern Perú, Uruguay
Taxonomy: Order Notoungulata: Suborder Typotheria: Family Mesotheriidae
Representative Taxa: Trachytherus, Altitypotherium, Eutypotherium, Typotheriopsis, Pseudotypotherium, Mesotherium
Selected References:

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.

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.
Cerdeño, E., and C.I. Montalvo. 2001. Los Mesotheriinae (Mesotheriidae, Notoungulata) del Mioceno
Superior de La Pampa, Argentina. Revista Española de Paleontología 16:63–75.

Francis, J.C. 1965. Los géneros de la subfamilia Mesotheriinae (Typotheria, Notoungulata) de la República Argentina. Boletín del Laboratorio de Paleontología de Vertebrados 1:7-31.

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.

 This page was last updated on July 22, 2010.