| Mesotheriids, also known as
mesotheres, have been known to
science longer than almost any other group of notoungulates. The first
was named by Serres
1867; only the toxodontid
had been described earlier. Mesotherium
"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
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.
Reconstruction of the head of Mesotherium (Savage and Long, 1986), one of the last-occurring
members of the family. For some
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.
include the largest of the typothere notoungulates; some of the
(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
the earliest Oligocene and
persist through to the Pleistocene, an interval of some 35 million
years. Like some other
mesotheres have greatly enlarged first incisors that are obliquely
and meet at their tips. In this respect, they superficially
rodents. Unlike rodents, however, mesotheriids have enamel on
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). 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 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).
|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.|
(early Oligocene) to Ensenadan
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
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.