Mesotheriidae

Mesotheriidae ("Middle Beasts") is an extinct family of notoungulate mammals known from the Oligocene through the Pleistocene of South America. Mesotheriids were small to medium-sized herbivorous mammals adapted for digging.

Characteristics

Mesotheriids were small to medium-sized notoungulates; larger forms were approximately the size of a sheep. Shockey et al., 2007 Additionally, the family is characterized by specializations of the teeth and skeleton. In the dentition, all mesotheriids have ever-growing incisors with enamel restricted to the anterior surface, a condition termed gliriform, as it also occurs in Glires (rodents and lagomorphs). The cheek teeth ( premolars and molars) of mesotheriids are high-crowned (hypsodont) and in advanced members of the family, the cheek teeth are also ever-growing. Mesotheriid skeletons are heavily built and show features associated with digging in living mammals. In particular, fossorial characteristics of mesotheriids include deeply fissured claws, presence of a sesamoid bone in the elbow and reinforcement of the pelvic girdle by addition of vertebrae to the sacrum and fusion of the sacrum and innominate.

Behavior

A biomechanical study of the skeleton of three mesotheriid genera ('' Trachytherus'', '' Plesiotypotherium'', and '' Mesotherium'') spanning the temporal range of the family indicates that most or all mesotheriids were adapted for digging. Mesotheriids likely dug for roots and tubers and were most similar in their diet and behavior to living wombats, although no living group is perfectly analogous. Extensive burrowing was considered possible but unlikely given the relatively large size of most mesotheriids.

Geographic and temporal distribution

As with almost all other notoungulates, mesotheriids are known only from the Cenozoic of South America. Unlike some other families, mesotheriid fossils are not found across the continent. Instead, mesotheriids are most abundant and diverse in faunas from middle latitudes in Bolivia and Chile, particularly the Altiplano. Flynn et al., 2005 Mesotheriid fossils are rare in high latitude Patagonian faunas and absent entirely from tropical faunas in northern South America.

The earliest secure records of the family come from the late Oligocene, when the family is represented by the genus '' Trachytherus'' from Argentina and Bolivia. The family reached its greatest diversity in the Miocene, and mesotheriids persisted into the middle Pleistocene, in the form of the type genus, '' Mesotherium''. Mesotheriidae was one of only three notoungulate families to persist into the Quaternary, the others being Hegetotheriidae and Toxodontidae.

Classification

Within the order Notoungulata, Mesotheriidae is placed in the suborder Typotheria. Cifelli, 1993 In fact, Typotheria is named for the genus '' Typotherium'', a synonym of ''Mesotherium''. In addition to Mesotheriidae, Typotheria traditionally includes other small bodied notoungulates in the families Oldfieldthomasiidae, Interatheriidae, and Archaeopithecidae. Simpson, 1967McKenna and Bell, 1997 Recent opinion, however, favors inclusion of two additional families in Typotheria, Archaeohyracidae and Hegetotheriidae. Croft and Anaya, 2006 These families have traditionally been placed in a separate suborder, Hegetotheria, but phylogenetic studies indicate that their exclusion would render Typotheria paraphyletic. Billet et al., 2007 Within Typotheria, mesotheriids are more closely related to archaeohyracids and hegetotheriids than to the remaining typotherian families. In fact, both Mesotheriidae and Hegetotheriidae may have originated from within Archaeohyracidae.

McKenna and Bell recognized three subfamilies within Mesotheriidae: Fiandraiinae, Mesotheriinae, and Trachytheriinae. However, Flynn et al. suggested that '' Fiandraia'', the only known fiandraiine, is not a mesotheriid and may represent a toxodontid instead. Of the remaining subfamilies, Trachytheriinae includes only '' Trachytherus'' and may be paraphyletic with respect to Mesotheriinae, which includes more derived genera from the Miocene and later. Reguero and Castro, 2004

Classification of Mesotheriidae:Trachytheriinae following Billet et al. (2008); Mesotheriinae following Anaya and MacFadden (1995) and Shockey et al. (2007). Temporal ranges following McKenna and Bell (1997) except where noted.

Family †Mesotheriidae

*Subfamily † FiandraiinaeInclusion tentative following Flynn et al. (2005).
**†'' Fiandraia'' (Miocene)
***†'' F. romeroi''
*Subfamily † Trachytheriinae
**†'' Trachytherus'' (?l. Eocene-l. Oligocene FAD following Cerdeño et al. (2006) and Shockey and Flynn (2007).)
***†'' T. alloxus''
***†'' T. spegazzinianus''
***†'' T. subandinus''
**†'' Trachytherus'' (?l. Eocene-l. Oligocene)

Notes

References

*Billet, G.A., Muizon, C. de, and Quispe, B.M. 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.
*Billet, G.A., Patterson, B., and Muizon, C. de. 2007. The latest archaeohyracids representatives (Mammalia, Notoungulata) from the Deseadan of Bolivia and Argentina; pp. 39–43 ''in'' E. Díaz-Martínez and I. Rábano (''eds.''), 4th European Meeting on the Palaeontology and Stratigraphy of Latin America. Instituto Geológico y Minero de España, Madrid.

*Cerdeño, E., González Riga, B., and Bordonaro, O. 2006. Primer hallazgo de mamíferos en la Formación Mariño (Mioceno) en Divisadero Largo (Mendoza, Argentina). ''Ameghiniana'' 43:205-214

*Cifelli, R. L. 1993. The phylogeny of the native South American ungulates. pp. 195–216 ''in'' F. S. Szalay, M. J. Novacek and M. C. McKenna (''eds.'') ''Mammal Phylogeny, Volume 2, Placentals''. Springer-Verlag, New York.
*Croft, D.A., and Anaya, F. 2006. A new middle Miocene hegetotheriid (Notoungulata: Typotheria) and a phylogeny of Hegetotheriidae. ''Journal of Vertebrate Paleontology'' 26:387-399.
*Croft, D.A., Flynn, J.J. and Wyss, A.R. 2004. Notoungulata and Litopterna of the Early Miocene Chucal Fauna, Northern Chile. ''Fieldiana Geology'' 50(1):1-52

*Flynn, J. J., Croft, D.A., Charrier, R., Wyss, A.R., Hérail, G., and García, M. 2005. New Mesotheriidae (Mammalia, Notoungulata, Typotheria), geochronology and tectonics of the Caragua area, northernmost Chile. ''Journal of South American Earth Sciences'' 19:55-74.
*McKenna, Malcolm C., and Bell, Susan K. 1997. ''Classification of Mammals Above the Species Level.'' Columbia University Press, New York, 631 pp.
*Reguero, M.A., and Castro, P.V. 2004. Un nuevo Trachytheriinae (Mammalia, †Notoungulata) del Deseadense (Oligoceno tardío) de Patagonia, Argentina: implicancias en la filogenia, biogeografía y bioestratigrafía de los Mesotheriidae. ''Revista Geológica de Chile'' 31:45–64

*Shockey, B.J., Croft, D.A., and Anaya, F. 2007. Analysis of function in the absence of extant functional homologues: a case study using mesotheriid notoungulates (Mammalia). ''Paleobiology'' 33:227-247.
*Shockey, B.J., and Flynn, J.J. 2007. Morphological diversity in the postcranial skeleton of Casamayoran (?middle to late Eocene) Notoungulata and foot posture in notoungulates. ''American Museum Novitates'' 3601:1-26

*Simpson, G.G. 1967. The beginning of the age of mammals in South America. Part 2, Systematics : Notoungulata, concluded (Typotheria, Hegetotheria, Toxodonta, Notoungulata ''incertae sedis''), Astrapotheria, Trigonostylopoidea, Pyrotheria, Xenungulata, Mammalia ''incertae sedis''. ''Bulletin of the American Museum of Natural History'' 137:1-259

External links

Darin Croft's page on Mesotheriidae

{{Taxonbar, from=Q754377

Typotheres
Oligocene mammals
Miocene mammals of South America
Pleistocene mammals
Chattian first appearances
Pleistocene extinctions
Pliocene notoungulates
Prehistoric mammal families