Mesotheriidae ("Middle Beasts") is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

notoungulate Notoungulata is an extinct order of ungulates that inhabited South America from the early Paleocene to the end of the Pleistocene, living from approximately 61 million to 11,000 years ago. Notoungulates were morphologically diverse, with forms re ...

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s known from the

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

through the

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

. 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 Sheep (: sheep) or domestic sheep (''Ovis aries'') are a domesticated, ruminant mammal typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus '' Ovis'', in everyday usage it almost always refers to d ...

. Shockey et al., 2007 Additionally, the family is characterized by specializations of the

teeth A tooth (: teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...

and

skeleton A skeleton is the structural frame that supports the body of most animals. There are several types of skeletons, including the exoskeleton, which is a rigid outer shell that holds up an organism's shape; the endoskeleton, a rigid internal fra ...

. In the

dentition Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiology ...

, all mesotheriids have ever-growing

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...

s with enamel restricted to the anterior surface, a condition termed gliriform, as it also occurs in

Glires Glires (, Latin ''glīrēs'' 'dormice') is a clade (sometimes ranked as a grandorder) consisting of rodents and lagomorphs ( rabbits, hares, and pikas). The hypothesis that these form a monophyletic group has been long debated based on morph ...

(

rodent Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and Mandible, lower jaws. About 40% of all mammal specie ...

s and

lagomorph The lagomorphs () are the members of the taxonomic order Lagomorpha, of which there are two living families: the Leporidae (rabbits and hares) and the Ochotonidae ( pikas). There are 110 recent species of lagomorph, of which 109 species in t ...

s). The cheek teeth (

premolar The premolars, also called premolar Tooth (human), teeth, or bicuspids, are transitional teeth located between the Canine tooth, canine and Molar (tooth), molar teeth. In humans, there are two premolars per dental terminology#Quadrant, quadrant in ...

s and molars) of mesotheriids are high-crowned (

hypsodont Hypsodont is a pattern of dentition characterized by with high crowns, providing extra material for wear and tear. Some examples of animals with hypsodont dentition are cows and horses; all animals that feed on gritty, fibrous material. The oppos ...

) 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

claw A claw is a curved, pointed appendage found at the end of a toe or finger in most amniotes (mammals, reptiles, birds). Some invertebrates such as beetles and spiders have somewhat similar fine, hooked structures at the end of the leg or Arthro ...

s, presence of a

sesamoid In anatomy, a sesamoid bone () is a bone embedded within a tendon or a muscle. Its name is derived from the Greek word for 'sesame seed', indicating the small size of most sesamoids. Often, these bones form in response to strain, or can be presen ...

bone in the elbow and reinforcement of the

pelvic girdle The hip bone (os coxae, innominate bone, pelvic bone or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three parts: the Ilium (bone) ...

by addition of

vertebra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...

e to the

sacrum The sacrum (: sacra or sacrums), in human anatomy, is a triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part of the pelvic cavity, ...

and fusion of the sacrum and innominate.

Behavior

A biomechanical study of the

of three mesotheriid genera ('' Trachytherus'', '' Plesiotypotherium'', and ''

Mesotherium ''Mesotherium'' ("middle beast") is an extinct genus of mesotheriid, a long-lasting family of superficially rodent-like, burrowing notoungulates from South America. It is one of the youngest notoungulates, spanning the Early-Middle Pleistocene ...

'') spanning the temporal range of the family indicates that most or all mesotheriids were adapted for digging. Mesotheriids likely dug for

root In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often bel ...

s and

tuber Tubers are a type of enlarged structure that plants use as storage organs for nutrients, derived from stems or roots. Tubers help plants perennate (survive winter or dry months), provide energy and nutrients, and are a means of asexual reproduc ...

s and were most similar in their diet and behavior to living

wombat Wombats are short-legged, muscular quadrupedal marsupials of the family Vombatidae that are native to Australia. Living species are about in length with small, stubby tails and weigh between . They are adaptable and habitat tolerant, and are ...

s, although no living group is perfectly analogous. Extensive

burrow file:Chipmunk-burrow (exits).jpg, An eastern chipmunk at the entrance of its burrow A burrow is a hole or tunnel excavated into the ground by an animal to construct a space suitable for habitation or temporary refuge, or as a byproduct of Animal lo ...

ing 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 The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

of South America. Unlike some other families, mesotheriid fossils are not found across the continent. Instead, mesotheriids are most abundant and diverse in

fauna Fauna (: faunae or faunas) is all of the animal life present in a particular region or time. The corresponding terms for plants and fungi are ''flora'' and '' funga'', respectively. Flora, fauna, funga and other forms of life are collectively ...

s from middle

latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...

s in

Bolivia Bolivia, officially the Plurinational State of Bolivia, is a landlocked country located in central South America. The country features diverse geography, including vast Amazonian plains, tropical lowlands, mountains, the Gran Chaco Province, w ...

and

Chile Chile, officially the Republic of Chile, is a country in western South America. It is the southernmost country in the world and the closest to Antarctica, stretching along a narrow strip of land between the Andes, Andes Mountains and the Paci ...

, particularly the

Altiplano The Altiplano (Spanish language, Spanish for "high plain"), Collao (Quechuan languages, Quechua and Aymara language, Aymara: Qullaw, meaning "place of the Qulla people, Qulla") or Andean Plateau, in west-central South America, is the most extens ...

. Flynn et al., 2005 Mesotheriid fossils are rare in high latitude

Patagonia Patagonia () is a geographical region that includes parts of Argentina and Chile at the southern end of South America. The region includes the southern section of the Andes mountain chain with lakes, fjords, temperate rainforests, and glaciers ...

n faunas and absent entirely from

tropical The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

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

, in the form of the type genus, ''

''. Mesotheriidae was one of only three notoungulate families to persist into the

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...

, the others being

Hegetotheriidae Hegetotheriidae is an extinct family of notoungulate mammals known from the Oligocene through the Pliocene of South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a c ...

and

Toxodontidae Toxodontidae is an extinct family of notoungulate mammals, known from the Oligocene to the Holocene (11,000 BP) of South America, with one genus, '' Mixotoxodon'', also known from the Pleistocene of Central America and southern North America (a ...

Classification

Within the order Notoungulata, Mesotheriidae is placed in the

suborder Order () is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and recognized ...

Typotheria Notoungulata is an extinct order of ungulates that inhabited South America from the early Paleocene to the end of the Pleistocene, living from approximately 61 million to 11,000 years ago. Notoungulates were morphologically diverse, with forms re ...

. 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 Oldfieldthomasiidae is an extinct Family (biology), family of Notoungulata, notoungulate mammals known from the Paleocene, Late Paleocene to Eocene, Late Eocene of South America. The family was classified by George Gaylord Simpson in 1945 and a s ...

Interatheriidae Interatheriidae is an extinct family of notoungulate mammals from South America. Interatheriids are known from the Middle Eocene (Mustersan) to the Early Pleistocene (Uquian).McKenna & Bell, 1997Linares, 2004 These animals were principally small- ...

, and

Archaeopithecidae Archaeopithecidae is an extinct family comprising two genera of notoungulate mammals, '' Teratopithecus'' and '' Archaeopithecus'', both known from the Eocene of Argentina.Simpson, 1967McKenna and Bell, 1997 Recent opinion, however, favors inclusion of two additional families in Typotheria,

Archaeohyracidae Archaeohyracidae is an extinct family of Notoungulata, notoungulate mammals known from the Paleocene through the Oligocene of South America.McKenna, Malcolm C., and Bell, Susan K. 1997. ''Classification of Mammals Above the Species Level.'' Colum ...

and

. 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 Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

. 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 ''Trachytherus'' is an extinct genus of mesotheriidae, mesotheriid notoungulata, notoungulate that lived from the Late Oligocene to the Early Miocene in what is now South America. Description ''Trachytherus'' had a relatively low, compact skull ...

. 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

† A dagger, obelisk, or obelus is a typographical mark that usually indicates a footnote if an asterisk has already been used. The symbol is also used to indicate death (of people) or extinction (of species or languages). It is one of the mo ...

Mesotheriidae *Subfamily † FiandraiinaeInclusion tentative following Flynn et al. (2005). **†'' Fiandraia'' (

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

) ***†'' F. romeroi'' *Subfamily †

**†'' Trachytherus'' (?l.

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

-l.

FAD A fad, trend, or craze is any form of collective behavior that develops within a culture, a generation, or social group in which a group of people enthusiastically follow an impulse for a short time period. Fads are objects or behaviors tha ...

following Cerdeño et al. (2006) and Shockey and Flynn (2007).) ***†'' T. alloxus'' ***†'' T. spegazzinianus'' ***†'' T. subandinus'' **†'' Trachytherus'' (?l.

-l.

)

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