Mesotheriidae
Encyclopedia
Mesotheriidae is an extinct family
of notoungulate
mammal
s known from the Eocene
through the Pleistocene
of South America
. Mesotheriids were small to medium-sized herbivorous mammals adapted for digging.
and skeleton
. In the dentition
, all mesotheriids have ever-growing incisor
s with enamel
restricted to the anterior surface, a condition termed gliriform, as it also occurs in Glires
(rodent
s and lagomorph
s). The cheek teeth (premolar
s and molar
s) of mesotheriids are high-crowned (hypsodont
) and in advanced members of the family, the cheek teeth are also ever-growing (Shockey et al., 2007). Mesotheriid skeletons are heavily built and show features associated with digging in living mammals. In particular, fossorial characteristics of mesotheriids include deeply fissured claw
s, 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
(Shockey et al., 2007).
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 (Shockey et al., 2007). Shockey et al. concluded that mesotheriids likely dug for root
s and tuber
s and were most similar in their diet and behavior to living wombat
s, although no living group is perfectly analogous. Extensive burrow
ing was considered possible but unlikely given the relatively large size of most mesotheriids.
of South America (McKenna and Bell, 1997). Unlike some other families, mesotheriid fossils are not found across the continent. Instead, mesotheriids are most abundant and diverse in fauna
s from middle latitude
s in Bolivia
and Chile
, particularly the Altiplano
(Flynn et al., 2005). Mesotheriid fossils are rare in high latitude Patagonia
n faunas and absent entirely from tropical
faunas in northern South America.
The earliest potential record of a mesotheriid is ?Trachytherus mendocensis from the late Eocene
or early Oligocene
Divisadero Largo Formation of Mendoza Province
, Argentina
(Shockey et al., 2007), but Cerdeño et al. (2006) have suggested that this specimen may actually derive from early Miocene
rocks that overlie the Divisadero Largo Formation. The earliest secure records of the family come from the late Oligocene, when the family is represented by the genera Anatrachytherus and Trachytherus from Argentina and Bolivia (Reguero and Castro, 2004). The family reached its greatest diversity in the Miocene (Flynn et al., 2005), and mesotheriids persisted into the middle Pleistocene
, in the form of the type genus, Mesotherium
(McKenna and Bell, 1997). Mesotheriidae was one of only three notoungulate families to persist into the Quaternary
, the others being Hegetotheriidae
and Toxodontidae
.
(Cifelli, 1993). In fact, Typotheria is named for the genus Typotherium
, a synonym of Mesotherium (Shockey et al., 2007). In addition to Mesotheriidae, Typotheria traditionally includes other small bodied notoungulates in the families Oldfieldthomasiidae
, Interatheriidae
, and Archaeopithecidae
(Simpson, 1967; McKenna 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 (Cifelli, 1993; Billet et al., 2007). Within Typotheria, both Cifelli and Billet et al. indicate that mesotheriids are more closely related to archaeohyracids and hegetotheriids than to the remaining typotherian families. In fact, Billet et al.'s analysis indicates that both Mesotheriidae and Hegetotheriidae originated from within Archaeohyracidae.
McKenna and Bell (1997) recognized three subfamilies within Mesotheriidae: Fiandraiinae, Mesotheriinae, and Trachytheriinae. However, Flynn et al. (2005) have suggested that Fiandraia, the only known fiandraiine, is not a mesotheriid and may represent a toxodontid
instead. Of the remaining subfamilies, Trachytheriinae includes earlier (Eocene and Oligocene) forms 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:
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
of notoungulate
Notoungulata
Notoungulata is an extinct order of hoofed, sometimes heavy bodied mammalian ungulates which inhabited South America during the Paleocene to Pleistocene, living from approximately 57 Ma to 11,000 years ago.-Taxonomy:...
mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s known from the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
through the Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
of South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
. 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 teethTooth
Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...
and skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
. In the dentition
Dentition
Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, the characteristic arrangement, kind, and number of teeth in a given species at a given age...
, all mesotheriids have ever-growing incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s with enamel
Tooth enamel
Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...
restricted to the anterior surface, a condition termed gliriform, as it also occurs in Glires
Glires
Glires is a clade consisting of rodents and lagomorphs . This hypothesis that these form a monophyletic group has been long debated based on morphological evidence, although recent morphological studies strongly support monophyly of Glires...
(rodent
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
s and lagomorph
Lagomorpha
The lagomorphs are the members of the taxonomic order Lagomorpha, of which there are two living families, the Leporidae , and the Ochotonidae...
s). The cheek teeth (premolar
Premolar
The premolar teeth or bicuspids are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant, making eight premolars total in the mouth. They have at least two cusps. Premolars can be considered as a 'transitional tooth' during chewing, or...
s and molar
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
s) of mesotheriids are high-crowned (hypsodont
Hypsodont
Hypsodont dentition is characterized by high-crowned teeth and enamel which extends past the gum line. This provides extra material for wear and tear. Some examples of animals with hypsodont dentition are cows, horses and deer; all animals that feed on gritty, fibrous material. The opposite...
) and in advanced members of the family, the cheek teeth are also ever-growing (Shockey et al., 2007). Mesotheriid skeletons are heavily built and show features associated with digging in living mammals. In particular, fossorial characteristics of mesotheriids include deeply fissured claw
Claw
A claw is a curved, pointed appendage, found at the end of a toe or finger in most mammals, birds, and some reptiles. However, the word "claw" is also often used in reference to an invertebrate. Somewhat similar fine hooked structures are found in arthropods such as beetles and spiders, at the end...
s, presence of a sesamoid bone in the elbow and reinforcement of the pelvic girdle
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
by addition of vertebrae to the sacrum
Sacrum
In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...
and fusion of the sacrum and innominate
Hip bone
The hip bone, innominate bone or coxal bone is a large, flattened, irregularly shaped bone, constricted in the center and expanded above and below...
(Shockey et al., 2007).
Behavior
A biomechanicalBiomechanics
Biomechanics is the application of mechanical principles to biological systems, such as humans, animals, plants, organs, and cells. Perhaps one of the best definitions was provided by Herbert Hatze in 1974: "Biomechanics is the study of the structure and function of biological systems by means of...
study of the skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
of three mesotheriid genera (Trachytherus, Plesiotypotherium, and Mesotherium
Mesotherium
Mesotherium ,better known by its synonym, "Typotherium," is the type genus of Mesotheriidae, a long-lasting family of superficially rodent-like, burrowing notoungulates from South America. It was first named by Étienne Serres in 1867, and through further finds now contains four species, M....
) spanning the temporal range of the family indicates that most or all mesotheriids were adapted for digging (Shockey et al., 2007). Shockey et al. concluded that mesotheriids likely dug for root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s and tuber
Tuber
Tubers are various types of modified plant structures that are enlarged to store nutrients. They are used by plants to survive the winter or dry months and provide energy and nutrients for regrowth during the next growing season and they are a means of asexual reproduction...
s and were most similar in their diet and behavior to living wombat
Wombat
Wombats are Australian marsupials; they are short-legged, muscular quadrupeds, approximately in length with a short, stubby tail. They are adaptable in their habitat tolerances, and are found in forested, mountainous, and heathland areas of south-eastern Australia, including Tasmania, as well as...
s, although no living group is perfectly analogous. Extensive burrow
Burrow
A burrow is a hole or tunnel dug into the ground by an animal to create a space suitable for habitation, temporary refuge, or as a byproduct of locomotion. Burrows provide a form of shelter against predation and exposure to the elements, so the burrowing way of life is quite popular among the...
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 CenozoicCenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
of South America (McKenna and Bell, 1997). Unlike some other families, mesotheriid fossils are not found across the continent. Instead, mesotheriids are most abundant and diverse in fauna
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...
s from middle latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
s in Bolivia
Bolivia
Bolivia officially known as Plurinational State of Bolivia , is a landlocked country in central South America. It is the poorest country in South America...
and Chile
Chile
Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...
, particularly the Altiplano
Altiplano
The Altiplano , in west-central South America, where the Andes are at their widest, is the most extensive area of high plateau on Earth outside of Tibet...
(Flynn et al., 2005). Mesotheriid fossils are rare in high latitude Patagonia
Patagonia
Patagonia is a region located in Argentina and Chile, integrating the southernmost section of the Andes mountains to the southwest towards the Pacific ocean and from the east of the cordillera to the valleys it follows south through Colorado River towards Carmen de Patagones in the Atlantic Ocean...
n faunas and absent entirely from tropical
Tropics
The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately N and the Tropic of Capricorn in the southern hemisphere at S; these latitudes correspond to the axial tilt of the Earth...
faunas in northern South America.
The earliest potential record of a mesotheriid is ?Trachytherus mendocensis from the late Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
or early Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
Divisadero Largo Formation of Mendoza Province
Mendoza Province
The Province of Mendoza is a province of Argentina, located in the western central part of the country in the Cuyo region. It borders to the north with San Juan, the south with La Pampa and Neuquén, the east with San Luis, and to the west with the republic of Chile; the international limit is...
, Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
(Shockey et al., 2007), but Cerdeño et al. (2006) have suggested that this specimen may actually derive from early Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
rocks that overlie the Divisadero Largo Formation. The earliest secure records of the family come from the late Oligocene, when the family is represented by the genera Anatrachytherus and Trachytherus from Argentina and Bolivia (Reguero and Castro, 2004). The family reached its greatest diversity in the Miocene (Flynn et al., 2005), and mesotheriids persisted into the middle Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
, in the form of the type genus, Mesotherium
Mesotherium
Mesotherium ,better known by its synonym, "Typotherium," is the type genus of Mesotheriidae, a long-lasting family of superficially rodent-like, burrowing notoungulates from South America. It was first named by Étienne Serres in 1867, and through further finds now contains four species, M....
(McKenna and Bell, 1997). Mesotheriidae was one of only three notoungulate families to persist into the Quaternary
Quaternary
The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...
, the others being Hegetotheriidae
Hegetotheriidae
Hegetotheriidae is an extinct family of notoungulate mammals known from the Eocene through the Pleistocene of South America...
and Toxodontidae
Toxodontidae
Toxodontidae is an extinct family of notoungulate mammals known from the Oligocene through the Pleistocene of South America, with one genus, Mixotoxodon, also known from the Pleistocene of Central America. They somewhat resembled rhinoceroses, and had teeth with high crowns and open roots,...
.
Classification
Within the order Notoungulata, Mesotheriidae is placed in the suborder TypotheriaTypotheria
Typotheria is a suborder of the extinct mammalian order Notoungulata and includes five families: Archaeopithecidae, Campanorcidae, Interatheriidae, Mesotheriidae, and Oldfieldthomasiidae...
(Cifelli, 1993). In fact, Typotheria is named for the genus Typotherium
Mesotherium
Mesotherium ,better known by its synonym, "Typotherium," is the type genus of Mesotheriidae, a long-lasting family of superficially rodent-like, burrowing notoungulates from South America. It was first named by Étienne Serres in 1867, and through further finds now contains four species, M....
, a synonym of Mesotherium (Shockey et al., 2007). In addition to Mesotheriidae, Typotheria traditionally includes other small bodied notoungulates in the families Oldfieldthomasiidae
Oldfieldthomasiidae
Oldfieldthomasiidae is an extinct family of notoungulate mammals known from the Paleocene and Eocene of South America....
, Interatheriidae
Interatheriidae
Interatheriidae is an extinct family of notoungulate mammals from South America. Interatheriids are known from the Paleocene or Eocene through the Miocene .-References:...
, and Archaeopithecidae
Archaeopithecidae
Archaeopithecidae is an extinct family comprising two genera of notoungulate mammals, Acropithecus and Archaeopithecus, both known from the early Eocene of South America .-References:...
(Simpson, 1967; McKenna and Bell, 1997). Recent opinion, however, favors inclusion of two additional families in Typotheria, Archaeohyracidae
Archaeohyracidae
Archaeohyracidae is an extinct family comprising four genera of notoungulate mammals known from the Paleocene through the Oligocene of South America....
and Hegetotheriidae
Hegetotheriidae
Hegetotheriidae is an extinct family of notoungulate mammals known from the Eocene through the Pleistocene of South America...
(Croft and Anaya, 2006). These families have traditionally been placed in a separate suborder, Hegetotheria
Hegetotheria
Hegetotheria is a suborder of the extinct mammalian order Notoungulata and includes two families, Archaeohyracidae and Hegetotheriidae. Cifelli indicated that the suborder Typotheria would be paraphyletic if it excluded members of Hegetotheria and he advocated inclusion of the hegetothere families...
, but phylogenetic studies indicate that their exclusion would render Typotheria paraphyletic (Cifelli, 1993; Billet et al., 2007). Within Typotheria, both Cifelli and Billet et al. indicate that mesotheriids are more closely related to archaeohyracids and hegetotheriids than to the remaining typotherian families. In fact, Billet et al.'s analysis indicates that both Mesotheriidae and Hegetotheriidae originated from within Archaeohyracidae.
McKenna and Bell (1997) recognized three subfamilies within Mesotheriidae: Fiandraiinae, Mesotheriinae, and Trachytheriinae. However, Flynn et al. (2005) have suggested that Fiandraia, the only known fiandraiine, is not a mesotheriid and may represent a toxodontid
Toxodontidae
Toxodontidae is an extinct family of notoungulate mammals known from the Oligocene through the Pleistocene of South America, with one genus, Mixotoxodon, also known from the Pleistocene of Central America. They somewhat resembled rhinoceroses, and had teeth with high crowns and open roots,...
instead. Of the remaining subfamilies, Trachytheriinae includes earlier (Eocene and Oligocene) forms and may be paraphyletic with respect to Mesotheriinae, which includes more derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
genera from the Miocene and later (Reguero and Castro, 2004).
Classification of Mesotheriidae:
- Family †ExtinctionIn biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
Mesotheriidae