Meridiolestida is an extinct clade of mammals known from the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

and

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configu ...

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the souther ...

and possibly

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest co ...

. They represented the dominant group of mammals in South America during the Late Cretaceous. Meridiolestidans were morphologically diverse, containing both small insectivores such as the "sabretooth-squirrel" '' Cronopio,supplementary information
'' as well as the clade Mesungulatoidea/Mesungulatomorpha, which ranged in size from the shrew-sized ''

Reigitherium ''Reigitherium'' was a mammal that lived during the Late Cretaceous, in the (Late Campanian-Maastrichtian). Its fossils have been found in the Los Alamitos and the La Colonia Formations of Argentina. Description The original specimen of ''Reig ...

'' to the dog-sized ''

Peligrotherium ''Peligrotherium'' is an extinct meridiolestidan, and the sole member of the family Peligrotheriidae, from the Paleocene of Patagonia, originally interpreted as a stem-ungulate (though it did co-exist with early meridiungulates). Its remains hav ...

.'' Mesungulatoideans had highly modified dentition with

bunodont The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...

(low and rounded) teeth, and were likely herbivores/omnivores. Cronopio

Meridiolestidans are generally classified within

Cladotheria Cladotheria is a clade (sometimes ranked as a legion) of mammals. It contains modern therian mammals (marsupials and placentals) and several extinct groups, such as the dryolestoids, amphitheriids and peramurids. The clade was named in 19 ...

, more closely related to living

marsupial Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a ...

s and

placental mammals Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...

(

Theria Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. C ...

) than to

monotremes Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals ( Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their bra ...

, barring one study recovering them as the sister taxa to

spalacotheriid Spalacotheriidae is a family of extinct mammals belonging to the paraphyletic group 'Symmetrodonta'. They lasted from the Early Cretaceous to the Campanian in North America, Europe, Asia and North Africa. Spalacotheriids are characterised by hav ...

symmetrodont Symmetrodonta is a group of Mesozoic mammals and mammal-like synapsids characterized by the triangular aspect of the molars when viewed from above, and the absence of a well-developed talonid. The traditional group of 'symmetrodonts' ranges in ...

s". However, more recent studies have stuck to the cladotherian interpretation. Within Cladotheria, they have often been placed in a group called

Dryolestoidea Dryolestida is an extinct order of mammals, primarily and possibly exclusively known from the Jurassic and Cretaceous. They are considered members of the clade Cladotheria, close to the ancestry of therian mammals. It is also believed that they d ...

(or Dryolestida) together with a few other taxa, such as

Dryolestidae Dryolestidae is an extinct family of Mesozoic mammals, known from the Middle Jurassic to the Early Cretaceous of the North Hemisphere. The oldest known member, '' Anthracolestes'', is known from the Middle Jurassic Itat Formation of Western Siber ...

and

Paurodontidae Paurodontidae is a family of Late Jurassic to Early Cretaceous mammals in the order Dryolestida. Remains of paurodontids have been found in the United States, Britain, Portugal, and Tanzania. The group likely represents a paraphyletic group of ba ...

. However, some analyses have found this group to be

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

, with the meridiolestidans being more or less closely related to therian mammals than other dryolestidans are. Meridiolestidans differed from northern dryolestoids in the absence of a parastylar hook on the molariform teeth and the lack of a

Meckelian groove The Meckelian groove (or Meckel's groove, Meckelian fossa, or Meckelian foramen, or Meckelian canal) is an opening in the medial (inner) surface of the mandible (lower jaw) which exposes the Meckelian cartilage.Lakotalestes'' from the Early Cretaceous of North America, originally identified as a dryolestid, was noted in one paper to have a tooth morphology closer to that of meridiolestidans. A possible meridiolestidan is known from a tooth fragment, now lost, found in the

La Meseta Formation The La Meseta Formation is a sedimentary sequence deposited during the Eocene. The formation is found on Seymour Island, Antarctica. Description La Meseta Formation lies unconformably on the Cretaceous Lopez de Bertodano Formation. It is an app ...

from the Eocene of the

Antarctic Peninsula The Antarctic Peninsula, known as O'Higgins Land in Chile and Tierra de San Martín in Argentina, and originally as Graham Land in the United Kingdom and the Palmer Peninsula in the United States, is the northernmost part of mainland Antarctic ...

. The latest surviving meridiolestidan was the mole-like insectivore ''

Necrolestes ''Necrolestes'' ("grave robber" or "thief of the dead") is an extinct genus of mammals, which lived during the Early Miocene in what is now Argentine Patagonia. It is the most recent known genus of Meridiolestida, an extinct group of mammals mo ...

'' from the

Miocene The Miocene ( ) is the first 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 means "less recent" ...

of Patagonia.

Taxa

* '' Amarillodon'' * ''

Austrotriconodon ''Austrotriconodon'' is a mammal genus from the Campanian and Maastrichtian of South America. It currently contains only the type species, ''A. mckennai''. Originally assumed to be a eutriconodont, more recent studies have recovered it as a meri ...

'' * '' Bondesius'' * '' Casamiquelia'' * '' Lakotalestes''? * '' Paraungulatum'' * '' Quirogatherium'' * Cronopioidea ** '' Cronopio'' ** ''

Leonardus ''Leonardus'' is an extinct mammal genus from the Late Cretaceous ( Late Santonian to Maastrichtian) of South America.

''? ** ''

'' * Mesungulatoidea ** ''

'' ** ''

'' **

Mesungulatidae Mesungulatidae is an extinct clade of meridiolestidan dryolestoid mammals from the Late Cretaceous of South America and possibly other Gondwannan landmasses. They are particularly notable for their ecological speciation and large size. Charac ...

*** ''

Coloniatherium ''Coloniatherium'' is a meridiolestid mammal from the Late Cretaceous of Argentina. The single species, ''Coloniatherium cilinskii'', was a large member of the family Mesungulatidae. Taxonomy ''Coloniatherium'' was named in 2009 by Guillermo ...

'' *** '' Mesungulatum'' *** ''

Orretherium ''Orretherium'' (meaning "Five teeth beast"; in part from the Aonikenk language) is a genus of mesungulatid mammal that lived in South America (Chile) during the Late Cretaceous period in what is now the Dorotea Formation The Dorotea Formatio ...

References

{{Taxonbar, from=Q15991176 Cladotheria Late Cretaceous first appearances Miocene extinctions Taxa named by Guillermo W. Rougier