Meridiolestida is an extinct clade of

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

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

and

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 ...

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 ...

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 (also known as the Antarctic Ocean), it contains the geographic South Pole. ...

. 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 ''Rei ...

'' to the dog-sized ''

Peligrotherium ''Peligrotherium'' is an extinct meridiolestidan mammal from the Paleocene of Patagonia, originally interpreted as a stem-ungulate (though it did co-exist with early meridiungulates). Its remains have been found in the Salamanca Formation.

.'' 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 tooth ...

(low and rounded) teeth, and were likely herbivores/omnivores. 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 1975 ...

, more closely related to living

marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...

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 distinguish ...

(

Theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...

) than to

monotremes Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

, 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 together with

Dryolestida Dryolestida is an extinct order of mammals, known from the Jurassic and Cretaceous. They are considered basal members of the clade Cladotheria, close to the ancestry of therian mammals. It is also believed that they developed a fully mammalian ...

, a group of mammals primarily known from the Jurassic and Early Cretaceous of the Northern Hemisphere. However, some analyses have found this group to be

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 ...

, with the meridiolestidans being more or less closely related to therian mammals than dryolestidans are. Meridiolestidans differ from dryolestidans 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 much of the Paleogene on Seymour Island off the coast of the Antarctic Peninsula. It is noted for its fossils, which include both marine organisms and the only terrestrial vertebr ...

from the Eocene of the

Antarctic Peninsula The Antarctic Peninsula, known as O'Higgins Land in Chile and Tierra de San Martin 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 Antarctica. ...

. The latest surviving meridiolestidan was the mole-like burrowing 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 mor ...

'' from the

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 ...

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 merid ...

'' * '' Bondesius'' * '' Casamiquelia'' * ''

Groebertherium ''Groebertherium'' is a genus of dryolestoid mammal from the Late Cretaceous Los Alamitos Formation, Los Alamitos and Allen Formations of Argentina. It is not closely related to other contemporary dryolestoids, all of which are part of the clade ...

''? * '' Lakotalestes''? * '' Paraungulatum'' * '' Quirogatherium'' * '' Solanutherium'' * Cronopioidea ** '' Cronopio'' ** ''

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

'' * Mesungulatoidea ** ''

'' ** ''

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 R ...

'' ** '' 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 Format ...

'' Cladogram after Lasseron et al. 2022:

References

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