''Anisolambda'' is an extinct genus of

litoptern Litopterna (from grc, λῑτή πτέρνα "smooth heel") is an extinct order of fossil hoofed mammals from the Cenozoic era. The order is one of the five great orders of South American ungulates that were endemic to the continent, until the ...

. It lived from the Late

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...

to the Middle

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

in what is now

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, t ...

Description

This animal is mostly known from fossils of its

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The ...

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bon ...

and

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, te ...

, and it is therefore difficult to speculate on its appearance. Its molars were primitive in shape, and closely resembled those of the enigmatic ''

Didolodus ''Didolodus'' is an extinct genus of mammals from Middle Eocene Argentina. It is an ungulate mammal of uncertain affinities, possibly related to Litopterna, though this is uncertain due to the lack of reliable post-cranial remains, and for now r ...

''. They were distinguished from the latter by the presence of a strong paraconid, in an internal position, almost identical in size to the metaconid, and separated from the latter by a narrow indentation. ''Anisolambda'' may have been similar to more recent genera of

Proterotheriidae Proterotheriidae is an extinct family of fossil ungulates from the Cenozoic era that displays toe reduction. Despite resembling primitive, small horses, they were only distantly related to them, and instead belonged to the native South America ...

, such as '' Diadiaphorus'' or ''

Proterotherium ''Proterotherium'' (meaning "first beast") is an extinct genus of litoptern mammal of the family Proterotheriidae that lived during the Late Miocene of Argentina and Chile. Fossils of this genus have been found in the Ituzaingó Formation of Ar ...

'', but without the characteristic limb specializations of the latter genera.

Classification

The genus ''Anisolambda'' was first described in 1901 by

Florentino Ameghino Florentino Ameghino (born Giovanni Battista Fiorino Giuseppe Ameghino September 19, 1853 – August 6, 1911) was an Argentine naturalist, paleontologist, anthropologist and zoologist, whose fossil discoveries on the Argentine Pampas, especiall ...

, based on a mandible with teeth from the Eocene of

. Ameghino latter described fossils of the maxilla, that he attributed to the genus ''Josepholeidya''. Subsequent discoveries in slightly older terrains from the Late Paleocene of

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

, including associated maxilla and mandible (which is now thought to belong to a different genus, ''

Paranisolambda ''Paranisolambda'' is an extinct genus of proterotheriid litopterns from the Early to Middle Eocene of Brazil. Fossils of ''Paranisolambda'' have been recovered from the Itaboraí Formation in the Brazilian state of Rio de Janeiro Rio de J ...

''), have allowed the researchers to understand that ''Anisolambda'' and ''Josepholeidya'' belonged to the same genus. The type species is ''Anisolambda fissidens'', from the Early and Middle Eocene of Argentina. ''Anisolambda amel'' was described by

George Gaylord Simpson George Gaylord Simpson (June 16, 1902 – October 6, 1984) was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern synthesis, contributing '' Tempo ...

in 1948. ''Anisolambda'' is one of the earliest litopterns, and its systematic position isn't clear due to its primitive dental characteristics. It is supposed that ''Anisolambda'' was one of the earliest and most basal members of

, a clade of small to medium-sized litopterns, which in the course of their evolution developed

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million ...

-like forms, specially regarding the specializations of their legs. ''Anisolambda'' belonged to a separate subfamily,

Anisolambdinae Proterotheriidae is an extinct family of fossil ungulates from the Cenozoic era that displays toe reduction. Despite resembling primitive, small horses, they were only distantly related to them, and instead belonged to the native South American ...

, including one the most basal proterotheres ; sometimes this clade is elevated to the rank of family,

Anisolambdidae Proterotheriidae is an extinct family of fossil ungulates from the Cenozoic era that displays toe reduction. Despite resembling primitive, small horses, they were only distantly related to them, and instead belonged to the native South American ...

References

* G. G. Simpson. 1967. The beginning of the age of mammals in South America. Part II. Bulletin of the American Museum of Natural History 137:1-260 * R. Cifelli. 1983. The origin and affinities of the South American Condylarthra and early Tertiary Litopterna (Mammalia). American Museum Novitates 2772:1-49 * M. O. Woodburne, F. J. Goin, M. S. Raigemborn, M. Heizler, J. N. Gelfo and E. V. Oliveira. 2014. Revised timing of the South American early Paleogene land mammal ages. Journal of South American Earth Sciences 54:109-119 {{Taxonbar, from=Q60977294 Proterotheriids Paleocene mammals of South America Eocene mammals of South America Paleogene Argentina Fossils of Argentina Fossil taxa described in 1901 Taxa named by Florentino Ameghino Prehistoric placental genera Golfo San Jorge Basin Sarmiento Formation