''Anisolambda'' is an extinct genus of

litoptern Litopterna (from "smooth heel") is an extinct order of South American native ungulates that lived from the Paleocene to the Pleistocene-Holocene around 62.5 million to 12,000 years ago (or possibly as late as 3,500 years ago), and were also pres ...

. It lived from the Late

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

to the Middle

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 ...

in what is now

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

Description

This animal is mostly known from fossils of its

maxilla In vertebrates, the maxilla (: maxillae ) 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 two maxil ...

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

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, tear ...

, and it is therefore difficult to speculate on its appearance. Its molars were primitive in shape, and closely resembled those of the enigmatic '' Didolodus''. 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 extinction, extinct family of Litopterna, litoptern ungulates known from the Eocene-Late Pleistocene of South America. Members of the group were small-medium sized cursorial Herbivore, herbivores with brachydont Tooth, teet ...

, such as ''

Diadiaphorus ''Diadiaphorus'' is an extinct genus of litoptern mammal from the Miocene of Argentina ( Ituzaingó, Pinturas, Chiquimil and Santa Cruz Formations) and Bolivia ( Nazareno Formation), South America. Description ''Diadiaphorus'' closely rese ...

'' 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 Argentina, officially the Argentine Republic, is a country in the southern ...

'', 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, especial ...

, 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, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

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

-like forms, specially regarding the specializations of their legs. ''Anisolambda'' belonged to a separate subfamily, Anisolambdinae, including one the most basal proterotheres ; sometimes this clade is elevated to the rank of family, Anisolambdidae.

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