''Polymorphis'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

of litopterns belonging to the family Macraucheniidae. It lived during 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'', "d ...

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 ...

Taxonomy

The type species of ''Polymorphis'' is ''Polymorphis lechei''. Both the genus and the species were named by Santiago Roth in 1899. The lectotype of ''P. lechei'' is the left half of a mandible with teeth. Two other species, ''Megacrodon planus'' and "''Lambdaconus''" ''alius'', have been previously assigned to ''Polymorphis'', but are poorly known and may not belong to the genus. ''Megacrodon'', ''Polyacrodon'' (which has the junior objective synonym ''Oroacrodon''), ''Decaconus'', and ''Periacrodon'' have been considered synonyms of ''Polymorphis''. ''Polymorphis'' is the most basal member of Macraucheniidae. It is considered to be either part of the paraphyletic subfamily Cramaucheniinae, or in a monotypic subfamily of its own, Polymorphinae. ''Polymorphis'' is known from the

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'', " ...

, belonging to the Mustersan

South American land mammal age The South American land mammal ages (SALMA) establish a geologic timescale for prehistoric South American fauna beginning 64.5 Ma during the Paleocene and continuing through to the Late Pleistocene (0.011 Ma). These periods are referred to as age ...

, and is the earliest known macraucheniid.

Description

This animal is only known from fossils of teeth and a mandible, and it is therefore impossible to faithfully restore its appearance. ''Polymorphis'' was a small to medium sized litoptern, probably with a rather slender build. The mandible was characterized by a still primitive complete dentition ; on the molars, the trigonid was complete ; the paraconid and the metaconid were nearly of equal size and separated by a narrow groove.

History of study

In 1899, Santiago Roth described several new genera and species of mammal from Patagonia. These taxa included ''Polymorphis'' and its type species ''P. lechei'', ''Megacrodon'' and its species ''M. prolixus'' and ''M. planus'', and ''Polyacrodon'' and its species ''P. lanciformis'' and ''P. ligatus''. ''Polymorphis'' and ''Megacrodon'' were both based on lower jaws with teeth, whereas ''Polyacrodon'' was based on upper molars. Roth did not specifically comment on the affinities of ''Polymorphis'' or ''Megacrodon'', but suggested that ''Polyacrodon'' may have been a predecessor of the toxodonts.

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 ...

believed that the name ''Polyacrodon'' was preoccupied by '' Polyacrodus'', a hybodont shark genus named ten years earlier by

Otto Jaekel Otto Max Johannes Jaekel (21 February 1863 – 6 March 1929) was a German paleontologist and geologist. Biography Jaekel was born in Neusalz (Nowa Sól), Prussian Silesia, the son of a builder and the youngest of seven children. He studied at t ...

, and proposed the replacement name ''Oroacrodon'' in 1904, viewing ''P. ligatus'' as the type species. In the same paper, he established a separate genus, ''Periacrodon'', for ''Polyacrodon lanciformis''. Ameghino classified both ''Oroacrodon'' and ''Periacrodon'' as phenacodontid condylarths, but noted the similarity between ''Oroacrodon'' and macraucheniids, and suggested it was close to the origins of the latter group. In 1936,

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 ...

noted that ''Polyacrodon'' was not preoccupied by ''Polyacrodus'', making ''Oroacrodon'' an unnecessary synonym of it. He also synonymized ''Periacrodon'' with ''Decaconus''. In 1948, as part of a review of South American mammal taxonomy, Simpson synonymized ''Megacrodon'' and ''Polymorphis'', selecting ''Polymorphis'', which was based on a better type specimen, as the valid name. However, he kept the type species separate as ''Polymorphis lechei'' and ''Polymorphis planus'', as he felt he lacked sufficient evidence to synonymize them. Simpson synonymized ''Trigonostylops wortmani'' with ''P. lechei'' and ''Didolodus multicuspis'' with ''P. planus''. He also assigned ''Lambdaconus alius'' to ''Polymorphis'' as a third species. Simpson suspected that ''Polyacrodon'' was synonymous with ''Polymorphis'', but did not formally synonymize the two genera. Simpson classified ''Polymorphis'' and ''Polyacrodon'' as members of Proterotheriidae. In 1982 and 1983, Miguel Soria and Richard Cifelli independently concluded that ''Polymorphis'' was a macraucheniid, the most basal member of the clade. Cifelli synonymized ''Polyacrodon'' and ''Polymorphis'' and selected ''Polymorphis'' as the valid name, but regarded their type species as probably distinct. Cifelli regarded ''Polyacrodon planus'' and ''Polyacrodon alius'' as inadequately characterized, but accepted ''Decaconus'' as synonymous with ''Polymorphis''. In 1997, McKenna and Bell listed ''Polyacrodon'', ''Megacrodon'', ''Decaconus'', and ''Periacrodon'' as synonyms of ''Polymorphis''. In research by Soria posthumously published in 2001, he viewed ''Megacrodon'' and ''Decaconus'' as didolodontids and ''Polymorphis'' as a cramaucheniine macraucheniid.

References

Works cited

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Taxonomy

Description

History of study

References

Works cited

Further reading