[1] Two other species, Megacrodon planus and "Lambdaconus" alius, have been previously assigned to Polymorphis,[2] but are poorly known and may not belong to the genus.

[9][5] Polymorphis is known from the Eocene of Argentina, belonging to the Mustersan South American land mammal age, and is the earliest known macraucheniid.

Roth did not specifically comment on the affinities of Polymorphis or Megacrodon,[12] but suggested that Polyacrodon may have been a predecessor of the toxodonts.

[15] In 1936, George Gaylord Simpson noted that Polyacrodon was not preoccupied by Polyacrodus, making Oroacrodon an unnecessary synonym of it.

[20] In 1982 and 1983, Miguel Soria and Richard Cifelli independently concluded that Polymorphis was a macraucheniid, the most basal member of the clade.

[23] In 1997, McKenna and Bell listed Polyacrodon, Megacrodon, Decaconus, and Periacrodon as synonyms of Polymorphis.

[5] In research by Soria posthumously published in 2001, he viewed Megacrodon and Decaconus as didolodontids and Polymorphis as a cramaucheniine macraucheniid.