[1] The type species, Ontocetus emmonsi, was named by Joseph Leidy in 1859 on the basis of a single tusk-like tooth (USNM 329064) collected by Ebenezer Emmons from the early Pliocene (Zanclean) Yorktown Formation of North Carolina.

[2] In the meantime, marine mammals fossils were being unearthed in Neogene deposits in the vicinity of Antwerp, Belgium as well as Suffolk, England.

[8][9][10][11] In 2008, all specimens of Pliocene odobenids from the North Atlantic region were reviewed following the rediscovery of the Ontocetus emmonsi holotype in the 1990s.

T. huxleyi, A. cretsii, A. antwerpiensis, A. antverpiensis, A. africanum, and P. alleni were declared junior synonyms of O. emmonsi based on comparisons with USNM 329064.

[13] O. oxymycterus was named by Remington Kellogg in 1925 on the basis of USNM 10923, collected from the middle Miocene (Serravallian) Monterey Formation in Santa Barbara, California.