The holotype specimen, a tooth fragment, was found by G. Natsume in the Sanuki Formation in the Kazusa Province of Honshu, Japan–an area dating back to the Middle Pleistocene–and described in 1937 by Japanese paleontologist Hikoshichiro Matsumoto.

Matsumoto noted that the teeth of O. paleorca are much larger and have more similar dimensions to the modern killer whale than those of the Pliocene O.

However, unlike the modern killer whale, O. paleorca had a circular tooth root as opposed to an oval, and the pulp extended more towards the back than the front.

[2] Orcinus species, like many other predatory marine lineages, may have fished up the food chain, with the apparently more primitive O. citoniensis able to hunt large fish, and the modern killer whale's ability to hunt large whales.

The marine mammal diversity comprises O. paleorca, an undetermined species of Orcinus, the baleen whale Mizuhoptera, an unidentified oceanic dolphin, the fossil false killer whale Pseudorca yokoyamai, the fossil walrus Odobenus mandanoensis, an undetermined species of Eumetopias sea lion, and the recently extinct Steller's sea cow (Hydrodamalis gigas).