A whale skull discovered just over 50 years ago in Osaka City is now thought to be the world's first known trace of an Eden's whale, dating from between 4,000 and 8,800 years ago.

The fossilized remains were reclassified in a study published recently in a paleontology journal by co-authors Yoshihiro Tanaka, curator of the Osaka Museum of Natural History, and Hiroyuki Taruno, a former curator of the same institution. The skull was originally thought to be of a Minke whale.

"The specimen adds a chronologically and geographically new record to the not well-known species" and suggests its modern distribution was established millennia ago, the study says.

During the time frame in which the whale is thought to have lived, much of the location of modern-day Osaka City was underwater due to the higher levels of Osaka Bay and, to its east, a historical feature known as Kawachi Bay.

The fossil of the Eden's whale (Balaenoptera edeni) was found at a depth of roughly 14 meters during underground construction in the eastern part of the city in 1966. Researchers had concluded in 1976 that it came from an ancient Minke whale.

Minke and Eden's whales are two types of baleen whale still existing around the world today, though the latter are less common. Japan has only in 12 previous instances been able to confirm a specimen as an Eden's whale through DNA and other testing, and only half of these cases include bone samples stored by one of the country's research institutions.

Tanaka and Taruno made the reassessment by analyzing the morphology of the skull, in particular the broad rostrum and the shape of rear cranial bones, and comparing their observations with the results of recent studies on baleen whale taxonomy.

In comments to Kyodo News, Tanaka noted that various other whale fossils have been unearthed in Osaka in past decades.

"We want to shed light on what kinds of whales were swimming in Osaka Bay in ancient times," he said.