World's biggest whale graveyard found in Indian Ocean off Australia

To come across a "whale fall" — the decomposing body of the marine mammal that has fallen to the sea floor — is a rare occurrence in the deep ocean.

So when a Chinese-led research expedition discovered a graveyard comprising five of these whale falls, along with hundreds of cetacean fossils from past falls, they knew it was globally significant.

On the one hand, that's because of the site's palaeontological record, which included the skull of a previously unknown extinct beaked whale species.

Some of the fossils found on the sea floor of the Diamantina Zone — a 1,200-kilometre-long area of underwater ridges and trenches in the Indian Ocean off Australia — were 5.3 million years old.

On the other hand, the discovery — published today in the journal Nature — is a biological wonder, with specialised communities of invertebrates and other marine creatures, many of them new to science, living on and devouring whale remains.

Together, these traits make up the deepest and largest collection of whale fossils and falls found to date in the ocean.

"These findings reshape our understanding of the limits and biogeography of whale-fall ecosystems and establish some deep-sea floors as a fossil archive for tracing cetacean evolution over geological time," study author and marine biodiversity researcher Xikun Song said.

How researchers found a whale graveyard

The whale graveyard was found during an expedition by the Chinese research vessel Tan Suo Yi Hao in March, 2023.

RV Tan Suo Yi Hao is often used for the Global Trench Exploration and Diving Program, a collaborative endeavour between China's Institute of Deep Sea Science and Engineering (IDSSE) and several countries, including New Zealand.

Chinese research vessel Tan Suo Yi Hao in Sanya. ( Pexels: Greek-China News )

IDSSE deputy director Xiaotong Peng said the program aimed to investigate the biodiversity, ecosystem, pollution, and geological processes in the deepest parts of the ocean.

These deep waters between 6,000m and 11,000m are known as the hadal zone.

"The Diamantina Zone is one of the major hadal trenches in the Indian Ocean," Professor Peng said.

Australia is the closest country to the Diamantina Zone, which sits about 1,600km to the west.

A human-piloted submersible vehicle, Fendouzhe, capable of reaching depths of 11,000m, was used during the expedition to explore the zone.

These depths ranged from 4,200m to 7,002m, with the first fossils found in a geological feature called Dordrecht Deep.

Fragmentary whale skeletal remains, some 5 million years old, colonised by stalked sea anemones, sponges, and sea stars. ( Supplied: Global TREnD/IDSSE )

"The first time we saw the whale fossils, we didn't recognise them as whale remains because they were coated with ferromanganese oxides," deep-sea geologist Peng Zhou, from IDSSE, said.

"Still, we could tell they were something special, so we collected the samples. Surprisingly, we found more and more similar whale fossils in later dives."

Thirty-three dives were conducted with Fendouzhe to map the extent of the fossils and whale falls.

There were 476 fossils of cetaceans found, including a new species.

What species of fossilised whale were found?

One fossil belonged to the baleen whale ( Balaenoptera borealis ), but five other fossil species were extinct beaked whales.

Three of the active whale falls belonged to species of modern beaked whales.

Dr Song, from IDSSE, said the beaked whale species still around today were primarily known from rare stranding events.

"Their [beaked whale] abundance, distribution and ecology remain poorly understood overall," Dr Song said.

The fossilised maxilla of a newly identified extinct whale species, Pterocetus diamantinae, is grabbed by the arm of a submersible. ( Supplied: Global TREnD/IDSSE )

Among the fossils was a skull fragment from an unknown species, which the new study named Pterocetus diamantinae .

Study author and palaeontologist Giovanni Bianucci, from the University of Pisa, said there was enough anatomical difference to distinguish the fossil from other species.

"The discovery improves our understanding of the evolutionary history of beaked whales and helps clarify how this highly specialised group evolved," he said.

Where the upper jaw of newly described, but extinct, beak whale species Pterocetus diamantinae would possibly be in its body. ( Illustration: Giovanni Bianucci )

UWA Oceans Institute marine mammal ecologist Kate Sprogis, who was not involved in the study, said that, with some of the fossils preserved for more than 5 million years, the research offered insights dating back to the Pliocene epoch.

But how did all these whales get there in the first place and stay so well preserved for so long?

Fossil skulls of three beaked whale species brought up from the Diamantina Zone, including a newly named but extinct species, on the left, Pterocetus diamantinae. ( Supplied: Global TREnD/IDSSE )

How to make a whale graveyard

There are several reasons hypothesised…