Rare fossil tooth find sheds a light on Australia’s distant past

Discovery of a rare fossil tooth reveals extinct group of seals from Australia’s deep past.

Take a long walk down the beach on Australia’s southern coastline and the chances are you will encounter a fur seal or sea lion. These charismatic, furry, semi-aquatic residents often haul out to rest on our beaches after long fishing trips at sea. But, if you also keep an eye down on the rocky shoreline, you may encounter something a little more… prehistoric.

An Australian Sea Lion.
An Australian Sea Lion. Photo: Authors

This is exactly what happened in 1998, when the tiny fossil was discovered on the beach at Portland, Victoria. It wasn’t until years later that the true identity of this specimen was discovered: it was the tooth from a 3-million-year-old seal. Yet this was no ordinary seal tooth, this unusual specimen was from a group of seals now long extinct, demonstrating that our current seals were not the first ‘sea dogs’ on the block.

The fossilised seal tooth, which is 3 million years old.
The fossilised seal tooth, which is 3 million years old. Photo: Authors

A rare discovery

A research team from Monash University and Museums Victoria compared the tooth to other pinnipeds: the group that includes earless seals, fur seals, sea lions, and the walrus. They found that the tooth belonged to an extinct earless seal. This means that millions of years before the arrival of eared seals, earless seals similar to the living tropical Monk and Antarctic Leopard seals lived in ancient Australia. The fossil tooth is only the second earless seal fossil ever discovered in Australia.

A Monk seal on the beach in Hawaii. These “earless” seals lack external ear pinnae.
A Monk seal on the beach in Hawaii. These “earless” seals lack external ear pinnae. Photo: Prof Robert Harcourt

This tooth possessed characteristics of the southern group of earless seals (known as the monachines). The morphology of a tooth can tell us a lot about and how seals lived. For example, the unique and bizarre teeth of Leopard and Crabeater seals help them eat everything from Penguins to Plankton. Fur seals and sea lions on the other hand, have pointed teeth that allow them to grip onto fishes and squid.

The teeth of the extinct monachine seal that once bounced along Victoria’s beaches was unlike any pinniped that exists in Australia today. But why, then, aren’t these earless monachines around anymore?

Where did these ancient seals go?

To understand where Australia’s extinct monachines went, you have to put them into a global context. The Portland tooth tells a story similar to what occurred in South Africa and South America in the past. Earless monachine seals used to dominate southern beaches and waters, and then suddenly disappeared, with eared seals replacing them. It may have been because of a change in their habitat.

In the past dramatic shifts in the Earth’s climate altered Australia’s environment. The formation of the polar ice caps caused a drop in sea level, eliminating the beaches that earless seals relied upon to rest. This likely meant they either had to leave Australia’s waters, or that they became locally extinct. Scientists think the elimination of shallow water beaches, and the appearance of more islands, resulted in eared seals colonising southern shorelines from the North Pacific Ocean.

Change in sea levels may have led to the disappearance of earless seals, and the rise of eared seals.
Change in sea levels may have led to the disappearance of earless seals, and the rise of eared seals. Photo: Authors

What does this mean for seals today?

The environmental shift in the past ended the earless seal’s three and a half million-year history in Australia. So what will happen with modern climate change?

As the Earth continues to warm, more polar ice will melt, leading to rising sea levels. This will eventually result in the loss of most of the islands that Australia’s Fur seals and Sea Lions rely upon to rest.

As a result, the same fate may await living species of seal, as our rapidly warming climate relentlessly assaults the world’s polar ice caps.

Find out more about the tooth on Museums Victoria's Collections website

Authors: James P. Rule, Dr David P. Hocking, Dr Erich M.G. Fitzgerald

This article is an edited version of an article that originally appeared in the Monash Lens.

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