Collections uncovered: Giant fossil shark’s tooth

Among the numerous objects and specimens on display at the Queensland Museum are some with truly extraordinary stories! Today Dr Jonathan Cramb from the Discovery Centre team joins us to share one of his favourite specimens from the museum’s displays.

When sharks ate whales

An isolated tooth of a megatooth shark (Otodus megalodon).
This tooth is huge, but is still smaller than the largest known specimens.
Megatooth sharks were much larger than modern Great White Sharks. The human figure is included to show the size of the sharks; megatooth sharks were already extinct for millions of years before our species evolved.

This is a single fossil tooth of a very large shark called Otodus megalodon. These sharks are commonly called megatooth sharks or simply ‘megalodons’. Fossil teeth of megatooth sharks are found all around the world in marine sedimentary rocks dating to the Neogene period (roughly 2.5-23 million years ago).

This particular fossil tooth was found on the Tweed River in northern New South Wales. This is proof that these enormous sharks would have once cruised the waters off the Gold Coast! Swimmers and surfers need not fear, however, as megatooth sharks have been extinct for over two million years.

This raises a question: why did the giant sharks become extinct? Furthermore, why were there giant sharks in the first place? To answer these questions we need to look beyond the sharks themselves to the world that they inhabited. Megatooth sharks evolved in a very different world to ours today. Earlier in the Cenozoic era (the timespan of the last 66 million years to today) the global climate was in a ‘greenhouse’ phase, meaning that temperatures were warmer across the globe, largely due to high levels of carbon dioxide in the atmosphere. This warm world had highly productive oceans (basically, there was lots of food in the seas). New groups of animals had evolved to forage in the seas at this time: marine mammals such as whales and seals.

Some sharks started to become specialist hunters of marine mammals, and as their prey evolved larger body sizes, so too did the sharks. During the Neogene period (2.58-23.03 million years ago) there was a very large variety of whales, but most were not particularly large, at least in comparison to modern whales. At the end of the Neogene, however, cooling of the global climate caused changes in the distribution of food in the oceans; instead of being plentiful everywhere, it was now scattered about in patches. These changes seem to have caused the extinction of most whales at this time. With the whales went their predators: the megatooth sharks. The giant whales we have today seem to have evolved such large bodies in part because this enables them to travel long distances in search of food. So a fossil that would fit in the palm of your hand is a small part of a big story, about the changing climate of our planet and the evolution of the largest animals of all time!

You can see this amazing fossil for yourself in the Discovery Centre at Queensland Museum, South Bank.