Mystery Molars Lead To Discovery Of Giant Crayfish In Ancient Aotearoa New Zealand

Giant freshwater crayfish, longer and bulkier than today’s species, roamed New Zealand 20 million years ago. They would have made a perfect snack for the crocodiles and trout-sized whitebait that cruised the waters of ancient New Zealand.

This world-first discovery could change the way fossils are identified across the globe.

The researchers – from Flinders University (Adelaide), the University of New South Wales (Sydney), Canterbury Museum and University of Canterbury – have found a new way to identify freshwater crayfish in the fossil record. Crayfish bodies rarely fossilise, but the researchers have discovered that a tiny molar on the crayfish jaw can be preserved. This means fossil hunters around the world can now work out if crayfish existed in prehistoric ecosystems.

Dr Paul Scofield, co-author and Canterbury Museum Senior Curator Natural History, says crayfish were an important creature in ancient environments.

“Crayfish bodies don’t fossilise because they are made from the wrong chemical element. But the molar on the jaw does fossilise as it’s made from a different material, much like mammal teeth,” he says.

“This means we can now say categorically where freshwater crayfish lived in prehistoric times. That will tell us much more about prehistoric ecosystems and how they worked.”

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The researchers identified the prehistoric crayfish by sifting through hundreds of thousands of tiny fossil fragments found near St Bathans in Central Otago, using a microscope to identify each piece.

The crayfish were identified from eight mandible (jaw) fragments, each about 4 mm long. An exciting observation was that there were three different kinds of mandible, indicating three species lived together. New Zealand now has only two species and they live in different parts of the country. The fragments are incredibly rare; one haul of 100 kg of fossil pieces recovered from St Bathans yielded just one fragment. The findings were recently published in the scientific journal Alcheringa.

Co-author Dr Trevor Worthy of Flinders University says they have also identified fossilised yabbie buttons for the first time. These are calcium deposits formed in crayfish stomachs.

“Historically, palaeontologists have often misidentified yabbie buttons as fossilised fish teeth,” says Trevor.

“Fragments like these have been sitting in boxes in museum collections around the world with a question mark over them. We can now say what they really are.”

Co-author Vanesa De Pietri of Te Whare Wānanga o Waitaha University of Canterbury says tiny fossils can reveal much about prehistoric life. “It’s not just about the big sexy bones. It is about the little things too. Every single fragment has a story to tell.”

Prehistoric freshwater crayfish grew up to about 25 cm long, compared to about 8 cm for today’s New Zealand species. The giant crayfish were similar in size and appearance to Australian yabbies or American crawdaddies.

A team of palaeontologists and volunteers have been excavating the large St Bathans site since 2001. The site, which was once at the bottom of a large prehistoric lake, offers the only significant insight into Aotearoa‘s non-marine wildlife from 20 million years ago.

The tiny fossil fragments were recovered by digging up mud and sieving it through a mesh of 1 mm holes. This process washes the mud away, leaving only the fossil fragments. One 2-week dig at St Bathans recovered 100 kg of fossil fragments. These were then sorted by hand and analysed under a microscope. The crayfish mandibles were identified by carefully comparing them to the teeth and bones of a range of present-day creatures, including sea creatures and unusual mammals like the platypus before finally finding a match after dissecting freshwater crayfish.

To cite this publication: Worthy, T. H., Scofield, R. P., Worthy, J. P., De Pietri, V. L., & Archer, M. (2025). Parastacid (Decapoda, Parastacidae) fossil mandible remains from the Early Miocene, New Zealand. Alcheringa: An Australasian Journal of Palaeontology, 1–8. https://doi.org/10.1080/03115518.2025.2488056

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