Fossil discovery shows freshwater crayfish measuring 10 inches from 20 million years ago

Researchers have unearthed fossils of massive freshwater crayfish that once scuttled through the lakes of southern New Zealand, 20 million years ago.

These crustaceans, which stretched to nearly 10 inches (25 centimeters), challenge the assumption that ancient aquatic life in this region was limited to smaller species.

The fossils came from jaw fragments that shed light on the animals’ unexpected size.

By analyzing the shape and mineral composition of these remains, paleontologists gained new knowledge about a waterlogged world teeming with more creatures than anyone had expected, including formidable crayfish that appear to have thrived alongside fish, reptiles, and even early mammals.

Parastacids crayfish fossils

These fossils are parastacids, a family of Southern Hemisphere crayfish that includes modern groups found in Australia, South America, and Madagascar.

“We report the first identification of fossil freshwater crayfish based on fragments of the molar ridge from their mandibles,” stated Dr. Trevor Worthy of Flinders University.

The secret behind these remains lies in apatite, a phosphate mineral that hardens part of the crayfish jaw.

That tough surface acted like a built-in shield, allowing small fragments to endure shifts in sediment and water flow across millions of years without being destroyed; this structural advantage is rare among crustaceans.

Revisiting an ancient habitat

Back then, large lakes in southern New Zealand supported fish, reptiles, and birds that dove beneath the surface for prey.

The presence of these bigger crayfish indicates a richer underwater network, where stronger predators and agile scavengers likely crossed paths in an ecosystem that balanced competition and survival.

Fossil jaw structures suggest that at least three distinct crayfish species shared these waters. This contrasts with modern New Zealand, where only two species remain, each confined to separate regions and never overlapping in the same habitat.

The newly-discovered shift in crustacean variety offers fresh clues about how environmental changes shape crayfish evolution.

Researchers also uncovered traces called yabbie buttons, which are hardened deposits used by crayfish to store calcium for shell regrowth.

These round structures were once mistaken for fish teeth, but they are now recognized as key evidence that large crayfish roamed the area and underwent regular molting cycles. This corrects decades of fossil catalog errors.

Piecing together crayfish fossils

These extinct crayfish likely scoured lakebeds for decaying plant material, keeping water quality in check.

Their size suggests they may have been valuable prey for fish and reptiles, fueling a cycle of consumption that drove ecological balance, and they might also have served as opportunistic feeders on smaller organisms whenever the chance arose.

Fossils from the St. Bathans Formation often reveal birds, fish, and occasionally mammals, painting a dynamic portrait of the Early Miocene.

Crayfish fossils, though rarely complete, offer snapshots of how crustaceans adapted to murky waters and shifting temperatures, and this insight ties them to broader climate changes that shaped southern New Zealand’s aquatic fauna.

“It is morphologically distinguished from all other species of Cherax by the presence of marginal mesial dactylar basal spines,” noted Robert McCormack, adding that this highlights the diverse evolutionary paths within the family. Scientists have described separate parastacids with unique adaptations. 

Why does any of this matter?

Having a body length of nearly 10 inches sets these New Zealand crayfish apart from typical freshwater crustaceans found in modern times.

Larger bodies often confer advantages in mating, competition for habitat, and the range of food sources the animal can handle, though they also demand robust resources to support growth and molting.

The crayfish likely influenced nutrient cycling by breaking down organic debris and releasing nutrients back into the water.

Their larger size meant they could tackle sturdier food items, accelerating the decomposition of plants and animals, and this role might have shaped local trophic dynamics.

What happens next?

Paleontologists plan to re-examine existing collections to spot more overlooked crayfish remains. They also hope to find better-preserved specimens that reveal details about the animals’ anatomy, to help shed light on how these ancient giants functioned in their watery domain.

Such discoveries may confirm links between extinct and surviving crayfish species across the Southern Hemisphere.

Modern conservation efforts can benefit from a deeper understanding of extinct crayfish lineages. By tracing how these animals handled earlier climate shifts, researchers may uncover strategies that could help current species cope with warming habitats and changing water flows.

Such knowledge might guide habitat management in regions where crayfish populations face pollution or competition from invasive species.

This discovery shows how small fossil pieces can shift our view of the natural world. With each new find, scientists refine the timeline of crayfish evolution, reminding us that ancient waters once held creatures that challenge our understanding of what we see in streams today.

The study is published in Alcheringa: An Australasian Journal of Palaeontology.

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