A large, flesh coloured creature swims deep in the ocean.

Radiodonts were one of the largest creatures to live 500 million years ago and dominated the oceans. They are classed as arthropods, which means they belong to the same group as spiders, insects and crustaceans. © Katrina Kenny

Fossils of sea monster eyes shed light on animal evolution

Scientists have unearthed fossilised eyes of an extinct group of sea creature called radiodonts. The findings suggest vision played an important role in an evolutionary race that has shaped today's ecosystems.

Radiodonts are an extinct group of sea creatures that were once widespread throughout the oceans.

Information about these arthropods - animals with jointed legs and no backbones - is incomplete but their fossils show they thrived during the Cambrian Period about 500 million years ago.

The Cambrian explosion was a crucial event in the history of evolution when dozens of major animal groups first appeared in the fossil record.

Museum palaeontologist Greg Edgecombe says, 'Over about 20 million years in the Cambrian, we see the appearance of arthropods, segmented worms, shelly animals like molluscs and even the earliest vertebrates. The Cambrian explosion marks the rise of animal-dominated marine ecosystems.'    

It was during this time that radiodonts flourished.

What did radiodonts look like?

The first radiodont fossils were discovered over a century ago. For a long time, researchers tried to picture what these ancient sea monsters looked like. They came up with an array of descriptions that likened the creatures to jellyfish, sea cucumbers and shrimps.

Over the past few decades, scientists have unearthed many more radiodont fossils, including several well-preserved full bodies. These have shed light on the anatomy of the marine animals, as well as their diversity, feeding habits and possible lifestyle.

Radiodonts mostly resembled elongated prawns with conical teeth arranged in a circle around the mouth. But each species varied in many ways.

Some were powerful apex predators such as Anomalocaris, which grew to over 50 centimetres and used its strong, spiny head appendages to catch its prey.

Others were gentle giants such as Aegirocassis, which grew to around two metres long and used the fringe-like spikes on its appendage to filter plankton.

A large, grey radiodont swims in water.

Radiodonts obtained their name from the Latin word 'radius' due to the ring of pointy teeth in a circular form around the mouth © Katrina Kenny

New samples of radiodont eyes found in South Australia

While fossils found over the past few decades have given scientists knowledge of these mysterious creatures, there is still little known about the eyes, as they rarely fossilise well.

Through 12 years of excavations, Greg and colleagues unearthed specimens of two different kinds of radiodont eyes in a Cambrian fossil site in South Australia. This helped them better understand radiodont vision and how it impacted their feeding habits.

Greg says, 'The Australian material is unique among the dozens of occurrences of radiodonts around the world in the Cambrian Period, because it's the only place that the visual surface of the eye is preserved.

'In other sites in China, Canada, the USA and elsewhere, only the outline of the eyes is known but there's no information on their lenses.'

The new fossils showed radiodonts evolved to have large, complex eyes consisting of thousands of lenses - more than almost all living species of their arthropod relatives.

Like all radiodonts, the two species had a large pair of jointed appendages at the front of the head with which they captured their food, but besides that, they vary largely from each other.

One species had robust barbed spines along the margin of its feeding appendage, which is typical of a predator. The other had slender, almost frilly spines, as seen in arthropods that filter their food from the water.

This shows the different ways these creatures fed which are associated with the differences in the eyes: the predator had its eyes on stalks attached to the head, whereas the filter feeder had its eyes closer to the head, surrounded by a rim of tough cuticle.

The predator also had uniform lenses across the surface of its compound eye, while the filter feeder had a zone of enlarged lenses, which in living arthropods are associated with especially acute vision and enhanced light capture.

'We never knew that the eyes of radiodonts could attach to the head any way other than by stalks,' says Greg. 'Based on how the eyes would have sat in the head, we infer that the acute zone looked upwards as the radiodont swam in the dim light of deep water.'

A fossil of a radiodont claw in black and white.

A fossil of a grasping claw from an Anomalocaris found in the famous Burgess Shale in British Columbia, Canada in 1892. Image by James St John (CC BY 2.0) via Wikimedia Commons

What do the new fossils tell us?

The new samples showed how the eyes changed as the animal grew. The lenses formed at the margin of the eyes grew bigger and increased in numbers in large specimens. This is also the case for many living arthropods.

The findings reveal that the way compound eyes grow has been consistent for more than 500 million years. This suggests that vision played an important role at a critical time in evolution.

Whether as predators or filter feeders, radiodonts placed selective pressure on prey species, forcing them to adapt or face extinction. This has shaped today's biodiversity and ecosystems.

The new information will allow scientists to revisit how these animals are classified. In the past, it had been based mostly on their tough, spiny appendages which are sometimes the only body parts found in a fossil site.

Now that different kinds of eyes, mouthparts and head plates have been found, there is more evidence to produce a more informed classification.