Cambrian Deep-Sea Arthropods Had Complex Compound Eyes
A team of paleontologists from Australia and the United Kingdom has found that ancient deep-sea creatures called radiodonts developed sophisticated eyes over 500 million years ago (Cambrian period), with some specially adapted to the dim light of deep water.
Radiodonts (meaning ‘radiating teeth’) are a group of arthropods that dominated the oceans around 500 million years ago.
The many radiodont species share a similar body layout comprising of a head with a pair of large, segmented appendages for capturing prey, a circular mouth with serrated teeth, and a squid-like body.
It now seems likely that some lived at depths down to 1,000 m (3,281 feet) and had developed large, complex eyes to compensate for the lack of light in this extreme environment.
“When complex visual systems arose, animals could better sense their surroundings, that may have fuelled an evolutionary arms race between predators and prey,” said lead author Professor John Paterson, a researcher in the Palaeoscience Research Centre at the University of New England.
“Once established, vision became a driving force in evolution and helped shape the biodiversity and ecological interactions we see today.”
In 2011, Professor Paterson and colleagues documented isolated eye specimens of up to 1 cm (0.4 inches) in diameter from the 515-million-year-old Emu Bay Shale on Kangaroo Island, but they were unable to assign them to a known arthropod species.
They also described the stalked eyes of Anomalocaris, a top predator up to 1 m (3.3 feet) in length, in great detail.
“The Emu Bay Shale is the only place in the world that preserves eyes with lenses of Cambrian radiodonts,” said Dr. Diego García-Bellido, a researcher in the School of Biological Sciences at the University of Adelaide and South Australian Museum.
“The more than thirty specimens of eyes we now have, has shed new light on the ecology, behavior and evolution of these, the largest animals alive 500 million years ago.”
In the new study, the researchers identified the owner of 515-million-year-old eye specimens: ‘Anomalocaris’ briggsi, representing a new genus that is yet to be formally named.
“We discovered much larger specimens of these eyes of up to 4 cm (1.6 inches) in diameter that possess a distinctive ‘acute zone,’ which is a region of enlarged lenses in the center of the eye’s surface that enhances light capture and resolution,” Professor Paterson said.
The large lenses of ‘Anomalocaris’ briggsi suggest that it could see in very dim light at depth, similar to amphipod crustaceans, a type of prawn-like creature that exists today.
The frilly spines on its appendages filtered plankton that it detected by looking upwards.
“These specimens have shown us that the animals’ feeding strategies previously indicated by the appendages — either for capturing or filtering prey — are paralleled by differences in the eyes,” said Dr. Greg Edgecombe, a paleontologist in the Department of Earth Sciences at the Natural History Museum, London.
“The predatory radiodont in our samples has the eyes attached to the head on stalks but one that filter feeds has them at the surface of the head.”
“The more we learn about these animals the more diverse their body plan and ecology is turning out to be.”
“The new samples also show how the eyes changed as the animal grew. The lenses formed at the margin of the eyes, growing bigger and increasing in numbers in large specimens — just as in many living arthropods. The way compound eyes grow has been consistent for more than 500 million years.”
The scientists also found that the Anomalocaris eyes described in 2011 are likely from a species called Anomalocaris aff. canadensis.
“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 where the visual surface of the eye is preserved,” Dr. Edgecombe said.
“In other sites in China, Canada, the United States and elsewhere, only the outline of the eyes is known but there’s no information on their lenses.”
The study was published in the journal Sciences Advances.
John R. Paterson et al. 2020. Disparate compound eyes of Cambrian radiodonts reveal their developmental growth mode and diverse visual ecology. Science Advances 6 (49): eabc6721; doi: 10.1126/sciadv.abc6721