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A team of paleontologists from Australia and the United Kingdom discovered that ancient deep-sea creatures, called radiodonts, developed sophisticated eyes over 500 million years ago (Cambrian period), some specially adapted for the dim light of the deep waters.
Reconstruction by an artist of Briggsi ‘Anomalocaris’. Image credit: Katrina Kenny.
Radiodonts (meaning “radiating teeth”) are a group of arthropods that dominated the oceans about 500 million years ago.
The many radiodont species share a similar body layout comprising a head with a pair of large, segmented appendages to capture prey, a circular mouth with jagged teeth, and a squid-like body.
It now seems likely that some lived at depths of up to 1,000m (3,281ft) and had developed large, complex eyes to make up for the lack of light in this extreme environment.
“When complex visual systems emerged, animals could sense their surroundings better, which could have fueled an evolutionary arms race between predators and prey,” said lead author Professor John Paterson, a researcher at the Palaeoscience Research Center from the University of New England.
“Once established, the vision became a driver of evolution and helped shape the biodiversity and ecological interactions we see today.
In 2011, Professor Paterson and his colleagues documented isolated eye specimens up to 1 cm (0.4 inch) in diameter in the 515 million year old Emu Bay shale on Kangaroo Island, but they were not able to attribute them to a known arthropod species. .
They also described the hunted eyes of Anomalocaris, a top predator up to 1 m (3.3 feet) in length, with great detail.
“The Emu Bay Shale is the only place in the world that preserves the eyes with Cambrian radiodont lenses,” said Dr. Diego García-Bellido, a researcher at the School of Biological Sciences at the University of Adelaide and the South Australian Museum.
“The more than thirty eye specimens we now have have shed new light on the ecology, behavior and evolution of these largest living animals 500 million years ago.
Acute area type eyes Briggsi ‘Anomalocaris’. Image credit: John Paterson, University of New England.
In the new study, researchers identified the owner of eye specimens 515 million years old: Briggsi ‘Anomalocaris’, representing a new genre that has not yet been officially named.
“We have discovered much larger specimens of these eyes, measuring up to 4 cm (1.6 inches) in diameter, which have a distinctive ‘sharp zone’, which is a region of enlarged lenses in the center of the surface of the eye. the eye which improves the capture and resolution of light, ”Professor Paterson said.
Large lenses of Briggsi ‘Anomalocaris’ suggest he could see in a very shallow light at depth, similar to amphipod crustaceans, a type of shrimp-like creature that exists today.
The frilly thorns on its appendages filtered out the plankton it detected looking up.
“These specimens showed us that the feeding strategies of animals previously indicated by the appendages – whether to capture or filter out prey – parallel differences in the eyes,” said Dr Greg Edgecombe, paleontologist in the Department. of Earth Sciences at the Museum of Natural History, London.
“The predatory radiodonate in our samples has eyes attached to the head on stems, but the filter feeder has them on the surface of the head.
“The more we learn about these animals, the more diverse their body plan and ecology.”
“The new samples also show how the eyes have changed as the animal has grown. Lenses formed at the margin of the eyes, enlarging and increasing in number in large specimens – just as in many living arthropods. The growth of compound eyes has been constant for over 500 million years. “
Scientists have also found that the Anomalocaris the eyes described in 2011 probably come from a species called Anomalocaris aff. canadensis.
“The Australian material is unique among the dozens of occurrences of radiodonts in the world during the Cambrian Period, because it is the only place where the visual surface of the eye is preserved,” said Dr Edgecombe.
“In other sites in China, Canada, the United States and elsewhere, only the eye area is known but there is no information on their lenses.”
The study was published in the journal Advances in Science.
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John R. Paterson et al. 2020. The disparate compound eyes of Cambrian radiodonts reveal their developmental growth pattern and diverse visual ecology. Scientific advances 6 (49): eabc6721; doi: 10.1126 / sciadv.abc6721
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