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Reconstruction by an artist of the briggsi “Anomalocaris” swimming in the twilight zone. Credit: Katrina Kenny
According to a new study published on December 2, 2020, ancient sea creatures called radiodonts had incredible vision that likely led to an evolutionary arms race.
The international study, led by Professor John Paterson of the Palaeoscience Research Center at the University of New England, in collaboration with the University of Adelaide, the South Australian Museum and the Natural History Museum (United Kingdom), has revealed that radiodonts have developed sophisticated eyes over 500 million years. there are, with some adapted to the low light of deep waters.
“Our study provides crucial new information on the evolution of early ecosystems of marine animals,” said Professor Paterson. “In particular, he supports the idea that vision played a crucial role during the Cambrian Explosion, a pivotal phase in history when most large groups of animals first appeared during a period of time. rapid burst of evolution more than half a billion years ago.
Radiodonts, meaning “radiating teeth,” are a group of arthropods that dominated the oceans around 500 million years ago. The many species share a similar body layout comprising a head with a pair of large, segmented appendages for capturing prey, a circular mouth with jagged teeth, and a squid-like body. It now seems likely that some have lived at depths of up to 1,000 meters and developed large, complex eyes to make up for the lack of light in this extreme environment.
Briggsi’s eye ‘Anomalocaris’. Complete left fossil eye (scale bar is 5 mm); close-up in the middle of the lenses (the scale bar is 0.5 mm); reconstruction by the artist on the right showing the acute area. Credit: University of Adelaide
“When complex visual systems arose, animals could sense their surroundings better,” explained Professor Paterson. “It may have fueled an evolving arms race between predators and prey. Once established, the vision became a driver of evolution and helped shape the biodiversity and ecological interactions we see today.
Some of the earliest radiodont fossils discovered over a century ago were isolated body parts, and early attempts at reconstruction resulted in “Frankenstein monsters.”
But over the past few decades, many new discoveries – including entire radiodont bodies – have provided a clearer picture of their anatomy, diversity, and possible lifestyles.
The radiodont Anomalocaris, with its large, stalked eyes, is considered one of the main predators that swam in the oceans more than 500 million years ago. Credit: Katrina Kenny
The co-author, Associate Professor Diego García-Bellido of the University of Adelaide and the South Australian Museum, said the rich fossil treasure of Emu Bay Shale on Kangaroo Island in South Australia in particular has helped build a clearer picture of Earth’s first animals.
“The Emu Bay shale is the only place in the world that preserves eyes with Cambrian radiodont lenses. The more than thirty eye specimens that we have today have shed new light on the ecology, behavior and evolution of these animals, the largest living animals half a billion years ago ” , A / Prof. Said García-Bellido.
In 2011, the team published two articles in the journal Nature on eyes made up of fossils from the 513 million year old Emu Bay shale on Kangaroo Island.
The first article on this topic documented isolated ocular specimens up to one centimeter in diameter, but the team were unable to attribute them to a known arthropod species. The second article detailed the stalked eyes of Anomalocaris, a top predator up to one meter in length.
“Our new study identifies the owner of the eyes of our first article from 2011: ‘Anomalocaris’ briggsi – representing a new genus that has yet to be officially named,” said Professor Paterson.
“We have found much larger specimens of these eyes, up to four centimeters in diameter, which have a distinctive ‘sharp zone’, which is a region of enlarged lenses in the center of the eye’s surface that improves capture. and the resolution of light. “
The large lenses of ‘Anomalocaris’ briggsi suggest that it could see deep in very low light, 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.
Dr Greg Edgecombe, a researcher at the Natural History Museum in London and co-author of the study, added that radio broadcasts from South Australia show that the different feeding strategies previously indicated by the appendages – either to capture or filter preys – are parallel by differences. in the eyes.
“The predator has eyes attached to the head on the 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, ”said Dr Edgecombe.
“The new samples also show how the eyes have changed as the animal has grown. Lenses formed at the margins 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. “
Reference: “The disparate compound eyes of Cambrian radiodonts reveal their developmental growth pattern and diverse visual ecology” by John R. Paterson, Gregory D. Edgecombe and Diego C. García-Bellido, December 2, 2020, Scientific advances.
DOI: 10.1126 / sciadv.abc6721
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