Incredible vision in ancient marine creatures drove an evolutionary arms race

An artist's reconstruction of 'Anomalocaris' briggsi swimming within the twilight zone. Credit: Katrina Kenny
An artist's reconstruction of 'Anomalocaris' briggsi swimming within the twilight zone. Credit: Katrina Kenny

According to new research, ancient deep-sea creatures called radiodonts had an incredible vision that likely drove an evolutionary arms race according to new research. Researchers study provides critical new information about the evolution of the earliest marine animal ecosystems but it supports the idea that vision played a crucial role during the Cambrian Explosion, a pivotal phase in history when most major animal groups first appeared during a rapid burst of evolution over 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 of ahead 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 1000 meters 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 but that may have fuelled an evolutionary arms race between predators and prey. Once established, the vision became a driving force in evolution and helped shape the biodiversity and ecological interactions we see today.

Some of the first radiodont fossils discovered over a century ago were isolated body parts, and initial attempts at reconstructions resulted in Frankenstein's monsters. But over the past few decades, many new discoveries including whole radiodont bodies have given a clearer picture of their anatomy, diversity, and possible lifestyles.

According to the co-author, Associate Professor Diego García-Bellido from the University of Adelaide and South Australian Museum said that the rich treasure trove of fossils at Emu Bay Shale on South Australia's Kangaroo Island, in particular, has helped to build a clearer picture of Earth's earliest animals. The Emu Bay Shale is the only place in the world that preserves eyes with lenses of Cambrian radiodonts. 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 half-a-billion years ago.

Researchers' new study identifies from their first 2011 paper: Anomalocaris briggsi representing a new genus that is yet to be formally named. Researchers discovered much larger specimens of these eyes of up to four centimeters 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. 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.

Dr. Greg Edgecombe, a researcher at The Natural History Museum, London, and co-author of the study said that the South Australian radiodonts show the different feeding strategies previously indicated by the appendages either for capturing or filtering prey are paralleled by differences in the eyes. The predator has the eyes attached to the head on stalks but the filter feeder has them at the surface of the head. The more we learn about these animals the more diverse their body plan and ecology are 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 new study, led by Professor John Paterson from the University of New England's Palaeoscience Research Center in collaboration with the University of Adelaide, the South Australian Museum, and The Natural History Museum (UK) found that radiodonts developed sophisticated eyes over 500 million years ago, with some adapted to the dim light of deep water.

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