Ancient deep-sea radiodonts had some of the world’s earliest complex eyes

December 14, 2020

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

Deep-sea animals called radiodonts developed large, complex eyes that enabled some to adapt to dim water 500 million years ago, new research shows, during a pivotal phase of rapid evolution in which vision is now understood to have been a crucial driving force.

The radiodont — meaning “radiating teeth,” in a nod to its circular mouth with serrated teeth — is a type of ancient animal with jointed legs but no backbone dubbed the arthropod, a classification that also includes insects, spiders and crabs. Radiodonts, which likely lived as deep as 3,280 feet below the ocean surface, are some of the oldest known animals with acute vision, although knowledge of their eyes has so far been limited.

The findings, published this month in Science Advances, provide important new information about the evolution of creatures in the earliest marine ecosystems, reinforcing the idea that sight played a critical role during the historic Cambrian Explosion, said John R. Paterson, a professor of earth sciences at the University of New England in Armidale, Australia, who led the study. That’s when most major animal groups first appeared, and radiodonts dominated the oceans.

“With the apparent advent of eyes at the beginning of the Cambrian, we see clear evidence of novel complex ecological interactions taking place, such as predator-prey relationships, suggesting that vision was a driving force during this significant evolutionary event,” he said. “The Cambrian is the time when we see the earliest undisputable fossil evidence of animals eating each other.”

Vision is a key sense throughout the animal kingdom because of its important role in ecosystem functions and interactions such as feeding, mating and avoiding danger.

Radiodonts’ incredible vision likely drove an “evolutionary arms race,” the researchers suggest, as the hunters and hunted continued to adapt to survive. Paterson invoked an example of a predator using claws to crack its prey’s hard shell: If over time the shell grows thicker because of predation pressure, the predator in turn needs to evolve stronger claws.

“Now imagine this happening over a timeframe of half-a-billion years,” he said. “So it is a multitude of interactions like this, which originated in the Cambrian, that has helped shape the animal diversity we see today.”

In addition to the notable oral features and eyes, the radiodont head has a pair of big, segmented appendages to capture prey, and the animals have squid-like bodies. 

More — and more diverse — radiodont fossils have surfaced in the last decade, mainly because more people are searching for Cambrian fossils worldwide, Paterson posited. 

“Also, with the occasional unearthing of fully or partially articulated specimens, we have a much better understanding of how various radiodont body parts fit together,” he added. “This, in turn, permits us to identify isolated fossils that may have once been palaeontological puzzles in museum drawers as now representing radiodont body parts.”

Paterson’s team has been excavating fossils in Emu Bay Shale on South Australia’s Kangaroo Island since 2007, with more recent discoveries bringing new clarity. The latest study documents how isolated eyes that they reported in 2011 — whose owner at the time could not be determined — were identified as "Anomalocaris" briggsi and should be assigned to a new genus that the researchers will name in a forthcoming article.

While the eyes from 2011 reached up to 1 centimeter in diameter, the team discovered specimens four times that size, with more than 13,000 lenses and a distinct “acute zone,” with enlarged lenses in the center of the surface of the eye that improve light capture and resolution. The radiodont eyes from the Emu Bay Shale are among the largest ever belonging to arthropods. 

While slightly older radiodonts with big eyes have been found at other fossil sites, such as China’s 518 million-year-old Chengjiang Biota, they don’t rival the level of anatomical detail in the “exceptionally preserved” Emu Bay Shale, Paterson noted.

“We still have some surprises in store with our Emu Bay Shale fossils,” he added. “Ultimately, I’d like to find much older evidence of eyes in the fossil record to better understand when vision first emerged among the earliest animal ecosystems.”

The study “Disparate compound eyes of Cambrian radiodonts reveal their developmental growth mode and diverse visual ecology,” published Dec. 2 in Science Advances, was authored by John R. Paterson, University of New England; Gregory D. Edgecombe, The Natural History Museum; and Diego C. García-Bellido, University of Adelaide and South Australian Museum.

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