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Deep sea animals can be the stuff of nightmares.
Many inhabit the twilight zone of the ocean (up to 1,000 meters deep), where sunlight has practically disappeared, and have adapted their vision to this dark alien world. Evolution has given them large, complex eyes to see in dim light – examples include the Vampire Squid, Sloane’s Viperfish, and various predatory crustaceans.
But how far back in prehistory do these scary, sharp-eyed creatures go?
Our study, published today in Science Advances, looked at radiodont (meaning “radiating teeth”), a type of ancient arthropod (animals with jointed legs but no backbones).
We found that they developed sophisticated eyes over 500 million years ago and some were adapted to the dim light of deep waters.
Our study provides new insights into the evolution of early marine animal ecosystems.
In particular, it supports the idea that vision played a crucial role during the Cambrian explosion, a pivotal phase in history where most major groups of animals (including the oldest fish) first appeared. during a rapid explosion of evolution.
Once complex visual systems arose, animals could better perceive their surroundings. This may have fueled 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.
A brief guide to radiodont
Radiodont are strange animals. Now extinct, they once dominated the oceans, especially during the Cambrian period (541 to 485 million years ago).
Some of the earliest radiodont fossils discovered over a century ago were isolated body parts, and early reconstruction attempts led to some “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. However, full radiodont still look like something out of science fiction!
There are many species of radiodont and they share a similar body structure.
The head has a pair of large segmented appendages for catching prey, a circular mouth with serrated teeth, and a pair of eyes. The rest of the body rather resembles that of a squid.
It might sound like a chimera of different animal parts, but the articulated appendages and compound eyes allow us to classify radiodons as arthropods, which include insects, spiders, and crabs.
Over the past decade, new radiodont fossils have revealed a surprising variety of forms and improved our understanding of how they lived and especially how they fed.
A kind of radiodont, Anomalocaris, has long been considered an apex predator, similar to the modern great white shark. It had a large body, more than 50 centimeters long, and very strong spiny head appendages which it used to catch prey. He swam swaying flaps on the sides of his body.
However, other radiodons were gentle giants, such as the two-meter-long genus Aegirocassis, which used its appendages to filter out plankton.
So much the better to see you with
Despite the recent surge in knowledge about these fantastic arthropods, little was known about radiodont eye optics. In 2011, we published two articles in the journal Nature on fossil compound eyes from the 513 million-year-old Emu Bay Shale on Kangaroo Island, South Australia.
The first document documented isolated eye samples (up to 1 cm in diameter) that could not be assigned to a known arthropod species. The second article reported the chased eyes of Anomalocaris in spectacular detail.
Since then, we have amassed a much larger collection of eyes from the Emu Bay Shale, shedding new light on radiodont vision.
Importantly, our new study identifies the owner of the eyes from our first document from 2011: “Anomalocaris‘ briggsi – the quotation marks indicate that it represents a new genus yet to be formally named.
We have discovered much larger specimens of these eyes (up to 4 cm in diameter). They have a distinctive “acute zone”: magnified lenses in the center of the eye surface that improve the capture and resolution of light.
Radiodont eyes are also extremely sensitive. One eye of Anomalocaris aff. canadensis – “aff.” which means “affinity”, as it is closely related to this Canadian species – with more than 24,000 lenses, it is only comparable to certain insects such as dragonflies. These make it a highly visual shallow-water predator, catching prey with appendages bearing thorny spines.
The great lenses of ‘Anomalocaris‘ briggsi suggest it could see in very dim light at depth, similar to the crustaceans of amphipods, a type of shrimp-like creature that exists today. The jagged spines on its appendages filtered out the plankton it detected when looking up.
The compound eyes of the two radiodont of the Emu Bay Shale are outliers among arthropods, alive or extinct. Their size places them among the largest arthropod eyes ever seen.
John Paterson, professor of earth sciences, University of New England
This article was republished by The Conversation under a Creative Commons license. Read the original article.
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