Scary ancient sea predator sported big, dangling eyes
With large claws and toothlike serrations in its mouth, it was already menacing enough
By Jennifer Welsh LiveScience updated 12/7/2011 5:28:31 PM ET 2011-12-07T22:28:31
The biggest, scariest predator of the ancient Cambrian oceans just got a lot more menacing: Researchers have found a pair of fossilized eyes that show the beast had excellent vision.
"The animal itself has been known for quite some time, but we've never known the detail of the eyes," study researcher John Paterson, of the University of New England in Australia, told LiveScience. "It can tell us a great deal [ http://www.livescience.com/17348-big-eyes-marine-predator.html ] about how it saw its world and it also supports that it's one of the key predators during the Cambrian period."
The group of predators in question, which belong to the genus Anomalocaris, could reach more than 3 feet (1 meter) long and lived in shallow oceans more than 500 million years ago. The researchers call it the "world's first apex predator [ http://www.livescience.com/8898-ancient-monster-shrimp-real-softie.html ]," because it had highly acute vision and was much larger than other animals in the ocean at that time. It also had large claws and toothlike serrations in its mouth to tear apart trilobites.
"When you look at the animal it has these really gnarly looking grasping claws at the top of its head, for grasping onto its prey," Paterson said. "It used these grasping claws at the front to shove its prey into its circular mouth, which is also fairly fearsome looking [ http://www.livescience.com/14320-largest-sea-predators-fossils.html ]."
Ancient predators
Previous fossils had preserved only the outlines of these creatures' eyes. Researchers knew the eyes were situated on stalks that protruded from its face, and they had thought the dangling eyes might be compound eyes, but weren't sure and couldn't tell how many lenses they might have had, or how sharp their vision might have been.
The eyes were discovered in a fossil from a 515-million-year-old deposit on Kangaroo Island, in South Australia. Other fossils discovered in this deposit show ancient eyes [ http://www.livescience.com/14832-super-sharp-ancient-eyes-discovered.html ] that aren't nearly as well developed, but still quite sharp compared with other animals of the day.
The fossils were pried from shale rock samples: "You split them along the really fine layers in the shale with hammer and chisel, like opening the pages of the book, and hopefully something will be looking back at you for the first time in 500 million years," Paterson told LiveScience. "I was actually the one that found the pair of eyes. That was a spine-tingling moment."
Excellent eyes
Compound eyes, the type of eyes seen in dragonflies and mosquitoes, are made up of multiple individual lenses. Dragonflies, one of the few living arthropods with similarly acute eyesight, have up to 28,000 lenses per eye, while a housefly may have 3,000. These 500 million-year-old creatures had around 16,000.
Like pixels in a digital image, for compound eyes, more lenses mean a clearer picture [ http://www.livescience.com/16829-early-eye-evolution-crustacean.html ]. Based on the structure, this animal might have had an exceptionally clear, almost 360-degree view of the world around it, the researchers said. Such precise vision would have given these predators an advantage over their prey, which would need to evolve their own visual capabilities to avoid being eaten.
"It would have been very aware of its environment. It would have been a very capable [ http://www.livescience.com/17348-big-eyes-marine-predator.html# ] predator, especially when you compare it to other animals in the same fossil sites that wouldn't have had as good of eyesight or could have even been blind," Paterson said. "Anomalocaris would have had a distinct advantage, I think."
The study [linked below] will be published in Thursday's issue of the journal Nature.
It's got 16,000 eyes on you—the vision of a Cambrian-era predator
This Cambrian predator was about a meter long. Katrina Kenny & University of Adelaide
By John Timmer | Published December 7, 2011
Those of you who get a bit weirded out by spiders and other arthropods would probably have a coronary if an Anomalocaris were to swim in your direction. The animals were about a meter long, and shaped as a flattened oval, a bit like a modern flounder. That's about the only similarity with a fish, though. Instead of fins, the Anomalocarids propelled themselves through the water using a series of elongated paddle-like structures running down both edges of the body. In front, a pair of appendages could shovel prey into a circular mouth located on its underside.
And then there were the large, bulging eyes, springing from each side of the animal's head. Until now, we could only guess at what the eyes looked like, but some spectacular, 515 million-year-old fossils from Australia have now shown that they had a huge number of small lenses, arranged much like those in modern insects and other arthropods. The finding suggests that the compound eyes evolved right at the origin of this branch of the evolutionary tree, long before the sorts of hard exoskeletons we now consider typical of arthropods.
First, the fossils themselves, which are absolutely spectacular. We've discovered a number of different Anomalocarid species in fossil deposits around the globe but, at best, these simply left behind an impression of the eyes. So, we knew they were roughly pear-shaped and where they appeared on the animal, but nothing about their internal structure. The eyes found in the new fossils clearly show details of the internal structure. They aren't actually attached to an Anomalocaris, but they match the impressions previously found with them, and we've not found anything else in these fossil beds big enough to support an eye of this size.
It takes a microscope to see them, but individual lenses were preserved in each eye. For someone who has seen countless images of the compound eyes of Drosophila, they are startling in how modern they look. Based on their density, the authors estimate that each eye housed 16,000 individual lenses, the most that have ever been seen on any animal we know about. Based on the curve of the eye and what we know about modern compound eyes, they suggest that the animal had very good visual acuity.
The fossilized remains of 515 million year old eyes. John Paterson
Those findings feed into a number of evolutionary arguments. For one, a number of researchers suggested that Anomalocarids were the apex predators of the Cambrian seas, able to shove many of the smaller creatures into their circular jaws—in fact, a number of fossils of smaller creatures have been discovered that appear to bear bite marks from an Anomalocarid. The visual capabilities suggested by these fossils supports the idea that these animals were hunters, using their visual system to spot prey.
The clear similarities between the visual system of the Anomalocarids and modern arthropods also strengthens previous indications of a close relationship between these groups. Previous phylogenetic trees place Anomalocarids as branching off earlier than the origin of all modern arthropods, and near the base of an entire phylogenetic group that includes extinct species and modern arthropods. Confirming that these animals had modern-looking compound eyes pushes the origin of those eyes back to near the base of the group, and suggests the possibility that the evolution of this mode of vision may be one of their defining features.
The final argument made by the authors is in support of an idea that's been around for decades: the proliferation of new phylogenetic groups in the Cambrian came about in part because the first large predators drove an evolutionary arms race that led to new forms of protection (shells and hard plates) and new modes of motion. The presence of a meter-long predator with excellent vision, in the authors opinion, certainly fits in nicely with the idea of an extremely competitive environment.
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