Alien-looking predators called anomalocaridids dominated the marine ecosystem over 500 million years ago, long before animals colonized the land. Now, researchers working in China have unearthed near-complete fossils belonging to a new member of this fearsome group. Their remarkably preserved nervous system displays a similar brain structure to a modern worm-like animal -- suggesting how features of ancient sea monsters are still lurking around today.
Discovered in the 19th century, anomalocaridids (“abnormal shrimp”) are considered the world’s first predators, feeding on primitive fishes in the Lower Cambrian. They can grow well over a meter (3 feet) long. Previous work has suggested how they're closely related to arthropods (spiders, lobsters, and friends), but features of their frontal appendages -- a pair of claws sticking out of their faces -- don’t match up with any corresponding structures around today. Their wicked-looking graspers, says Nicholas Strausfeld from the University of Arizona, “are totally different from the antennae of insects and crustaceans.”
Last year, an excavation in Cambrian sediments dating back 520 million years unearthed anomalocaridid remains that so well preserved, their nervous system is in full view -- allowing detailed comparisons with today’s arthropods for the first time. Strausfeld, Xianguang Hou of Yunnan University, and colleagues describe the new species, Lyrarapax unquispinus -- Latin for "spiny-clawed lyre-shaped predator” -- in Nature this week. At about 13 centimeters (5 inches) long, Lyrarapax was a wee predator.
"It turns out the top predator of the Cambrian had a brain that was much less complex than that of some of its possible prey and that looked surprisingly similar to a modern group of rather modest worm-like animals,” Strausfeld says in a news release. These frontally disposed appendages aren't found in any other living creature, except velvet worms, or onychophorans. ("I said, Holy shit, that’s an onychophoran brain!’” he tells Nature.)
Onychophorans have stubby, unjointed legs that end in a pair of tiny claws, and they roam the leaf litter looking for small bugs. They have a simple brain located in front of the mouth and a pair of ganglia (collection of nerve cells) located in the front of the optic nerve and at the base of their long feelers.
"And -- surprise, surprise -- that is what we also found in our fossil," Strausfeld says. Their neuroanatomy resemble each other in several ways, but instead of feelers, anomalocaridids have pinchers in front of the eyes. Long nerves extend to paired ganglia in front of the optic nerve and connect to the main brain mass in front of the mouth. In this comparison, the onychophoran is green, Lyrarapax is gray, and their nervous systems are in blue.
If anomalocaridid predators are distant relatives of velvet worms, that suggests that as arthropods evolved, they retained the ancestral connection between the brain and the front appendages. "With the evolution of dedicated and highly efficient predators, the pressure was on other animals,” Strausfeld adds. That may have driven the evolution of more complex brain circuitry.”
Here, you can see traces of neural structures in the head. The X-like structure denotes the fossilized brain; the dark, round spots represent the optic ganglia with nerves leading from the eyestalks into the head.
Images (from top to bottom): Peiyun Cong, Nicholas Strausfeld/University of Arizona, Nicholas Strausfeld, Peiyun Cong and co-authors
9