Mysterious 500 Million-Year-Old Predators Are the Ancestors of Spiders

Anomalocarids are one of the oldest families of animals on Earth, and they looked like nightmarish sea scorpions. But a new fossil discovery actually contains traces of their brain structure — and amazingly, their half-billion-year-old brains look a lot like an arthropod's.

Anomalocarid illustration by John Meszaros

Though some anomalocarids may have been as big as 7 feet long, these newly-discovered specimens are closer to the size of today's large insects. The critters you see fossilized below are about 8 cm long. Still, they look pretty insane — especially when you consider that their segmented heads are so similar to what we'd see in an arthropod today. A team of paleontologists led by Peiyun Cong found the three gorgeously-preserved anomalocarid fossils in Yunnan Province, and described them today in Nature.

The fossils have been photographed with a blue digital filters, which enhance physical features like the muscles (the dark blobs next to the vascular system running from the head down), the eye stalks (branching off the head), and turns the mouth cone a reddish brown. Because of course there is a "mouth cone," along with those eye stalks and enormous things that the paleontologists call "frontal appendages."

Mysterious 500 Million-Year-Old Predators Are the Ancestors of Spiders

Mysterious 500 Million-Year-Old Predators Are the Ancestors of Spiders

520 million-year-old fossils of Lyrarapax unquispinus. Image by Peiyun Cong

The connection between anomalocarids and today's animals has long been a source of debate, and some paleontologists believed that there was no way to classify them properly. But thanks to Cong and his team, we have some strong evidence that arthropods — a group containing spiders, crabs, scorpions, and centipedes — are the distant descendants of these anomalocarids.

The main overlap between the two groups, separated by 500 million years of evolution, is their brain structure. Arthropods have segmented, compartmentalized heads like anomalocarids. Plus, their brains connect to those "front appendages" which are called mouth flaps or antennae in today's arthropods. So both groups of creatures shared a very specific physical characteristic, which was a connection between those appendages and the brain.

Below, another blue filter shows regions of brain and the animals' nervous systems.

Mysterious 500 Million-Year-Old Predators Are the Ancestors of Spiders

Images: Peiyun Cong and co-authors.

Write the researchers about the image above:

At right, a nearly complete specimen of the Cambrian anomalocaridid Lyrarapax ungusipinus, from the Chengjiang biota, China. The three images at the left depict traces of neural structures in the head, including the brain. These are highlighted with a blue digital filter that cancels colours except for dark neural regions (top left image), shown as carbon-rich domains by energy dispersive X-ray spectroscopy (centre left image), and with oblique lighting (lower left image).

Mysterious 500 Million-Year-Old Predators Are the Ancestors of Spiders

Illustration by Nicholas Strausfeld

And above, you can see a side-by-side comparison of one of the fossils (gray) with a modern arthropod called an onychophoran or velvet worm (green).

Write the researchers:

Long nerves from the frontal appendages extend to paired ganglia lying in front of the optic nerve and connect to the main brain mass in front of the mouth. Anomalocaridids had a pair of clawlike grasping appendages instead of feelers.

OK, so "clawlike grasping appendages." Evolution has definitely spared those of us living in the Quaternary from having to see some things that sound marvelously terrifying. Meanwhile, Cong and his team are well on their way to solving an evolutionary mystery.

Read the full scientific paper at Nature

Correction: An earlier version of this article incorrectly identified onychophorans as arachnids. They are arthropods. The author has been soundly beaten by a phylogeny textbook.