How scientists got a 410-million-year-old arachnid to walk again

Some 410 million years ago, a tiny creature walked the Earth, standing with its eight legs at the top of the terrestrial food chain.  

Today, researchers have recreated its stroll on video.

The creature belonged to the order Trigonotarbid, the genus Palaeocharinus, both now extinct, and the class Arachnida, which includes modern-day spiders, ticks, mites, and scorpions.  

The scientists scanned fossils of this ancient arachnid and compared it with living arachnids. With that combined information, they created a simulation of the creature flexing, stretching, and crawling.

"When I started working on fossil arachnids we were happy if we could manage a sketch of what they used to look like," said study coauthor Jason Dunlop, curator at the Museum für Naturkunde, Berlin, in a news release. "Now we can view them running across our computer screens."

These animals appear much like spiders alive today, but without the ability to produce a characteristic web-making silk.

What was its life like?

This Palaeocharinus lived during the Devonian, when most of the globe was covered by a vast ocean. The landmasses were clustered together, forming two supercontinents, called Gondwana and Euramerica. The fossils that the researchers relied on for their study were unearthed in Scotland, at sites known to harbor remains of some of the oldest terrestrial ecosystems.

Back then, land-dwelling life was just beginning to diversify. Plant diversity exploded during this time, with the first trees emerging by the end of the period.

The animals on land largely consisted of arthropods, invertebrate animals with tough exoskeletons, segmented bodies, and jointed limbs. Tiny insects and other invertebrates were likely sources of food for the Palaeocharinus.

"When it comes to early life on land, long before our ancestors came out of the sea, these early arachnids were top dog of the food chain," said study author Russell Garwood, a paleontologist at the University of Manchester in a news release.

These spider-cousins may have been a ambush-style predator, stalking, running, and jumping on their prey. It was one of the first land predators ever to exist, say the researchers.

"We can see from its mouth parts that it pre-orally digested its prey – something that most arachnids do – because it has a special filtering plate in its mouth. So, that makes us fairly sure it vomited digestive enzymes on to its prey and then sucked up liquid food," Dr. Garwood told BBC News.

Fearsome as all this may sound, Palaeocharinus would not seem particularly menacing today: the specimens examined by the Manchester researchers were just about a tenth of an inch.

"They are now extinct, but from about 300 to 400 million years ago, seem to have been more widespread than spiders," said Garwood in a news release.

Turning rock into motion

These fossils have been in the Natural History Museum in London since the 1920s. Scientists had cut the rocks into extremely thin slices, making them easier to examine.

In a paper published this week in the Journal of Paleontology, the scientists describe how they brought this ancient creature back to life.

First, they took X-rays of the fossils imprinted in slices of rock to determine the  arachnid's features. From these scans, they created a three-dimensional virtual model that displayed the leg joints.

By comparing this model to living arachnids, the paleontologists could recreate the ancient creature's gait. Using the open-source 3D animation program, Blender, they then overlaid this information on the range of motion allowed by the joints in the model.

"[W]hat's really exciting here," said Dr. Dunlop in a news release, "is that scientists themselves can make these animations now, without needing the technical wizardry – and immense costs – of a Jurassic Park-style film."