Scientists at Imperial College London have brought ancient relatives of spiders to “life” with ultra-detailed three-dimensional computer models.
The images help reveal some of the physical traits that helped these “pre-spiders” hunt for prey and evade predators.
Pictures of the 300-million-year-old creatures are published today in the journal Biology Letters. The 3D models depict two species — Cryptomartus hindi and Eophrynus prestvicii — that were closely related to modern-day spiders.
The researchers created their images by using a CT scanning device, which enabled them to take 3,000 X-rays of each fossil. These X-rays were then compiled into precise 3D models, using custom-designed software.
Both Cryptomartus hindi and Eophrynus prestivicii were around the size of a 50-pence piece. Both roamed the Earth during the Carboniferous period — 359 – 299 million years ago — well before dinosaurs. This was a time when life was first emerging from the oceans to live on land. During this period, the world’s continents came together near the equator to form one supercontinent, and the first tropical rainforests became home to a diverse range of species.
Previous studies of the fossilised remains of Cryptomartus hindi allowed scientists to see some features of the creature, which had four pairs of legs and looked similar to a spider.
In the new study, the researchers’ computer models reveal that Cryptomartus hindi’s first two legs were angled towards the front of the body, which suggests that it used its legs to grab its prey before killing them. The researchers believe this find suggests Cryptomartus hindi was an ambush predator, living in logs and fronds, waiting for prey such as insects to walk by before catching and killing them. The stance is similar to that of modern-day crab spiders, which sit on the edges of flowers and wait for insects to land so that they can grab them.
The scientists also discovered that Cryptomartus hindi had ball-like growths at the base of its limbs, called coxal endites. The scientists believe the coxal endites could be an evolutionary hang-over from their last common ancestor, which probably used the growths to help grind their food. These coxal endite-type growths can still be seen today in species such as horseshoe crabs, which use them to grind up prey before pushing it into their backward-facing mouths.
The computer models also revealed that Cryptomartus hindi’s mouth appendages, called pedipalps, had tiny “tarsal” claws attached at the end to help the creature to manipulate its prey. These claws are seen in rare modern-day arachnids such as the Ricinulei. The researchers say the existence of this common physical feature lends further weight to the theory that Cryptomartus hindi was closely related to today’s arachnids.
The models also reveal new information about Eophrynus prestivicii. Previous studies of fossilised remains of this creature suggested it could have hunted on the open forest floor. It had long legs that enabled it to run through leaf litter to chase, catch and kill its prey.
The new models reveal for the first time that Eophrynus prestivicii had defensive spikes on its back. The researchers say the spikes may have been a defensive adaption to make the creatures a less tempting meal for the amphibians that would have recently emerged from the oceans onto land.
“Our models almost bring these ancient creatures back to life and it’s really exciting to be able to look at them in such detail,” said lead author Russell Garwood. “Our study helps build a picture of what was happening during this period early in the history of life on land. We think one creature could have responded to increasing predation from the amphibians by growing spikes, while the other responded by becoming an ambush predator, hiding away and only exposing itself when it had to come out to eat.”
Currently, most palaeontologists analyse fossils by splitting open rocks and looking at the creatures encased inside. This means scientists can often only see part of a fossil and can’t explore all of the fossil’s physical features.
The Imperial College London researchers believe their new technique could be used to re-explore previously analysed fossils to provide a much clearer picture of how ancient extinct species survived on early Earth.