500-Million-Year-Old Minibeast Is Oldest Relative Of Scorpions And Spiders Ever Found


Rachel Baxter

Copy Editor & Staff Writer


Scuttle, scuttle. Joanna Liang © Royal Ontario Museum

The Aragogs and Shelobs of this world have been scuttling about for more than 500 million years – well, at least their ancestors have. Scientists have uncovered a new ancient species called Mollisonia plenovenatrix, the oldest example of a chelicerate we’ve ever found. Chelicerata is a large group of creepy-crawlies that includes spiders, scorpions, sea spiders, and horseshoe crabs.

The new species was identified from fossils in the Burgess Shale rock deposit in Canada’s Rocky Mountains. The area contains a plethora of fossilized critters and is known for preserving soft tissues incredibly well. Dating back 508 million years, the creatures it bears tell us about the life that existed towards the end of the Cambrian explosion, an event 541 million years ago that saw the rise of most major animal phyla.


Fossil members of the Mollisonia genus have been found before, first discovered over a century ago by American palaeontologist Charles Doolittle Walcott. However, previous specimens have only included hard exoskeletons; the new finds give us much more detail.

Mollisonia plenovenatrix as seen from above, showing the large eyes, the walking legs, and the small chelicerae (pincers) at the front. Jean-Bernard Caron © Royal Ontario Museum

“It is the first time that evidence of the limbs and other soft-tissues of this type of animal are described, which were key to revealing its affinity,” said the Royal Ontario Museum’s Jean-Bernard Caron in a statement.

“I would not have imagined that we could, in a way, rediscover the Burgess Shale like this, a hundred years later, with all the new species we are finding.”

  1. M. plenovenatrix was small – about the size of a thumb – but nevertheless mighty, and would have been a fearsome predator. It had big eyes, long legs, and many pairs of limbs for sensing, grabbing, and crunching up prey. The creature is described as having a “multi-tool head”, but one set of tools stood out to the researchers. The minibeast was equipped with chelicerae, or pincers, which are only found in chelicerates and are what scorpions and spiders use to nab and kill prey.


The ancient species would have lived in water and employed its long legs to hunt close to the seafloor, filling a niche not occupied by other similar animals at the time. It had a breathing apparatus likened to gills, similar to what we see in certain chelicerates today.


“Chelicerates have what we call either book gills or book lungs,” explained lead author Cédric Aria. “Their respiratory organs are made of many collated thin sheets, like a book. This greatly increases surface area and therefore gas exchange efficiency. Mollisonia had appendages made up with the equivalent of only three of these sheets, which probably evolved from simpler limbs.”

The researchers think that Mollisonia expanded pretty quickly as the ancient fossils share traits with modern chelicerates. While the new study, published in Nature, shows chelicerates existed over 500 million years ago, the team reckons they probably arose right “at the heart of the Cambrian explosion.”

It seems this group of nightmare-inducing beasties has been around for a very, very long time.