Back when the ancient seas teemed with trilobites, the little ones may have had nightmares about this fearsome predator.

It now appears that Habelia optata, which lived about 508 million years ago in what is now eastern B.C., was a relative of spiders and scorpions, researchers at the Royal Ontario Museum and the University of Toronto have discovered.

Those animals belong to a group of animals called chelicerates, and the new study on Habelia tells us a lot about what their ancestors may have been like.

"If you're looking for a scary Hollywood creature, it probably would be the perfect one," said Cédric Aria, lead author of the study published this week in BMC Evolutionary Biology. "It's like a centipede or perhaps an insect that would have not one pair of mandibles, but five."

Each of those powerful pairs of jaws was equipped with sharp teeth and designed to crush the protective shells of its prey, likely small trilobites. In fact, a similar fossil predator was recently found in Australia with chopped up trilobite remains in its gut, said Aria, who studied Habelia during his PhD at the University of Toronto and is now a postdoctoral researcher at the Nanjing Institute of Geology and Paleontology in China.

Fossil specimen of Habelia optata from the Royal Ontario Museum. This specimen shows some of the very large jaws under the head shield. Note also the long dorsal spines on the thorax. (Jean-Bernard Caron/Royal Ontario Museum)

Predator and prey

But despite its fearsome features, Habelia was only two centimetres long and likely an appetizing snack for larger predators, as evidenced by its defensive features, said Jean-Bernard Caron, senior curator of invertebrate paleontology at the Royal Ontario Museum and Aria's supervisor.

"It has a crazy number of spines along the body — pretty much from tail to head it has spines everywhere."

The first specimens of Habelia were collected from the Burgess Shale of Yoho National Park in B.C. and described in 1912 by Charles Walcott. He was the paleontologist who first discovered the 508-million-year-old fossil beds that contain beautifully preserved remains of a huge variety of creatures that lived in a shallow sea during the middle Cambrian period.

New specimens of Habelia optata were collected during field expeditions to a fossil site called the Walcott Quarry in Yoho National Park in B.C. in the 1990s. (Desmond Collins/Royal Ontario Museum)

But for more than a century, paleontologists were puzzled as to what, exactly, Habelia was. Its mandibles made it look like it could be related to insects, which belong to a group of creatures called mandibulates. But many of its other features were unusual.

In the 1990s, ROM paleontologist Desmond Collins led several expeditions to the Walcott Quarry and collected dozens of other specimens.

Caron chose specimens in different orientations and, peering through a microscope, used microengraving tools to painstakingly chisel the rock away from its minuscule appendages to reveal hidden features.

Unflattening the fossils

The creatures had been squashed flat during the fossilization process, so figuring out what they looked like in 3D was a challenge.

Aria carefully measured the distances between different appendages and worked with artist Joanna Liang, a master's student in the biomedical communications program at the University of Toronto, to reconstruct the animal in 3D. In some ways, he said, it's similar to the way a dinosaur fossil might be put back together from its individual bones. In this case, he said, the art was a crucial step in the science.

The work revealed seven pairs of appendages on its head:

Five consisting of the mandible-like pincers, each with antennae attached.

A small pair at the back of the head similar to those seen on horseshoe crabs.

A very small pair at the very front called "chelicerae" that are found in all chelicerates, including spiders. They were key to placing Habelia in what researchers believe is its rightful place in the tree of life.

Simplified phylogeny (tree of life) showing the relationship of Habelia with other groups of arthropods. The study shows that it is an early relative of chelicerates — a group including spiders, scorpions, horseshoe crabs and mites. (Cedric Aria)

While the head appendages on modern chelicerates are used for walking, Habelia appeared to use them for grasping and crushing prey.

It had another five pairs of legs for walking on its thorax, a part of the body where spiders and scorpions don't have any appendages.

While animals that lived far in the past and are high up in their family trees are often described as primitive, Aria says that word is definitely not appropriate for Habelia: "It's more complex than a lot of chelicerates that live today."

The study was funded by the University of Toronto and the Natural Sciences and Engineering Research Council of Canada.