Paleontologists have unearthed fossils of a giant trilobite species that inhabited Australian waters approximately 500 million years ago (Cambrian period).

Trilobites are a group of extinct marine arthropods that resemble modern-day horseshoe crabs and are related to modern crustaceans and insects.

These creatures appeared in ancient oceans in the Early Cambrian period, about 540 million years ago, and disappeared in the mass extinction at the end of the Permian period, about 252 million years ago.

They were extremely diverse, with about 20,000 species, and their fossil exoskeletons can be found all around the world.

Dubbed Redlichia rex, the newly-discovered species is the largest Cambrian trilobite discovered in Australia.

It grew to around 12 inches (30 cm) in length, which is almost twice the size of other Australian trilobites of similar age.

“We decided to name this new species of trilobite Redlichia rex — similar to Tyrannosaurus rex — because of its giant size, as well as its formidable legs with spines used for crushing and shredding food, which may have been other trilobites,” said James Holmes, a PhD student with the University of Adelaide.

The preservation of ‘soft parts’ — such as the antennae and legs — in fossilized trilobites is extremely rare.

Redlichia rex was discovered at the Emu Bay Shale on Kangaroo Island, South Australia, a world-renowned deposit famous for this type of preservation.

“Interestingly, trilobite specimens from the Emu Bay Shale — including Redlichia rex — exhibit injuries that were caused by shell-crushing predators,” said Dr. Diego García-Bellido, a researcher at the University of Adelaide and the South Australian Museum.

“There are also large specimens of fossilized coprolites containing trilobite fragments in this fossil deposit.”

“The large size of injured Redlichia rex specimens and the associated coprolites suggests that either much bigger predators were targeting Redlichia rex, such as Anomalocaris, an even larger shrimp-like creature, or that the new species had cannibalistic tendencies.”

One of the major drivers of the Cambrian explosion was likely an evolutionary ‘arms race’ between predators and prey, with each developing more effective measures of defense and attack.

“The overall size and crushing legs of Redlichia rex are a likely consequence of the arms race that occurred at this time. This giant trilobite was likely the terror of smaller creatures on the Cambrian seafloor,” Holmes said.

The discovery is described in a paper published online this week in the Journal of Systematic Palaeontology.

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James D. Holmes et al. The trilobite Redlichia from the lower Cambrian Emu Bay Shale Konservat-Lagerstätte of South Australia: systematics, ontogeny and soft-part anatomy. Journal of Systematic Palaeontology, published online June 12, 2019; doi: 10.1080/14772019.2019.1605411