New species of ancient Cambrian predator named after Mothra

A finger-sized fossil has shed new light on the diversity of ancient arthropods.

More than 500 million years ago, many major animal groups first appeared on Earth in a burst of evolution known as the Cambrian Explosion. Among these were the arthropods, which today include everything from spiders and scorpions to crabs and insects.

Some of the earliest members of the group were the radiodonts, whose members include famous species like Anomalocaris. While this half-metre-long predator was one of the largest animals of its time, most of its relatives were much smaller – including the newly described Mosura fentoni.

The species reminded researchers of a ‘sea moth’, so much so that its genus has been named in honour of the character Mothra – a giant moth-like kaiju that has fought Godzilla in a number of films.

Dr Jean-Bernard Caron, the co-author of the research, says that the new species can help researchers to understand how arthropods have evolved.

“Radiodonts were the first group of arthropods to branch out in the evolutionary tree, so they provide key insight into ancestral traits for the entire group,” Jean-Bernard says. “The new species emphasizes that these early arthropods were already surprisingly diverse and were adapting in a comparable way to their distant modern relatives.”

The findings of the study were published in the journal Royal Society Open Science.

What are radiodonts?

Radiodonts are a group of arthropods that lived for more than 100 million years between 520 and 400 million years ago. Until fairly recently, only a few species of these animals were known. Dr Greg Edgecombe, one of our experts in fossil arthropods, says that this has all changed in the past couple of decades.

“After a huge flurry of research, we now know of 38 named species of radiodonts from all over the world,” Greg explains. “They’re stem-group arthropods, meaning that they branched off before the most recent common ancestor of the living group. So, radiodonts are not members of any of the major groups of today’s arthropods.”

“What makes them interesting is their distinct body plan, which shares some of the characteristics of living arthropods but not others. They have compound eyes, for example, but only just the one pair of jointed appendages.”

While some of the most famous radiodonts, like Anomalocaris, used their distinctive appendages to grab prey, other species used them for filter feeding. Having examined Mosura’s fossils, the palaeontologists believe it’s more likely to have been a hunter focusing on small, shrimp-like prey to avoid competing with the larger radiodonts living alongside it.

Mosura would have pursued its prey using rows of paddle-like fins along its sides that allowed it to glide through the ocean. As it moved in for the kill, hook-like segments on its feeding appendages would have been able to grab this soft prey and move them to its mouth.

To help do this, the researchers have identified what appears to be a median eye. This is an organ that detects changes in light and the seascape, and is used by modern arthropods like dragonflies to orient themselves during high-speed hunts. While Mosura’s median eye is different to the ones in living arthropods, the paper speculates that it played a similar role.

What does Mosura reveal?

While further research is needed into Mosura’s median eye, other aspects of its fossils provide more certain insights into the evolution of arthropods. One feature that has particularly intrigued the study’s authors are its ‘tonguelettes’ – dark, reflective patches in the fossil.

These structures have been interpreted in different ways over the years, from parts of the nervous system and gut to the remains of appendages. In Mosura, the well-preserved tonguelettes suggest that these structures are actually part of the circulatory system instead.

Arthropods like Mosura have an open circulatory system, which means that the heart pumps a blood-like substance known as haemolymph through arteries and out to different parts of the body. There aren’t any veins, however, so the haemolymph returns to the heart through a series of body cavities, known as lacunae.

Dr Joe Moysiuk, the study’s lead author, explains that the tonguelettes appear to be the remnants of the lacunae, and should help our understanding of similar features found in other species.

“Their identity has been controversial, but the well-preserved lacunae in Mosura help us to interpret similar, but less clear features that we’ve seen before in other fossils,” Joe says. “It turns out that preservation of these structures is widespread, confirming the ancient origin of this type of circulatory system.”

Mosura also has a distinctive arrangement of specialised segments at the back of its body, which have fewer swimming flaps than in other radiodonts but more densely packed gills. It’s thought this might have allowed it to live a more active lifestyle, or live in low-oxygen environments.

“Until Mosura, all radiodonts have organised their body in a similar way,” Greg adds. “They’ve all had a similar head composition, a neck composed of a few segments, and then a body with swimming flaps that ends with a crustacean-like tail fan.”

“The narrow, gill-dense segments of Mosura have been compared to a similar structure in horseshoe crabs known as the opisthosoma. That’s not to say that horseshoe crabs and radiodonts are especially closely related, but it’s instead an example of convergent evolution.”

Studying these examples of convergence between ancient and living arthropods will provide deeper insights into how these animals evolved, and the wider process of evolution itself.