Nature
PUBLISHED
What you’ll discover in this article
- The Cambrian oceans were once filled with a diverse range of ancient invertebrates called radiodonts that were the top predators of the oceans.
- A new rare fossil of a radiodont shows what appears to be a bite wound on the side of the animal.
- Dr Stephen Pates told IFLScience that “we can be confident that the fossil shows damage that has been repaired while the animal was alive because it has a healed margin.”
Scientists have unearthed a 520 million-year-old mystery, in which they think an ancient early animal was attacked whilst swimming in the Cambrian sea.
The animal in question, Amplectobelua symbrachiata, belonged to a now-extinct group of creatures called the radiodonts. These were a highly successful group of arthropods that thrived during the Cambrian, and included some of the earliest large predators and filter feeders.
Looking a bit like aquatic woodlice with long, grasping arms, they were soft-bodied with a frilly fringe that ran along each side. One of their most famous members is Anomalocaris, but evidence of radiodonts has been found around the world.
Amplectobelua was one of the largest of these early marine beasties, growing to about 90 centimeters (3 feet) long. While that might be small in comparison to modern-day marine predators, it made the animal one of the largest in the Cambrian seas.
The top predators in the Cambrian seas were radiodonts, whereas in modern oceans they are mostly vertebrates
Dr Kun-sheng Du
“In the oceans where it lived, arthropods were the most numerous and species-rich animals, having colonzed the seas of the time, while animals from other phyla were far fewer,” Dr Kun-sheng Du, a researcher at the Research Center of Natural History & Culture, Qujing Normal University, China, and first author of the new preprint, told IFLScience.
“The top predators in the Cambrian seas were radiodonts, whereas in modern oceans they are mostly vertebrates, which were tiny filter-feeders in the Cambrian.”
An ancient encounter
Because of the soft-bodied nature of these animals, the preservation of most of their remains requires some very specific conditions. This means they are only found in a handful of locations.
One of these is the famous Burgess Shale in Canada, but another is the Maotianshan Shales in Chengjiang, China. This formation contains some of the most exceptional soft-bodied fossils dating to the Cambrian explosion.
It was while exploring this fossil site that Du and his colleagues came across part of an Amplectobelua that showed an interesting mark.
“This fossil is an appendage of Amplectobelua called a flap,” explains Du. “Previous fossil records on healed injuries were represented by hard structures such as the exoskeletons of arthropods (for example, the calcified cuticle of trilobites) and the shells of brachiopods.”
“This specimen is the first Cambrian fossil to preserve evidence of healed injury in a soft appendage, supporting the existence of high-level feeding loops, analogous to those in modern marine ecosystems.”
Identifying a predator is very difficult from fossil material.
Dr Stephen Pates
But there were questions about whether this distinctive mark on the soft body of the animal was genuinely a healed wound, or maybe a feature of the preservation. To figure this out, the researchers looked in detail around the edge of the mark, and they saw that there were significant differences.
“We can be confident that the fossil shows damage that has been repaired while the animal was alive because it has a healed margin,” co-author Dr Stephen Pates, from University College London, told IFLScience.
“Other parts of the fossil have broken, and you can see in the elemental maps a big change, whereas the margin with the wound is complete. W-shape wounds like this are common in trilobites, and from trilobite studies are confidently identified as predation wounds”.
Solving an (almost) murder mystery
This begs the question of what might have been chomping down on the top predator of these seas. And, amazingly, the researchers think they have a pretty good idea of that too.
“Identifying a predator is very difficult from fossil material,” explains Pates. “However, the size of the wound and a census of other animals living in the deposit and the similar Chengjiang suggests that either a large radiodont - perhaps another Amplectobelua symbrachiata - or the large worm Omnidens might have been the predators.”
So it might have been a bit of Amplectobelua-on-Amplectobelua action, or maybe it was a close call with another of the Cambrian’s fearsome predators.
What it does provide, however, is a rare glimpse into the behavior and interactions between animals living in these waters over half a billion years ago.
The preprint, which is yet to undergo peer review, has been posted to bioRxiv.
