Even top predators weren鈥檛 safe in Cambrian seas around 520 million years ago. An extraordinary fossil shows a healed wound where one predator took a bite out of the side of another.
鈥淚 would never have expected to see something like this preserved in the fossil record,鈥� says at University College London.
The predator with a bite taken out of it is called 础尘辫濒别肠迟辞产别濒耻补听蝉测尘产谤补肠丑颈补迟补, which belonged to a long-extinct group of strange arthropods known as the radiodonts that looked a bit like a cross between squid and lobsters. They had two vicious, grasping appendages on their heads for catching prey, and a line of swimming flaps protruding from their sides that decreased in size towards their rear.
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They grew to nearly a metre long, which might be tiny compared with modern marine predators, but back in the Cambrian Period, it made them one of the biggest predators around.
The swimming flaps of radiodonts were softer than their grasping appendages, resulting in very few fossils of them. But a team recently found a single flap at a site in China called the Yu鈥檃nshan Formation, where soft tissues are exceptionally preserved.
It is recognisable as a radiodont flap because of a characteristic pattern of lines on its surface. If it was the largest swimming flap, the animal would have been at least 23 centimetres long. If it was one of the smaller flaps, it could have been much larger.
The researchers noticed an odd W-shaped feature on the edge of the flap 鈥� the kind of shape typically left when two fangs tear through flesh.
But is it really a bite wound? The researchers in China got in touch with Pates because he has studied similar injuries in trilobites. 鈥淵ou want to rule out that it鈥檚 been broken during the fossilisation process,鈥� says Pates.
X-ray fluorescence mapping shows that the chemical composition of the W-shaped part is consistent with the rest of the fossil, which means it鈥檚 not an artefact caused by flaking. 鈥淭his is telling us that this is likely a flap that was preserved like this, rather than altered later,鈥� says Pates.
What鈥檚 more, there is a black band visible around the edges of the W-shape that appears to be a sign of healing. 鈥淚n modern arthropods, you see this melanisation, this darkening around areas where they鈥檙e regrowing,鈥� he says. 鈥淚 think we can be fairly confident this happened while the animal was alive.鈥�
It is an extraordinary find because only around 10 isolated radiodont flaps like this have ever been found, says Pates. 鈥淲ith more complete specimens, even if these wounds were there, you wouldn鈥檛 necessarily be able to see them.鈥�
So, what took a chunk out of this ancient predator? The team can鈥檛 be sure, but it could have been another species of radiodont, or even a member of the same species, says Pates.
Hundreds of likely predation wounds have been found on trilobites, but their fossils are much more common because of their hard exoskeletons and because they survived until 250 million years ago, whereas radiodonts were all extinct by around 480 million years ago. Trilobites were also much smaller and nearer the bottom of the food chain.
鈥淭here have been other reports of healed injuries in Cambrian animals, though never in a radiodont,鈥� says at the Manitoba Museum in Canada. 鈥淲ith only one specimen, I don鈥檛 think we can rule out other potential causes.鈥�
For instance, it鈥檚 not unusual for arthropods to suffer injuries while shedding an exoskeleton, and so this flap might have been damaged during moulting, says Moysiuk.
鈥淵ou can鈥檛 rule it out, but it鈥檚 [a] balance of probabilities,鈥� says Pates. For instance, a moulting wound is most likely to be a singular tear, whereas the W-shape points more towards a bite mark, he says. The location on the flap is also what would be expected if the radiodont was fleeing an attacker, whereas a moulting wound could occur anywhere on the animal鈥檚 body.
鈥淔or us, the best interpretation for this injury is a predation injury,鈥� says Pates.
bioRxiv
