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Three-eyed ‘fossil monster’ could help explain how insects evolved
A marine predator living over 500 million years ago might bridge the gap between insects and their ancient relatives.
Fossils from China have revealed the head of Kylinxia zhangi in unprecedented detail, helping to reveal how modern arthropods evolved.
Insects might have inherited their distinctive multisegmented heads from an ancestor living more than half a billion years ago.
Newly described fossils of Kylinxia zhangi, an ancient animal that lived during the Cambrian Period around 518 million years ago, reveal that it had a head split into six segments, with three eyes and a pair of enlarged limbs used to grapple with prey.
While Kylinxia isn’t a direct ancestor of modern arthropods, the group containing the crustaceans, insects and spiders, it shares some traits thought to be present in the ancestor of these animals at this point in time.
Robert O’Flynn, a PhD student who led a new study of Kylinxia, says, ‘The preservation of this fossil animal is amazing. After CT-scanning we could digitally turn it around and stare into the “face” of something that was alive over 500 million years ago.’
‘As we spun the animal around, we could see that its head possessed six segments, just as in many living arthropods.’
The findings of the study were published in the journal Current Biology.
Scans of new Kylinxia specimens have allowed previously unidentified details to be recognised. Image © Professor Yu Liu.
Digging into the Chengjiang Biota
Fossils of Kylinxia were first found in Chengjiang county, located in southwest China. Back in the Cambrian Period, this region would have been a wide river delta, supporting a range of wildlife.
When storms formed further upstream, the resulting floodwater would have swept down the river and rapidly buried the animals under a pile of sediment. This quick burial was crucial to keep the bodies away from oxygen and predators, making it more likely soft tissue would survive in fossils.
As a result, many animals without skeletons which haven’t survived elsewhere in the fossil record are found here, while researchers have also found the remains of organs like eyes. These fossil species are collectively known as the Chengjiang Biota and have been vital in improving our understanding of early animals.
Dr Greg Edgecombe, an expert in the evolution of arthropods at the Natural History Museum and co-author of the new research, says, ‘The preservation of the Chengjiang Biota is a result of rapid burial and lack of oxygen.’
‘It’s thought that seawater saturated with calcium carbonate caused thin crusts of calcite to form at the top of a sediment layer that buried the animals, sealing the bodies off from oxygen. This slowed down decay and allowed for the soft tissues to be preserved.’
When Kylinxia was named in 2020, this exquisite preservation allowed scientists to pick out a pair of appendages with blade-like spines emerging from one of the head segments. These were probably used for grappling with prey as either a predator or scavenger.
At the time, it was also thought that the creature might have had five eyes, which is similar to another Cambrian fossil known as Opabinia. However, using 3D models of new fossils the team found that this wasn’t quite the case.
Instead, what were thought to be the fourth and fifth eyes are actually two parts of a head plate to which the other eyes are connected. They found that the animal had two compound eyes located on either side of the head, similar to modern insects, while another structure known as a median eye sits in the middle.
‘The median eye isn’t well understood in this animal,’ Greg says. ‘Its shape and position suggest it is an eye, but without the lenses, which are uncommonly preserved in Chengjiang Biota, it’s difficult to tell exactly what it was like.’
Other structures on the head, however, have proven more insightful. Together, the variety of appendages have helped to firm up Kylinxia’s position in the overall tree of life, and the evolution of the arthropods more widely.
The segmentation of Kylinxia's head is shared by modern insects and some of their relatives. Image © piotreknik/Shutterstock.
Searching for the origins of the arthropods
Modern arthropods have complex heads made up of many different segments, each of which contain specialised structures such as eyes, antennae or mouthparts. The exact combination changes between different groups, however, so its evolution remains a matter of scientific debate.
Kylinxia helps to offer new evidence in the search for an answer to this question. By using CT scans, the researchers could peer inside the rock containing the fossils to get a better look at its head.
‘The original description of Kylinxia found that it had two pairs of single-branched appendages in addition to those it used to grasp prey,’ Greg says. ‘We found there are actually four pairs, each with two branches.’
‘While Kylinxia doesn’t have the variety of specialised appendages that modern arthropods do, it does have a full complement of them. Along with other arthropods described by Robert, this helps us to understand how these characteristics are distributed in the family tree.’
The unique combination of Kylinxia’s characteristics suggest that it might bridge the gap between ancient animals such as Anomalocaris and modern arthropods.
This would make Kylinxia a stem arthropod, one of the last branches of the arthropod family tree before the last common ancestor of all living members of the group evolved. It suggests that many shared characteristics of these animals, such as their head, evolved long before the arthropods themselves did.
As more well-preserved fossils of the Chengjiang Biota are uncovered, they will provide scientists with an even better understanding of how the major animal groups came into being.
