Fossil of ancient three-eyed insect relative fills gap in animal evolution

A 520-million-year-old fossil of an arthropod relative of insects and crustaceans with 3 eyes fills a gap in our understanding of how these invertebrate animals evolved.

Kylinxia zhangi was found near the town of Chengjiang in the Yunnan province of southern China. The Chengjiang biota is made up of more than 250 species of well-preserved organisms from the Cambrian period (roughly 540–485 million years ago).

The animal is about the size of a large shrimp. It also has two large front limbs presumed to be used to catch prey.

Ct scan image of arthropod fossil with extended front limbs
CT images of the fossil animal Kylinxia zhangi from southern China, courtesy of Professor Yu Liu, Yunnan University. The image shows the large frontal limbs extended. Credit: Professor Yu Liu, Yunnan University.

This period of Earth’s history saw the “Cambrian explosion” – the sudden appearance in the fossil record of a slew of new organisms including the evolution of all the major body plans in the animal kingdom.

The first eyes date from the end of the Ediacaran period which predated the Cambrian. During the Cambrian explosion, eyes became more complex and diverse. This is partly bound up with what is sometimes referred to as “evolutionary experiments” as diversity spikes and natural selection leads to the persistence of traits that are beneficial to different species.

Like “Blinky” the three-eyed fish from The Simpsons, Kylinxia had one more eye than humans and most vertebrates. Many invertebrates have different numbers of eyes today, including the tadpole shrimp (neither a tadpole, nor a shrimp, but belonging to a family called Triospidae meaning “three eyes”) which has 3 eyes. Previous descriptions of the animal wrongly counted 5 eyes.

Kylinxia tells us much more about the evolution of arthropods – the family to which crabs, spiders, insects and other animals with exoskeletons belong.

Arthropod bodies are divided into segments, most of which bear a paired of jointed limbs.

While arthropods such as trilobites are common in the fossil record, usually only the hardest parts of their bodies are fossilised.

The new Kylinxia fossil is nearly complete. This allowed researchers to image its head using a CT scanner and reveal its soft anatomy. The findings are published in the journal Current Biology.

The report says, the animal’s head is divided into six segments: the front one bearing eyes, the second with a pair of large grasping limbs and the other four with jointed limbs.

Micro ct scan image of arthropod fossil
Micro-CT model of Kylinxia showing features in the head. Credit: Professor Yu Liu, YKLP.

“The preservation of the fossil animal is amazing,” says lead author Robert O’Flynn, a PhD student at the University of Leicester in the UK. “After CT scanning, we can digitally turn it around and literally stare into the face of something that was alive over 500 million years ago.”

“Most of our theories on how the head of arthropods evolved were based on these early-branching species having fewer segments than living species,” says Dr Greg Edgecombe from the Natural History Museum, London. “Discovering two previously undetected pairs of legs in Kylinxia suggests that living arthropods inherited a six-segmented head from an ancestor at least 518 million years ago.”

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