A weird, ancient shrimp has changed scientists' understanding of evolution

During the Cambrian period, the world’s oceans were teeming with strange, swimming, segmented creatures.

Over the course of millions of years, these elongated, millipede-like animals would eventually evolve to become modern arthropods: crustaceans like crabs, arachnids like scorpions, and insects like bees and ants.

But some 500 million years ago, these aquatic Cambrian beasts were rather more experimental when it came to their physical traits and body plans than their more familiar descendants. There was the 3-inch-long Opabinia, with 5 eyes balanced on stalks and an elephant trunk-like mouth, and the predatory Radiodonta species, replete with two spine-laden, curving and segmented appendages designed to capture prey.

But there was another creature swimming around Earth's oceans, showcasing both of these strange features, and so much more. Enter the Kylinxia zhangi.

This newly discovered, shrimp-like species is described in a paper published Wednesday in the journal Nature.

In the paper, the researchers reveal that rather than just one special feature, this small fossil arthropod possesses a melting pot of physical traits, including five eyes on stalks and curved, spiny hooks extending upward from the front of its body like talons. It appears to have been armored, too. The fossil has a fused head shield, an armored, segmented body, and other claw-like appendages down its shell.

The discovery helps scientists understand how Cambrian arthropods may have been related to each other, and how their legacy lives on in crabs and insects today.

Diying Huang is a co-author on the paper and professor at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. He tells Inverse that while modern arthropods may seem like a diverse bunch, they have nothing on their ancient ancestors.

“Modern arthropods are abundant and diverse. They are everywhere and familiar [to] people," he says. “The Cambrian arthropods are also complex, not only the diversity, [but] also the morphology, anatomy, and functional morphology.”

In a nod to its strange physiology, the researchers dubbed the new shrimp Kylinxia zhangi, named after Kylin, a Chinese mythological chimera. And while its mismatched body is curious, it is also a rare sign of a pivotal moment in ancient animal evolution. Kylinxia may be a “transition” species, able to shed light on the evolutionary relationships between the other animals that lived at the same time.

“[These] groups would [have] lived in the Cambrian sea at the same time or similar time,” Huang says. “They could have [the] same ancestor in an earlier time.”

Researchers use a process called phylogenetic analysis to try and reconstruct, on the basis of its strange physiology, Kylinxia's evolutionary pathway among arthropods. The analytic technique pays attention to details like how many segments a creature has, the shape of its head, or how spiky its appendages are. It assumes that similarities between animals are more likely to be based on evolutionary relationships than chance.

The fact that Kylinxia possess spiny, hunting appendages shaped like those of Radiodonta, combined with the fact these are upturned, like another arthropod, Megacheira, and a Megacheira-like body, leads the researchers to believe that Radiodonta and Megacheira inherited their appendages from a common ancestor, rather than evolving independently in each creature.

In other words, it’s the same creepy appendage, just flipped.

Adding Kylinxia to the ancient tree also sheds light on the evolution of modern arthropods. Previous work had suggested that Megacheira, with their spiny appendages, were closely related to Chelicerata, the group that includes modern scorpions and spiders, as well as a group of ancient animals with antennas, which may have evolved into insects, like bees and ants.

The spiny appendages on Megacheira, the mouth pinchers on scorpions and spiders, and antennas on bees are all similarly located on these animals' bodies, leading researchers to believe they may have all evolved from a creature or creatures with similar structure.

But researchers didn’t know what came first. Did the spiny appendages evolve into antennas, which evolved into pinchers? Or were mouthparts first? The discovery of Kylinxia ages the spiny appendages, suggesting they gave rise to both pinchers and antennae along separate lineages, rather than mouthparts arising from antennae, or vice versa.

The undersea world of little Kylinxia, swirling with segmented, many appendage beasts, is very unlike that of the modern oceans. But the diverse morphological and ecological experimentation by early arthropods like it “probably laid the foundation for their later evolutionary successes," the researchers say.

Abstract: Resolving the early evolution of euarthropods is one of the most challenging problems in metazoan evolution1,2 . Exceptionally preserved fossils from the Cambrian period have contributed important palaeontological data to deciphering this evolutionary process3,4 . Phylogenetic studies have resolved Radiodonta (also known as anomalocaridids) as the closest group to all euarthropods that have frontalmost appendages on the second head segment (Deuteropoda)5–9 . However, the interrelationships among major Cambrian euarthropod groups remain disputed1,2,4,7 , which impedes our understanding of the evolutionary gap between Radiodonta and Deuteropoda. Here we describe Kylinxia zhangi gen. et. sp. nov., a euarthropod from the early Cambrian Chengjiang biota of China. Kylinxia possesses not only deuteropod characteristics such as a fused head shield, a fully arthrodized trunk and jointed endopodites, but also fve eyes (as in Opabinia) as well as radiodont-like raptorial frontalmost appendages. Our phylogenetic reconstruction recovers Kylinxia as a transitional taxon that bridges Radiodonta and Deuteropoda. The most basal deuteropods are retrieved as a paraphyletic lineage that features plesiomorphic raptorial frontalmost appendages and includes Kylinxia, megacheirans, panchelicerates, ‘great-appendage’ bivalved euarthropods and isoxyids. This phylogenetic topology supports the idea that the radiodont and megacheiran frontalmost appendages are homologous, that the chelicerae of Chelicerata originated from megacheiran great appendages and that the sensorial antennae in Mandibulata derived from ancestral raptorial forms. Kylinxia thus provides important insights into the phylogenetic relationships among early euarthropods, the evolutionary transformations and disparity of frontalmost appendages, and the origin of crucial evolutionary innovations in this clade.

Correction: A previous version of this article misstated the length of Opabinia. We regret the error.