Some spider stars give their arms and legs (and yet another appendage) for reproduction. When lacking mates, these starfish-like sea creatures split their bodies in half. He then creates two identical clones of the original animal, with each side regenerating its lost half.
This process, known as clonal fragmentation, is carried out by nearly 50 extant species of spider star and its related sea stars. But scientists have found it difficult to pinpoint when the spider star, a group of echinoderms, began to reproduce in this way.
A recently discovered fossil in Germany traces the origins of starfish cloning back more than 150 million years. In a paper published Wednesday in Proceedings of the Royal Society B, a team of scientists describes a fossil spider star in which three of its six limbs were petrified during regeneration.
“This is the first fossil evidence of this phenomenon,” said Ben Thuy, a paleontologist at the Luxembourg National Museum of Natural History and author of the new study. He added that the specimen shows that “clonal fragmentation is actually much older than people previously thought.”
Spider star fossils were discovered in the Nuschlingen limestone deposit in southern Germany. During the Late Jurassic Period, 155 million years ago, this region was a refreshing lagoon home to marine crocodiles, sharks, and pterosaurs. When some of these creatures died, they sank to the bottom and were buried in mud. Low oxygen levels slowed decomposition and prevented scavengers from picking off the carcasses.
These conditions preserved the fossil in incredible detail, capturing delicate structures such as dragonfly wings and dinosaur feathers. The newly described spider star is another treasure carved into the site's limestone slabs. “This spider star is all in its original location, as if it washed up on the beach a day ago,” Dr Tui said.
The spider star fossil was discovered during excavations in 2018 by researchers at the National Museum of Natural History in Stuttgart, Germany. Dr. Tui worked with researchers from across Germany and Austria to study the fossil.
The discordant anatomy of the spider star was striking. Three of his arms were thin wavy lines, and his other three arms were large and studded with spines.
Scientists placed the spider star inside a micro-CT scanner and studied its structure. They also compared the animal's anatomy to other spider star species.
The researchers concluded that the fossil is the oldest known member of the extant arachnid family, called the Ophiactidae. They named the fossil spider star in the genus Ophiactis and added the species name Hex, after its six arms and as a nod to Hex, the magical supercomputer created by fantasy author Terry Pratchett. In Pratchett's book Discworld, Hex has an unimaginable imagination.
For scientists, discovering a fossilized creature that clones itself was unimaginable.
In the past, researchers discovered fossilized starfish that regenerated a single limb. A spider star unearthed from Jurassic deposits in Switzerland had regenerated multiple limbs when it fossilized. However, the irregular growth patterns of these early fossils likely represent starfish recovering limbs lost to injury. In contrast, O. hex appears to regenerate limbs along a plane of symmetry, making it the only known echinoderm fossil frozen due to cloning.
This new fossil provides evidence that spider stars have been splitting themselves into two since at least the late Jurassic period. According to Gordon Hendler, curator of echinoderms at the Natural History Museum of Los Angeles County, about half of the extant Ophiactis spider stars are capable of cutting themselves in half. Asexual reproduction helps spiny scavengers quickly colonize environments such as sponge meadows and algae patches.
More spider star clones may be found in the Nüschlingen Limestone, as spider stars usually live in close densities. But Hendler says finding fossils like this O.hex specimen is a blessing in disguise.
“The chances of something like this 'ancient link' ever being discovered again seem very small,” he said in an email. “I hope I'm wrong!”
