Some brittle stars give an arm and a leg (and another appendage) to reproduce. When mates are scarce, these starfish-like sea creatures split in half. Each party then re-grows its missing half, creating two identical clones of the original animal.
This process, known as clonal fragmentation, is practiced by almost 50 species of extant brittle stars and their starfish relatives. However, scientists have found it difficult to determine when brittle stars, a thin group of echinoderms, began to reproduce in this way.
A recently discovered fossil from Germany pushes the origin of starfish cloning back more than 150 million years. In a paper published Wednesday in The Proceedings of the Royal Society B, a team of scientists describes a fossil of a brittle star that was fossilized while regenerating three of its six limbs.
“This is the first fossil evidence of this phenomenon,” said Ben Tui, a paleontologist at the National Museum of Natural History in Luxembourg and author of the new study. The specimen, he added, shows that “clonal fragmentation is actually much older than people previously thought.”
The fragile star fossil was discovered in the Nusplingen limestone deposit in southern Germany. In the late Jurassic period, 155 million years ago, this area was a soft lagoon, home to marine crocodiles, sharks and pterosaurs. When some of these creatures died, they sank to the bottom and were buried by silt. Low oxygen levels slow their decomposition, preventing scavengers from picking up the carcasses.
These conditions preserve fossils in incredible detail, capturing delicate structures like dragonfly wings and even dinosaur feathers. The newly described brittle star is another treasure imprinted on the limestone slabs at the site. “You have this fragile star with every single piece in its original place, as if it had washed up on the beach a day ago,” Dr Tui said.
The fragile star fossil was discovered during a 2018 excavation by researchers at the State Museum of Natural History in Stuttgart, Germany. Dr Tui teamed up with researchers from across Germany and Austria to study the fossil.
The fragile star’s mismatched anatomy stood out. Three of his arms were thin curves compared to his other three arms which were larger and studded with thorns.
The scientists placed the fragile star in a micro-CT scanner to study its structure. They also compared the animal’s anatomy to other types of brittle stars.
The researchers concluded that the fossil is the oldest known member of a still-living family of brittle stars called Ophiactidae. They placed the fossil brittle star in the genus Ophiactis and added the species name hex, both in reference to its six arms and as a nod to Hex, a magical supercomputer created by fantasy writer Terry Pratchett. In Pratchett’s Discworld books, Hex is able to imagine the unimaginable.
For scientists, finding a fossilized creature while it has cloned itself is unimaginable.
In the past, researchers have found fossils of starfish regenerating single limbs. A brittle star from a Jurassic site in Switzerland even regrew multiple limbs when it was fossilized. But the irregular growth patterns of these earlier fossils appear to represent starfish regenerating limbs lost to injury. In contrast, O. hex appears to regenerate limbs along a symmetrical plane, making it the only known echinoderm fossil frozen after cloning.
The new fossil provides evidence that brittle stars have been splitting into two since at least the late Jurassic period. According to Gordon Handler, curator of echinoderms at the Los Angeles County Museum of Natural History, roughly half of all living Ophiactis stars are capable of splitting in two. Asexual reproduction helps slime scavengers quickly colonize environments such as mushroom meadows and kelp patches.
As they usually live in dense populations, it may be possible to find more fragile star branches in the Nusplingen limestone. But Dr Handler says finding a fossil like this O. hex specimen requires luck.
“The chances of another find like this ‘ancient connection’ seem vanishingly small,” he said in an email. “I hope I’m wrong!”
