Some brittle stars provide arms and legs (and another joint) to reproduce. When there are few mates, sea creatures like starfish split in half. Each side replants its missing half, creating two identical clones of the original animal.
This process, known as clonal fragmentation, is carried out by about 50 species of extant brittle stars and their starfish relatives. However, scientists have struggled to determine when brittle stars, a gangly group of echinoderms, began to reproduce in this way.
A recently discovered fossil from Germany pushes the origin of cloning sea stars back more than 150 million years. In a paper published Wednesday in The Proceedings of the Royal Society Ba team of scientists describes the fossil of a brittle star that melted while creating three of its six legs.
“This is the first fossil evidence for this phenomenon,” said Ben Thuy, a paleontologist at the National Museum of Natural History in Luxembourg and an author of the new study. The specimen, he added, shows that “clonal fragmentation is actually much older than people thought.”
The brittle 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 calm lagoon home to marine crocodiles, sharks and pterosaurs. When some of these creatures died, they sank to the bottom and were covered in mud. Low oxygen levels slowed their decomposition, preventing scavengers from picking up the corpses.
These conditions preserve fossils in incredible detail, capturing delicate structures such as dragonfly wings and even a dinosaur fur. The newly described brittle star is another treasure stamped into the site’s limestone slabs. “You have a brittle star with every piece in its original place, as if it washed up on the beach the other day,” said Dr. Tuy.
The brittle star fossil was discovered during a 2018 excavation by researchers from the State Museum of Natural History in Stuttgart, Germany. Collaborated with Dr. Thuy to researchers from all over Germany and Austria to study the fossil.
The brittle star’s mismatched anatomy is striking. Three of its arms are thin squiggles compared to its other three arms, which are larger and have thorns.
The scientists placed the brittle star inside a micro-CT scanner to examine its structure. They also compared the animal’s anatomy to other brittle star species.
The researchers concluded that the fossil is the oldest known member of an extant 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 can imagine the unimaginable.
For scientists, discovering a fossilized creature while it cloned itself was unthinkable.
In the past, researchers have discovered fossils of starfish that reproduce single legs. A brittle star from a Jurassic deposit in Switzerland regrows multiple limbs when fossilized. But the irregular growth patterns in these early fossils appear to represent sea stars regenerating limbs lost to injury. In contrast, O. hex seems to regenerate limbs along a symmetrical plane, making it the only known echinoderm fossil that froze after cloning.
The new fossil provides evidence that brittle stars have been dividing themselves into two since at least the late Jurassic period. According to Gordon Hendler, an echinoderm curator at the Natural History Museum of Los Angeles County, about half of all living Ophiactis brittle stars are capable of cutting themselves in two. Asexual reproduction helps spindly scavengers quickly conquer environments such as sponge meadows and patches of algae.
Since they usually live in dense populations, it is possible to find more brittle star clones in the Nusplingen limestone. But said Dr. Hendler that finding a fossil like this O. hex specimen is luck.
“The chances of another find like this ‘ancient link’ seem very slim,” he said in an email. “I hope I’m wrong!”
