Caught in action: Newly found fossil shows 155 million old brittle star mid-regeneration

German scientists have discovered the fossil ofa six-armed brittle star that was petrified while it was still partially renewing.

This very unique fossil was discovered in 2018 in a limestone deposit in southern Germany, which was originally a deep lagoon teeming with coral meadows and sponge beds. These days it is a fossil garden full of shark teeth and the skeletal remains of extinct pterosaurs, crustaceans, and crocodile-like animals from the late Jurassic period.

The fossil is the first and only example of Ophiactis hex, a new species of brittle star that scientists have identified.

Fissiparity, in which the star "splits" in two and each side regenerates its missing pieces, is the reproduction mechanism used by many contemporary brittle stars and certain starfish.

Because the two new stars are clones of the original, it is sometimes referred to as clonal fragmentation. The species that use this method tend to have six arms, instead of five, to make for an even split.

"While the biology and ecology of clonal fragmentation are comparatively well understood, virtually nothing is known about the evolution and geological history of that phenomenon," the paper published in the journal Proceedings of the Royal Society B: Biological Sciences explains.

The Ophiactis hex fossil, which is 155 million years old and has been preserved to the point that you can see its hook-shaped arm spines, offers compelling evidence of the deep evolutionary roots of clonal fragmentation in star-shaped echinoderms.

It was given that name in honour of the supercomputer that can conceive the unimaginable, in one of Terry Pratchett's Discworldnovels. It appears that the six-armed brittle star discovered by this group of paleontologists is an undiscovered object, proving that fissiparity is an old asexual reproduction strategy connected to that symmetrical body design.

"While skeletons of ophiuroids with individual arms frozen in the process of regeneration are relatively common in the fossil record, cases of individuals with a regenerating body half are exceedingly rare," the study read.

"To the best of our knowledge, the specimen described in the present paper is only the second case known so far, and the first one for which regeneration seems indeed linked to six-fold symmetry and clonal fragmentation," it continued.

The researchers acknowledge that it is hard to determine the precise appearance of the species before its division and whether or not it was six-armed, given that only one specimen of Ophiactis hex has been discovered so far.