27.05.2024

When stars divide – Oldest fossil brittle star with asexual reproduction found

Science News
Fossil eines Schlangensterns mit sechs Armen. Die drei Arme der rechten Körperhälfte sind dünner und kürzer als an der linken Körperhälfte.
Fossil of the brittle star Ophiactis hex. (Copyright: Günter Schweigert, SMNS)

Brittle stars have excellent regenerative abilities that allow them to recover well from injuries and even regrow lost arms. Some species of these echinoderms even go one step further and can reproduce asexually by dividing down the middle of their body and regenerating the missing halves. Through this process, a clone of the original animal is created. This "ability" is known as fissiparity and enables the brittle stars to expand into new habitats more quickly.

Of the more than 2000 species of brittle stars today, asexual reproduction by division is only known in a total of around 30 species, most of which are not closely related to each other. Nevertheless, many of these species show astonishing similarities in their anatomy and behavior. For example, they usually have small bodies and often have six arms rather than the typical five. Both adaptations are believed to be closely related to fissiparity: the six-fold symmetry allows easier separation into two equal halves and the small size facilitates rapid regeneration of the missing body parts after division. While the biological process and ecological significance behind this type of reproduction are well understood, little is known about the evolutionary origin of this ability.

An international team led by paleontologists Dr Ben Thuy from the National Museum of Natural History Luxembourg and Dr Günter Schweigert from the Natural History Museum Stuttgart has recently described the fossil of a brittle star from the Nusplinger Plattenkalk, which is not only a previously unknown species, but also provides interesting insights into the evolution of fissiparity in brittle stars. 

While it is quite easy to find evidence of fissiparity in today's brittle stars, it proves difficult in the case of exclusively fossilised species. This makes the fossil from Nusplingen described by Dr Ben Thuy and Dr Günter Schweigert all the more astonishing! The fossilised, six-armed brittle star shows that one half of the animal's body was in the process of regenerating after a reproductive division. Other body features also show astonishing parallels to today's brittle stars, which can reproduce asexually through division. This makes it the first and oldest (around 150 million years old) fossil find of a brittle star with proven fissiparity. 

The authors named the new brittle star species Ophiactis hex. The species name hex is derived from it having six-armes and also from the magical super-computer of the same name (from the Discworld fantasy novels by British author Terry Pratchett) - a machine or creature that can think the unthinkable.

The article was published in the scientific journal "Proceedings of the Royal Society B".

Original publication