Where are echinoderms found

Missing link in starfish evolution discovered

Extraordinary find: In Morocco, paleontologists have discovered one of the oldest ancestors of the starfish. The 480 million year old fossil already resembles a five-pointed star, but it also has some features of sea lilies. This makes it an important link in the evolution of echinoderms and sheds new light on the starfish family tree, as the researchers report.

Like the brittle stars, sea urchins, sea lilies or sea rollers, starfish belong to the group of echinoderms. The five-pointed symmetry of her body is typical for her. Thanks to the enormous strength of their arms, the mostly predatory starfish can even crack mussels. Some of the species can evert their stomachs to digest larger prey, others graze on coral reefs and are therefore feared as coral pests.

The roots of the starfish go back to around 480 million years ago. "If you were to go back in time and dive in the sea of ​​the Ordovician, almost all animals there would be completely alien to us - except for the starfish," explains first author Aaron Hunter of the University of Cambridge. "Because they are among the oldest representatives of modern animal groups."

Find in Morocco's fossil "treasure chest"

However, the origin and early evolution of the starfish-like species are still unclear. Paleontologists neither know exactly when and how the starfish got their typical star shape, nor how their ancestors were related to the other echinoderms found in the Ordovician and Cambrian.

This could change now. Because a fossil discovered in Morocco gives completely new insights into the evolution of starfish and their unique body shape. Hunter and his colleague Javier Ortega-Hernandez found it in the 480 million year old Fezouata slate formation in southeast Morocco. There, the particularly fine sediment of what was once a deep seabed itself has preserved fine structures and fragile soft tissues of living beings - among them the unusual original starfish.

Characteristics of starfish and sea lilies in one animal

“The level of detail in this fossil is amazing - its structure is so complex that it took us a while to figure out what it meant,” explains Hunter. Because the primal starfish, baptized Cantabrigiaster fezouataensis, shows the typical five-pointed symmetry of its current descendants and has the ambulacral system of fluid-filled canals that is typical for echinoderms.

At the same time, however, the fossil lacks some essential starfish features, including an arm rim clearly separated by limestone flakes or ribs. Instead, his arm plates run out in a feather-like frond structure, as the paleontologists report. In this respect, Cantabrigiaster is more similar to sea lilies, a very primitive and ancient group of echinoderms.

Link of the starfish evolution

According to the researchers, Cantabrigiaster is thus a crucial link in the evolution of starfish. They classify this echinoderm at the base of the branch of the family tree, which includes all of today's starfish and brittle stars as well as the already extinct primeval starfish (Somasteroidea). At the same time, Cantabrigiaster is the most original representative of the early starfish-like species to date and is therefore the closest to the closely related sea lilies.

“Finding that missing link to the starfish ancestors is incredibly exciting,” says Hunter. Because the new fossil now offers the opportunity to explore the development and relationships of the early echinoderms more closely. In addition, there is information about when the typical starfish physique originated and what its preliminary stages were. (Royal Society Biology Letters, 2021; doi: 10.1098 / rsbl.2020.0809)

Source: University of Cambridge

January 21, 2021

- Nadja Podbregar