A paleontological discovery in Morocco has provided direct clues as to how the symmetry of starfish, a five-ray body organization that distinguishes them in the animal kingdom. The key piece is a Cambrian echinoderm that documents, in unusual detail, the transition from a bilateral body towards pentaradial architecture.
The protagonist of the study is Atlascystis acantha, a fossil of about 510 million years recovered in the Moroccan Anti-Atlas and analyzed by an international team co-led by the IGME-CSICThe research, published in the journal Current Biology, shows that this organism combined bilateral traits with characters that anticipated the five-radius symmetry of modern echinoderms.
A discovery that reorders the origin of stellar symmetry
In present-day echinoderms, such as starfish, pentaradial symmetry It dominates the adult stage, although its larvae are bilateral. Atlascystis acantha breaks this scheme because it maintained bilaterality in maturity, while exhibiting structures that point to the transition towards a radial body plan, which makes it a evolutionary link essential.
To understand the contrast, it is worth remembering that the pentameric symmetry divides the body into five equivalent sectors, something evident in starfish. This organization is not exclusive to them: it also characterizes sea urchins, crinoids and holothurians, all members of the group of echinoderms, Unlike the cnidarians with radial symmetry.
What Atlascystis acantha reveals and why it matters
The study identifies Atlascystis acantha as the oldest known echinoderm with bilateral symmetry in the adult phase and documented at different stages of development. Its anatomy combines primitive features with elements characteristic of radial forms, placing it at a key intermediate point for reconstructing the morphological transition.
Among these elements, the following stand out: ambulacra, ducts associated with the water vascular system with which current echinoderms move, feed and attach to the substrate. The presence of ambulacra in such an ancient bilateral form helps to directly link the first echinoderms with their modern descendants.
From the bilateral plane to five rays: the role of the ambulacra
The evolutionary key lies in how the organisms were reorganized and multiplied. ambulacra over time. Evidence indicates a path in which the duplication and subsequent reconfiguration of these structures ended up establishing the five-spoke symmetry characteristic of the sea ​​stars.
- The oldest forms show two ambulacra, a simple, bilateral pattern.
- In certain lineages a reduction to one, followed by further duplications.
- The series of duplications led first to three and finally to five spokes, establishing the pentaradial architecture.
This itinerary explains why current echinoderms, despite their apparent radial symmetry, retain a bilateral imprint in their development and how the pentardiality emerged as a stable solution in the group.
Where and how the fossil was studied
The specimens come from deposits of the Lower Cambrian in the Anti-Atlas (Morocco), an exceptional region for investigating the early diversification of animal life. This geological context provides a time frame of around 510 million years, key to understanding the emergence of new body plans.
The team applied Synchrotron radiation to obtain high-resolution three-dimensional reconstructions without the need for mechanical preparation, thus avoiding alteration of the material. Thanks to this technique, it was possible to precisely observe internal and external details that reveal how the structures of the material were organized. fossilized organism.
Implications for understanding modern-day starfish
By documenting an intermediate stage between ancestral bilaterality and pentaradial symmetry, the work fills a gap in the fossil record and offers an anatomical basis for interpreting why starfish have five arms. The link provided by the ambulacra connects the early diversity of echinoderms with their remarkable later success.
The combination of a key fossil, a precise geological context and advanced imaging techniques allows to reconstruct the change of a bilateral body plane to another radial, shedding new light on how the symmetry that characterizes the sea ​​stars and their close relatives.
