What new fossil species, named Atlascystis acantha, helped scientists in December 2025 solve a 500-million-year-old mystery about how starfish got their arms?

The discovery of the 500-million-year-old fossil species, Atlascystis acantha, was a significant breakthrough in understanding the evolutionary history of starfish. This ancient echinoderm, unearthed in Morocco, provided scientists with crucial insights into how these iconic marine animals developed their distinctive multi-armed body plan. Unlike modern starfish, which typically exhibit pentaradial (five-fold) symmetry, Atlascystis acantha possessed bilateral symmetry, much like humans and most other animals.

For a long time, the transition from bilaterally symmetrical ancestors to the radially symmetrical starfish was a perplexing mystery in the fossil record. Atlascystis acantha, as the oldest known bilaterally symmetrical echinoderm, filled a critical gap, acting as a "missing link" that revealed the step-by-step evolutionary process. Researchers determined that the evolution of starfish arms was not a direct path but a more intricate journey involving structures called ambulacra.

The research suggests that early echinoderms, like Atlascystis acantha, began with two ambulacra. Over time, this was reduced to a single ambulacrum, which then underwent duplication events, first to three and eventually to the five ambulacra seen in most modern starfish. This remarkable fossil, with its flattened, spiny body and specialized skeletal plates, allowed scientists to reconstruct the complex evolutionary route that led to the familiar stellar shape of starfish, offering a clearer picture of their deep ancestral past. While the "how" of their arm development is now largely understood, the specific reasons "why" starfish ultimately settled on five-fold symmetry continue to be a subject of ongoing scientific inquiry.