In December 2025, a 500-million-year-old mystery was solved regarding how starfish got their arms, thanks to the discovery of what new species?

For centuries, the distinctive five-armed shape of starfish has presented a fascinating evolutionary riddle. Most animals, including humans, exhibit bilateral symmetry, meaning they have a clear left and right side. Starfish, however, display pentaradial symmetry, with their body parts arranged around a central axis, typically in fives. The challenge for scientists was understanding how such a unique body plan could have evolved from bilaterally symmetrical ancestors.

This enduring 500-million-year-old mystery began to unravel thanks to the discovery of a new species named Atlascystis acantha. Unearthed from fossils in Morocco and described in late 2025, this ancient creature is considered a crucial "missing link" in the evolutionary tree of echinoderms, the group that includes starfish, sea urchins, and sea cucumbers. Unlike modern starfish, Atlascystis acantha possessed bilateral symmetry in its adult form.

What makes Atlascystis acantha so significant is its combination of bilateral symmetry with early versions of ambulacra, the structures from which starfish arms eventually developed. Researchers were able to trace a step-by-step evolutionary path, showing how echinoderms transitioned from having two ambulacra, then a reduction to one, followed by duplications that led to three and ultimately the familiar five-fold arrangement. This suggests that starfish didn't suddenly appear with five arms, but rather gradually reshaped an ancestral two-sided body plan over millions of years, even retaining their larval body axis into adulthood.

Further genetic research has even proposed that what we perceive as starfish "arms" are, in a genetic sense, more akin to extensions of a head-like body, rather than traditional limbs. The discovery of Atlascystis acantha has thus provided invaluable fossil evidence, bridging a critical gap in the understanding of how these captivating marine animals acquired their iconic star shape.