By Taormina Lepore

The fossils of helicoplacoids don’t look like much more than fingerprints. But these 500-million-year-old smudges are all that remain of the inch-long, bumpy, balloon-shaped animals that thrived in Earth’s oceans for 13 million years before going extinct. Helicoplacoids are more than just a blip in the grand saga of evolutionary history; they actually hold a surprising secret.

When paleontologist James Wyatt Durham began to examine helicoplacoid fossils in the 1960s, he realized that they were echinoderms, like sea stars and sea urchins, modern animals that also make their living on the seafloor. But Durham noticed a key difference between extinct helicoplacoids and living echinoderms.

Why were they so different? Think of your own human body for a moment. We generally have two arms, two legs, two eyes, a body plan that mirrors itself on either side. This is called bilateral symmetry, meaning we are symmetrically divided into “two sides” (bi– means “two” and latus means “side” in Latin).

Now think of a sea star, with its five (or more) limbs arranged in a circular pattern. Almost all living echinoderms exhibit this radial symmetry as adults, specifically five-fold or pentaradial symmetry (penta– means “five” in Greek, and radius means “circle” in Latin). Helicoplacoids, however, have three-fold or triradial symmetry, which divides their balloon-shaped bodies into three symmetrical parts from top to bottom. This might seem minor, but in terms of the ways that animal bodies are arranged, it is as strange as if Durham had discovered a group of fish that all had three heads!

Durham thought that the helicoplacoids and their triradial symmetry were so different from other echinoderms that they belonged in their own sub-group. He published his description of this new animal class with his colleague Kenneth E. Caster in 1963. Durham’s description of these surprising fossils helped revise our scientific understanding of the entire group of echinoderms.

Biologists now think that when echinoderms first arose, they had bilateral symmetry, and then went through a series of evolutionary changes that affected their symmetry, with mainly the pentaradial lineages surviving. Helicoplacoids are our only fossil snapshot of a time in echinoderms’ evolutionary history when triradial species were around – or at least our only snapshot so far! Future generations of paleontologists may yet discover other triradial fossil species and use them to further fine tune our scientific understanding of Earth’s ancient life.