“Living fossil” is often misused, but chitons really haven’t changed much in about 300 million years.
Acanthochitona feroxa joins the 940 known species of chiton that have barely changed over the last 300 million years. The name of this living armored shield is derived from the Latin ferox, which translates to “fierce” or “bristling” and was inspired by the tufts that make chewing on this creature a rather unpleasant experience for predators. Usually found clinging to rocks, chitons of the genus Acanthochitona are recognizable by the tufts of bristles protruding from the edges of their flattened oval shells.
Some visible features of A. feroxa occur throughout Polyplacophora. Chitons typically have pointed spicules on the backs of their shells and eat with a radula, a tonguelike structure covered in tiny sharp teeth that help them scrape algae off rock surfaces, though they also feed on microorganisms like diatoms and bryozoans. Previously thought to be a subspecies of Acanthochitona defilippii because of morphological similarities, the slightly different dorsal spicules observed on the shell of A. feroxa raised questions. Zooming in with a scanning electron microscope revealed that these were rounded instead of pointed, and its radula and shell plates showed geometric differences. Molecular genetic tools sealed the case that it was a new species. “These molecular techniques have been proven potent in uncovering cryptic species within groups that exhibit morphological similarities,” the researchers said.
There is a reason A. feroxa, with its brownish-greenish shell and distinguishing tufts, could almost pass for A. defilippii. The two are closely related even without one being a subspecies of the other. Polyplacophora began to diversify around 378 million years ago during the Devonian Period, at the same time proto-tetrapods (our earliest land ancestors) were emerging from the seas. The genus Acanthochitona developed about 92 million years ago, during the Late Cretaceous, when rising sea levels and spreading shallow marine habitats are thought to have created more niches for mollusks, including chitons, which have adapted to both the shallows and depths.