Spiraltrochoid

Spiraltrochoid is a term used in paleontology to describe a group of extinct marine invertebrates belonging to the class Stelleroidea, which also includes modern starfish and brittle stars. These fossils are characterized by their typically pentamerous (five-armed) radial symmetry, although some variations exist. The arms of spiraltrochoids are usually distinct from the central disc and can vary in shape and size depending on the species. These organisms are known from the Paleozoic Era, with fossil evidence found in various marine sedimentary rocks.

The exact phylogenetic placement and evolutionary history of spiraltrochoids are subjects of ongoing scientific study and debate. They represent an early radiation of echinoderms, and their morphology provides clues about the diversification of this phylum. Paleontologists study spiraltrochoid fossils to understand ancient marine ecosystems, evolutionary trends, and the paleobiogeography of the time. Their presence in certain rock formations can also serve as an indicator of paleoenvironmental conditions. The term itself is derived from ancient Greek words reflecting their coiled or spiral-like structures, though this refers to specific features in some forms and not all members of the group.