Echinoids are divided into two subgroups: regular echinoids, with nearly perfect pentameral (five-part) symmetry; and irregular echinoids with altered symmetry.
The oldest echinoids belong to an extinct regular taxon called the Echinocystitoidea.
They first appeared in the fossil record in the Late Ordovician. Cidaroids or pencil urchins appear in the Mississippian in the Carboniferous and were the only echinoids to survive the mass extinction at the Permo-Triassic boundary.
Echinoids did not become particularly diverse until well after the Permo-Triassic mass extinction. True sea urchins first appear in the Late Triassic, cassiduloids in the Jurassic, and spatangoids or heart urchins in the Cretaceous. Sand dollars, a common and diverse group today, do not make an appearance in the fossil record until the Paleocene.
In echinoids, the skeleton is almost always made up of tightly interlocking plates that form a rigid structure or test — in contrast with the more flexible skeletal arrangements of starfish, brittle stars, and sea cucumbers. Test shapes range from nearly globular, as in some sea urchins, to highly flattened, as in sand dollars.
Living echinoids are covered with spines, which are movable and anchored in sockets in the test. These spines may be long and prominent, as in typical sea urchins and most have lovely raised patterns on their surface.
In sand dollars and heart urchins, however, the spines are very short and form an almost felt-like covering. The mouth of most echinoids is provided with five hard teeth arranged in a circle, forming an apparatus known as Aristotle’s lantern.
This wee urchin was photographed near Rosh Hanikrah, Israel on the Lebanese border.
