.40" Edrioasteroid On Brachiopod Shell- Ontario

This is a .40" Edrioasteroid attached to an unidentified brachiopod shell from the Veralum Formation, Ontario, Canada. It is still in the rock in which it was fossilized. Other fossils can be seen including crinoid stem sections and other edrioasteroids.

Edrioasteroids are an extinct class of echinoderms. Distantly related to starfish and sea urchins, they have a body laid out in a pentaradial pattern. They also had a water vascular system and a skeleton made of calcite plates. They were filter feeders who lived permanently attached to an object or the seafloor. Some are thought to have had short stems like crinoids but most lived flat on whatever object they had attached to as larva. Edrioasteroids appear in the Cambrian Period about 515 million years ago. They zenith of their diversity during the Late Ordovician Period. By about 275 million years ago, during the Permian Period, Edrioasteroids are extinct.

Edrioasteroids were small organisms from a few millimeters to a couple centimeters wide. They look like a tiny cushion attached to a substrate. The mouth was in the center of the theca (body) and from it, five ridges radiate out in a pentaradial pattern. These ambulacra channel food along the body to the mouth. There is little fossil evidence of how this was done, but by looking at modern echinoderms, it is likely Edrioasteroids had cilia or tube feet along the ambulacra that moved the food to the mouth. The ambulacra radiate out from the mouth in either straight lines or curving to form a whorl. Usually they all curve in the same direction but in a few species the curve in different directions.

Brachiopods are members of the phylum Brachiopoda. They are clam-like with wide shells composed of two halves called valves. They are filter feeders that live a-fixed to rocks or on the seafloor. Brachiopods first appear in early Cambrian. These were simple forms with non-articulating shells. Their diversity peaked during the Devonian and there are currently 12,000 described fossil species of Brachiopoda from 5,000 genera. Most species of brachiopod died out during the Permian-Triassic Extinction but there are about 450 species living today. They live in cold, marine environments, like polar seas or the continental shelf and slope. The largest fossil Brachiopod is 7.9 inches (200 mm). Most are 2-4 inches (3-8 cm). Living Brachiopods also fall into this range.

Brachiopods are more closely related to Bryozoans than Mollusks. The easiest differences to identify are in the shells of clams and Brachiopods. Mollusk shells are divided into left and right while Brachiopod shells are divided top (dorsal) and bottom (ventral). The shells of mollusks are usually equal on the right and left. In Brachiopods, the bottom shell is larger than the top. The other big difference is in how they feed. Both are filter feeders but mollusks extend their filter into the water and pull food into its shell. Brachiopods have internal feeding structures. Water is drawn into the shell where the food is filtered out before expelling it out.
Isorophusella incondita, Uidentified Brachiopod
Gamebridge, Ontario
Verulam Formation
.40" wide
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