Your Daily Fossil

Rhynchotrema

When: Ordovician (490 - 440 million years ago) 

Where: Fairly cosmopolitan, with major concentrations known from Midwestern USA

What: Rhynchotrema is a brachiopod.  Brachiopods are a clade of marine invertebrates that superficially resemble clams, but have no close relationship, as clams are in the phylum Mollusca. This resemblance is only in the bi-valve shells that protects the soft bodied organisms inside. While clams have a left and right shell, brachiopods shells are a top and bottom, with the axis of symmetry cutting though the shells, not between them. Inside of this shell brachiopods have a feeding structure called a lophophore, a collection of tentacles that circulate water and drive small particles of food towards the mouth of the brachiopod. 

Brachiopods were the dominate filter-feeders and a major component of reef-building during much of the Paleozoic, but were among the most heavily hit clades during the Permian-Triassic extinction event, 250 million years ago. Though the group did not go extinct, only about a third of their morphological diversity made it though this event, and brachiopods never regained their former diversity or dominance.