Orthid brachiopods of the Whitewater, Saluda and Liberty Formations in southeastern Indiana (section authored by Aaron Shear)

Brachiopods are the most abundant and diverse of the skeletonized invertebrates of the Paleozoic, making them the most common fossil found in many Paleozoic localities. Brachiopods are solitary, not colonial, animals that secrete a two-valved shell around their soft bodies and lophophore (fan-like filter-feeding device). One valve is usually larger than the other. This valve is called the ventral (or pedicle) valve, due to the fact that it usually has an opening for a fleshy stalk called the pedicle. The pedicle is what the brachiopod uses to attach itself to the substrate. The opposite, and smaller, valve is known as the dorsal (or brachial) valve, because the lophophore (also called the brachidium) attaches to it. Most brachiopods have a plane of symmetry that runs through both shell, ventral and pedicle, making one half of the shell the mirror image of the other. Within the shell are two sets of muscles that open and close the shell. The adductor muscles pull the two valves together, closing the shell. The diductor muscles pull the dorsal valve around its hinge line causing the brachiopod to open. Brachiopods are divided into two classes by differences in dentition style. Class Inarticulata, or inarticulate brachiopods , lack a tooth-and socket articulation between the two valves. Class Articulata, articulate brachiopods, have well-developed hinges with teeth and sockets between the valves.

Aaron Shear, Junior Geology major at The College of Wooster. Aaron is from Upland, California.

Dorsal valve of Platystrophia acutilirata, found in the Whitewater formation. This orthid brachiopod is characterized by having a low, rounded fold, three nearly equally developed plications in the sulcus, and by having ten or more plications on each side of the sulcus.

Phylum BRACHIOPODA Class Articulata Order Orthida Glyptorthis insculpta Hebertella occidentalis Platystrophia moritura Platystrophia acutilirata Platystrophia clarksvillensis Platystrophia cypha

Orthid brachiopods first appeared in the middle Lower Cambrian and are the most primitive articulate brachiopods. There are 340 genera of orthid brachiopods. Phylogenetic analysis suggests that orthid brachiopods are polyphyletic. Orthid brachiopods have an unequally biconvex shell with a wide, straight (strophic) hinge line. The fold and sulcus of orthid brachiopods are usually very shallow and sometime are even absent. Distinct interarea are found on each valve of orthides. Like the other articulates, orthid brachiopods bear teeth and sockets, have a blind intestine system (no anus), and make their shells out of calcite. Through the Cambrian and Ordovician orthid brachiopods were the dominant articulates, making them one of the more common brachiopods. They almost entirely died out due to the Late Ordovician extinctions but did not fully succumb to extinction until the Permian catastrophe.

Found in the Liberty formation this is the dorsal valve of a Platystrophia clarksvillensis, with another dorsal valve of a Platystrophia clarksvillensis in the background (upper-right corner). They differ from other Platystrophia by only having 8 or 9 plications on each side of the sulcus.

This is a ventral valve of a Platystrophia cypha, and is characterized by having a strong median plication in the sulcus and having a very wide (strophic) hinge line. This orthid brachiopod is also characterized by having 9 or more plications on each side of the sulcus. This specimen was taken from the Whitewater formation.

Orthid brachiopods, amongst the other brachiopods, are exclusively benthic marine invertebrates. They are incapable of searching for food which makes them sessile-benthic filter feeders. Due to this, orthid brachiopods must stay near the surface so that the current is strong enough to eliminate wastes while also bringing fresh food and oxygen. Orthid brachiopods are typically well preserved, and shell material is commonly intact when found, making them very abundant in the limestones and shales of the Ordovician. Glyptorthis insculpta, an orthid brachiopod, can be found in the base of the Liberty formation. Platystrophia is present in both the Whitewater and Saluda formations, along with Hebertella occidentalis.