Were you able to solve Monday’s mystery fossil? They aren’t little poop balls, nor are they clams, although they are often mistaken for them.
This photo can be found at the Arkansas Geological Survey website under “Brachiopod.” They look a lot like clams. Brachiopods, often called lamp shells, have two shells and live in shallow marine environments just like clams and the occupy the same niche, feeding on organics filtered from the water. But unlike clams, which are molluscs, just like snails and squid, brachiopods are lophophorates, most closely related to bryozoans, the “moss animals.”.
So what is a lophophorate? Lophophorate means “crest or tuft bearer, so named for their feeding apparatus called a lophophore, which is shaped like a roughly circular or semi-circular ring of tentacles. These tentacles lazily wave through the water passing through the lophophore, catching small particles of food suspended in the currents. Thus, everything in this group are what is known as suspension feeders. These lophophores serve not only to collect food, but for gas exchange as well. In addition, the animals are headless, with the lophophore surrounding the mouth. The food enters the mouth and passes through the digestive tract, which makes a U-turn and dumps out what it can’t digest just outside the ring of tentacles. Clams do essentially the same thing, only they use an entirely different apparatus to do so.
Clams attach themselves to surfaces by secreting a collection of what are called byssal threads. Most brachiopods, on the other hand, form a pedicle, a stalk that holds them in place. Some do not make pedicles, instead just gluing themselves down directly onto the rock.
Another difference that can usually be seen between clams and brachiopods is the symmetry of their shells. Brachiopods are symmetrical from side to side, their left side is the same as their right side. Clams follow a different pattern. They usually have two identical shells, but the shells themselves are not symmetrical. This is not always true though. The Cretaceous oyster, Exogyra ponderosa, has an huge, thick shell on one side and a thin lid for a shell on the other. But as a general rule, this usually works. Another difference that is sometimes stated is that brachiopods use their muscles to close their shells, while clams use their muscles to open their shells, closing them by the use of ligaments; thus making brachiopods more susceptible to predators. This, however, is not true. In truth, brachiopods use their muscles to both open and close their shells. Clams have large adductor muscles that function to close the shells and they have ligaments that open them when the muscles relax.
Brachiopods are quite diverse, with many different types. They range in size from less than a dime to almost 40 cm (15″). There are two general groups, the Articulates, which have toothed hinges holding the shells together, and the Inarticulates, which do not have teeth, so they fall apart easily after death. Probably the most commonly found in Arkansas are spirifers, known for being somewhat wing-shaped , with a prominent sulcus, or depression in the center. Many brachiopods prefer solid substrates, like rock, others were adapted for softer substrates like sand or mud. Productids, like the ones in our mystery fossil, often grew spines, which helped secure them to muddy surfaces. Others, like strophomenid brachiopods, handled muddy substrates by developing large, very flat shells, which floated on the mud like a snowshoe. Still others, like the modern-day lingulids, developed long pedicles, allowing them to burrow down into the sediment.
Brachiopods have been around since at least the Cambrian, over 520 million years ago. They were most abundant in the Paleozoic Era, but suffered greatly during the Permo-triassic extinction event. They recovered to some extent, but never reached their previous abundance due to the appearance of clams, which began taking over some of the spaces they occupied. Nevertheless, there are still several different kinds in the modern ocean and can often be seen clinging to rocks near shore or buried in the sand. In Arkansas, you won’t find any living specimens, but you can find numerous fossil brachiopods in the Paleozoic rocks throughout the Ozarks and Boston Mountains, even in some places of the Arkansas Valley. Stop by any outcrop along Highway 65 between Conway and the north edge of the state, particularly limestone outcrops, and you are likely to find some. You can find a few in the Bigfork Chert in the Ouachitas, but they are not nearly so common as they are farther north.