Inarticulate Brachiopods

Phylum Brachiopoda is comprised of three groups: the inarticulates, the articulates, and a group intermediate between the two. The classification of this phylum is currently in a state of flux, and for more details, "Google" the paper: Brachiopods: origins and early history---by Harper et al., 2017---for reading purposes or for a free pdf download.

Brachiopods have bivalved shells (valves) that can superficially resemble clams.

This post is the first of two parts and concerns the inarticulates and the aforementioned intermediate group. 

Inarticulate brachiopods have a long geologic history of approximately 520 million years, ranging from the Early Paleozoic (early Cambrian Period) to modern day. Their two valves are held together by muscles. Most inarticulate brachiopods lived/live as burrowers (infaunal), and their shells consist of chitinophosphatic composition. 

The burrows of inarticulate brachiopods can be as deep as 30 cm. A pedicle, which serves as an anchor to the floor of a vertical burrow is flexible, thus the brachiopod can extend itself upward or downward. The shell is very thin but is "supported" by the surrounding mud.

Lingula anatina, a modern-day inarticulate brachiopod, which lives in black mudflats (intertidal zone) in tropical and subtropical waters of Japan. The specimen shown above is embedded in epoxy for the purpose of scientific study. The exterior of both valves (height 33 mm) are shown, as well as a portion of the long, fleshy pedicle. 

Lingula hians is a modern-day inarticulate brachiopod from Queensland, Australia. Both valves (height 38 mm) are shown, exterior and interior views. Their interior has a sheen to it because of the chitinophosphatic composition. You can see that modern-day lingulid inarticulates (i.e., those belonging to family Lingulidae) look very similar. I surmise that the similarly is because that they have lived in the same type of stressful environment (mudflats, coastal lagoons) throughout their entire geologic history; thus, providing an example of how the environment can influence strongly the morphology of a shell. Said another way: once an animal has adapted to a stressful environment, it has great endurance potential.

Side view of the two valves (combined thickness 5 mm) of specimen of L. hians, shown above. The valves are paper thin and fragile.

An example of a Cambrian inarticulate brachiopod (height 20 mm), in black siltstone.


The intermediate group, mentioned above, is relatively uncommon in the fossil record, although this group ranges from the late Cambrian to modern day. The intermediate group has calcareous shells that attach themselves to other shells found on the ocean floor. Thus, the intermediate group consists of epifaunal dwellers (i.e., living on a hard substrate). 

exterior

interior

Craniscus wilsoni Squires, 1994, is an example of an intermediate- group brachiopod, whose shell would have consisted of two calcareous valves. When I collected this specimen, I could find only its substrate-free valve, shown above (exterior and interior views, shell 7 mm wide). It is of early Eocene age (about 45 million years old) epifaunal brachiopod from the southern Olympic Mountains, Washington State. The other valve (an attached one) was not found; it was most likely attached (epifaunally) to a shell of a mollusk or some other hard ground (pebble, coral, etc.).