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.).

A middle Eocene heart urchin

Heart urchins, also called spatangoids, are echinoderms (sand dollars, sea stars, etc.), which are generally characterized by having 5-rayed (pentameral) symmetry. This post focuses on a middle Eocene heart urchin known as Schizaster diabloensis Kew, 1920. It was named for its occurrence in sedimentary layers near Mount Diablo, just east of San Francisco.

A hand specimen of siltstone rock from the Llajas Formation has three specimens of
S. diabloensis on the same bedding plane. The hand specimen is 5 cm (2 in.) wide.

This species of heart urchin was common in northern and southern California during the middle Eocene (approx. 47 million years ago). The specimens shown here are from the Llajas Formation in Simi Valley, California. This formation was deposited in shallow-marine, warm-water conditions. The entire geologic time range for this species is late Paleocene through middle Eocene.

Five specimens of S. diabloensis from the Llajas Formation. The largest specimens are
  2 cm (0.8 in.) wide. All are top-side up.

Echinoderms, past and present, are strongly gregarious and can occur in great numbers on the ocean floor. Spatangoids have a fossil record extending back to the Cretaceous. They are burrowers and living below the surface provides protection against predators. During the Cretaceous, many new forms of predators evolved, which, which gave the force for some echinoderms (like spatangoids) to adapt to these adverse conditions by becoming infaunal (i.e., burrowers), mainly in fine-grained deposits, like siltstone.

You can readily see the five-rayed symmetry of the feeding grooves on the dorsal (top) surface of each specimen. The central groove, called ambulacrum III, is the longest and is sunken on most spatangoids, whereas the two posterior grooves are smaller.