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.).

Hyolithus: a Cambrian mystery solved

Preliminary comments: The fossil Hyolithus has long been an enigma. Is it a mollusk, a worm, or what? When I first started teaching, it was strongly favored to be an early mollusk. It remained in that uncertain mode until recently when a team of paleontologists (Moysiuk et al., 2017) collected over 1,500 specimens from British Columbia. Remarkable specimens were found that show, for the first time, soft-part anatomy impression. Now the mystery is solved. Read on to found out what the answer is.

Impression of a fossil specimen of the hyolithid Hyolithus cecrops Walcott, 1917 (19 mm height, early middle Cambrian, northwestern Montana). Found with specimens of trilobites [Albertella helena], brachiopods, and trace fossils.

Hyolithids are a group of small, somewhat flat-shaped, shells whose biologic affinities have long been uncertain. The most common assumption was that they were gastropods, which had a planktonic (floating) existence, like the pteropod gastropods (so-called "sea butterflies") found in modern seas. Their most perplexing morphologic feature were the two "oars," which are now named "helens." In the image shown above, only the left helen is preserved.

Most of the several hundred species belong to genus Hyolithus, which had a worldwide distribution, starting in Early Cambrian time. They became diminished during Ordovician and lingered thereafter until the Middle Permian.

Dorsal view of exterior of H. cecrops showing the major morphologic characters.  

Side view of exterior of H. cecrops. The operculum (lid) could open slightly, and the feeding apparatus (probably similar to the lophophore of some modern animals), could then be extended into the water column. The helens were used as stilts/props for holding the animal above the muddy bottom substrate. 

Dorsal view of interior of H. cecrops, showing critically important soft-tissue impressions: the presumed lophophore, the location of the mouth, and the U-shaped gut. These impression are indicative that hyolithids were not planktonic mollusks. Instead, they were benthonic (bottom dwellers) animals that had their closest relatives being the lophophore-bearing brachiopods and bryozoans. 

For more information see:
Moysiuk, J. et al. 2017. Nature v. 54:394–397 (like other pay-for- subscription journals, the abstract is free, but it costs to read or download the actual paper).

"Blind" trilobite Itagnostus from Utah

Trilobites like the one shown above are well known to most collectors of fossil invertebrates (animals without backbones). This trilobite, which is 4 cm long, has a cephalon or head (with eyes), a thorax (body with many segments), and a short pygidium ("tail"). This particular specimen is an Elrathia kingi of Middle Cambrian age (see time diagram below) from the Wheeler Shale near Delta, Utah.

Another type of trilobite found alongside E. kingi is Itagnostus interstictus, until recently known as Peronopsis interstrictus, which belongs to one group of so-called "blind" trilobites. Technically speaking, "blind trilobites" were not blind because they never had eyes. Itagnostus is an agnostid trilobite, which are characterized by having a thorax consisting of only two segments and a cephalon and pygidium of approximately the same size.

This is a "cluster" of two specimens of Itagnostus interstices. The largest specimen is nearly 1 cm  long.