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 clam for the ages

Venericardia is a widely distributed genus of shallow-marine bivalves (clams) in the family Carditidae. This genus was abundant during the Paleocene and Eocene epochs (a cumulative range of 66 to 34 million years ago). The highest biodiversity of Venericardia was in the Eocene. The genus needs detailed classification studies of all its various subgenera found throughout the world. Detailed studies are much needed to evaluate the likely possibly of over naming of species/subspecies found on the west coast of North America. The genus is now extinct, with the last survivors of this genus dying out apparently in the early Miocene.

Lamarck (1801) first described the genus Venericardia based on well preserved shells of this bivalve in the Paris Basin, France. The aragonite shells of this genus are commonly large and very sturdy, with wide radial ribs crossed by concentric growth lines on the exterior. The interior of the shells are characterized by long posterior teeth, much shorter anterior teeth, and two prominent elliptical-shaped muscle scars connected ventrally by a continuous line. Some species have prominent nodes along the inner margins of the shell.

These two images show the plaster replicas of the exterior and interior surfaces of a left-hand valve (6.5 cm high and 7 cm wide) of Venericardia planicosta Lamarck, 1801 from an Eocene shell bed at Grignon, Paris Basin, France. 

Venericardia (Pacificor) lutmani Turner, 1938, lower Eocene (Ypresian Stage), southwestern Oregon; plaster replicas. Left image is the exterior of a left valve, and the right image is the exterior of the right valve of a single specimen (9.25 cm high and 10 cm wide) which, upon burial, became separated from one another. 

These two images are the corresponding interior views of the same plaster replica shown immediately above.

Venericardia (Pacificor) hornii calafia Stewart, 1930, middle Eocene (Lutetian Stage), southern California. Exteriors of left-hand and corresponding right-hand valves of the same specimen (whose valves [9 cm high and 9 cm wide] are closed very tightly).

Dorsal (hinge) view of same specimen of V. (P.) h. calafia shown immediately above. Left valve is on the left side of image, and right valve is on the right side of image

Venericardia was an infaunal (burrowing) suspension feeder that lived buried just beneath the surface-water interface. Its optimum habitat was in relatively deep, shallow-marine (shelfal) environments, but its shells are commonly found as transported remains in coastal-storm beds.

WHITE SANDS, NEW MEXICO

White Sands is in southern New Mexico, 16 mi west of the town of Alamogordo. White Sands, which covers 275 square miles, was established as a National Monument in 1933. It recently became a National Park. The elevation of the park is 4,235 feet.

White Sands is geologically situated between the San Andres Mountains to the west and the Sacramento Mountains, just east of Alamogordo, to the east. [Google Earth photo, 2018].

Aerial view, looking northwest. White Sands is in the distance, below a cloud layer on the horizon and "between" the two 
pointed-metal objects on the airplane wing.

White Sands consists of white gypsum sand dunes, the largest of their kind on Earth. 

During the Late Permian Period, about 250 million years ago, shallow seas covered the area. Evaporating seas left behind deposits of white gypsum crystals, consisting of hydrous calcium sulfate. Gypsum is very soft: 2 on the Moh's Hardness Scale. Fingernails can scratch gypsum. See my previous post on "Some Varieties of Gypsum" (Sept. 30, 2017).

Subsequent tectonism uplifted the San Andreas and Sacramento Mountains, and, over time, rain dissolved the gypsum deposits, and rivers transported the dissolved material to the nearby Tularosa Basin, which had no outlet the sea. The trapped water evaporated and gypsum was deposited (once again). Over time, weathering and erosion broke down the gypsum crystals into sand-size grains. In the last million years or so, prevailing winds from the southwest transported these crystals and dunes formed. There are transverse, parabolic, and barchan dunes present. 

It is quite a wonderful experience to visit White Sands. You can walk barefoot and not get burned by the sand, and you can roll around in the gypsum sands without getting abraded, like you would if the dunes were made of quartz grains (hardness 7).

Photography is a challenge because of the glare. It is like photographing a snow field.

Maclurites, an early gastropod with a confusing shell

The gastropod genus Maclurites Le Sueur, 1818 is characterized by large flat-spiral shells. It lived in shallow, subtidal, warm-marine waters and was widespread where carbonates were deposited. Locally, it can be abundant and associated with algal fossils.   
Maclurites is restricted to Ordovician time, thus it is a guide [or index] fossil for the Ordovician Period. This genus belongs to family Macluritidae, which includes 10 genera.

Maclurites has a type of shell coiling called hyperstrophic, in which the animal is anatomically dextral (its genitalia are on right), but its shell is falsely sinistral, being actually ultradextral. If there is an associated operculum (the lid-like, partial or complete covering of the aperture), the operculum exhibits counter-clockwise coiling.  

Most gastropods have dextral (right hand) clockwise coiling of the shell, and any associated operculum exhibits counter-clockwise coiling (see image immediately below). A few gastropods are mirror opposites and have sinistral (left hand) counter-clockwise coiling of the shell, and any associated operculum exhibits clockwise coiling.

In my Nov. 20, 2014 post, entitled "Gastropod operculum," I provided views of the dextrally coiled, gastropod shell Megastrea undosa. In this present post, I provide another view of the exterior side of its operculum (38.7 mm hight), which shows counter-clockwise coiling.

Although hyperstrophy is best determined using soft parts for anatomical study, in the case of extinct gastropods, like Maclurites, molluscan paleontologists have to rely on the coiling direction of its calcareous operculum. As shown below, the operculum of Maclurites is coiled counter-clockwise, thus it cannot be a sinistral gastropod.

The above image is a sketch of the apertural view of a complete specimen of Maclurites logani [86 mm diameter], of Middle Ordovician age from Quebec, Canada. [Fascimile of fig. 105 in The Treatise on Invertebrate Paleontology, Part 1, Mollusca (1964), p. I188].

The following four views are of a specimen of Maclurites sp. [68.4 mm diameter, 21 mm height] from the Lower Ordovician Lebanon Limestone, near Nolensville, Tennessee. 

   front or apertural view, aperture (poorly preserved) is to the right

                                  back or abapertural view

                                            basal view

                            dorsal view (the "tip" is missing)

The shell rested on its very flat base, which provided great stability for the shell, thereby resisting being flipped over by waves or currents. The flatness of one side of the shell is inferred to be related to its sessile (unmoving), filter-feeding mode of life. 

The Earliest Undoubted Gastropods (Snails)

The earliest undoubted gastropods [Phylum Mollusca, Class Gastropoda] known in the fossil record are of Early Ordovician age (about 485 to 480 million years old). They are from shallow-marine Lower Ordovician rocks in Missouri.

Four views of a specimen of Sinuopea regalia (37.3 mm high [tip missing], 40 mm wide): front view (apertural), back (abapertural), bottom, and top. This specimen is from the Gasconade Formation near Sullivan, Missouri.

Four views of a specimen of Rhacopea grandis 32.2 mm high [tip missing], 80 mm wide: front (apertural), bottom, and top. This specimen is from the Gasconde Formation, near Bourbon, Missouri. Warm-water, shallow-marine stromatolites (fossil algal structures) are found also in these rocks.

Both specimens were given to me by Dr. Michael Vendrasco, and I sincerely appreciate his generosity.

Gastropod-like fossils occur throughout Cambrian-age rocks but all, except those of the latest-most Cambrian are questionable and might belong to another extinct molluscan class.

Bellerophont Mollusks: A Paleozoic Enigma

Bellerophonts have a shell that is coiled in a single plane (no spire). Their shell superficially resembles that of a nautiloid cephalopod, except that a bellerophonts lack the chambers that characterize nautiloids. Bellerophonts have a spiral band (selenizone) that encircles the middle of the shell. This band indicates successive growth positions of where the anal slit was located on the shell. Some gastropods (e.g., pleurotomariids (= the deep-sea "slit shells)," abalones, scissurellids, and some limpets) have also a selenizone. Interestingly, most of these other shells, unlike bellerophonts, have aragonite shells with mother-of-pearl luster.

The geologic range of bellerophonts is broadly given as Late Cambrian to Early Triassic, but their main range is Ordovician to Permian. There are several families and subfamilies of bellerophonts, as well as many genera. Two of these genera (Knightites and Pharkidonotus) are shown below.

Specimen C4 (width 7.7 mm): Three views (dorsal, aperture, and left side) of Knightites (Cymatospira) sp. of Pennsylvanian age:

Specimen 319 (width 18 mm): Three views (dorsal, aperture [filled with silt], and right side) of Knightites multicoronuatus of Pennsyslvanian age from the Ozawkie Limestone in Kansas. Genus Knightites ranges in age  from Devonian to Middle Permian. The slit band, which extends down (as shown here) along the middle of the shell, is bordered on both sides by several prominent nodes (weathered somewhat on this specimen). These nodes grade into narrow, closely spaced ridges along the sides of the shell:

Specimen 96 (width 14.4 mm): Three views (dorsal, aperture, and right side) of Pharkidonotus percarinatus of Pennsylvanian age from Grayford, Wise County, Texas: 

The largest known bellerophontid is approximately 190 mm width, from the Carlsbad Caverns area in New Mexico, and of Permian age. This specimen lived in a reef environment.

Bellerophonts represent a group whose undoubted evolutionary relationship with other mollusks remains indeterminate. Are bellerophonts gastropods? Are they part of an even more primitive group of mollusks? There are more questions than answers, when it comes to trying to understand the origin of bellerophonts. 

Fossil Bony Fish Ear Bones

Otoliths ("ear bones") occur in all vertebrates, including humans, whales, and dolphins [see my previous posts: Fossil whale ear bone, August 15, 2014; Fossil dolphin ear bones, Oct. 4, 2015] for these latter two examples) and allow for perception of linear acceleration, both horizontally and vertically. Otoliths are not skeletal components; rather they form as three separate bones (in each ear) within the auditory capsules. Their composition is calcium carbonate (mainly the mineral aragonite). They can also be fossilized and many are microscopic in size. Some, however, can be macroscopic in size.

Otoliths of modern-bony fish (teleosts) can be common in Tertiary freshwater and marine environments. The shapes and proportions of otoliths vary with fish species. Fish from reef or rocky bottom habitats (snappers, groupers) have larger otoliths than open-clean fish (tuna, mackeral). The growth rings in otoliths are similar to tree rings, in that they can be used to estimate the age of fish, as well to determine early life transitions from planktic larvae to benthic juveniles. The stable isotopes of oxygen and carbon found in the otoliths can be used to determine ancient temperatures of their environments. 

Front, side, and back views (in vertical order) of a matched pair of macroscopic left-and-right otoliths (24 mm long and 7 mm thick) from a 1-meter long, 40-pound White Sea Bass from the northern Gulf of California, Mexico.