Ilymatogyra, the Late Cretaceous cork-screwed-shaped oyster

Ilymatogyra is an odd-shaped oyster genus that belongs to an extinct group known as the exogyrine oysters, which were common during Mesozoic times. The genus is Late Cretaceous (Cenomanian Stage) in age and has only a few known species. It occurs in some Gulf Coast states (e.g., Texas, Mississippi), Oklahoma, Mexico, and Africa.

Ilymatogyra is characterized by having a left (lower) valve with a cork-screw shape and a right (upper) valve that is much smaller and “trap-door” like. Ilymatogyra lived on soft bottoms in mid-shelf environments about 25 to 50 m in depth, and the high-spiral shape of the valves most likely helped the oyster from becoming buried, thereby preventing its gills from being clogged up during intervals of high sedimentation.

The following three pictures are three views (apertural, abapertural, and side) of a specimen (41 mm height) of Ilymatogyra arietina from Mississippi.

The calcite shells of this genus are commonly replaced by pyrite (“fools gold”), which is made up of iron and sulfur. The following two pictures are of a pyritized I. arietina from the Del Rio Formation near Austin, Texas.

Sheared cobbles

A few years ago, one of the graduate students in my geology department gave me some very interesting cobbles that have been sheared (fractured) and then the pieces were cemented together (thus they do not fall apart). All of this took place by natural processes. The result is that it looks like they have been sliced up, with the pieces shifted relative to one another.

The cobbles occur in the Upper Cretaceous Trabuco Formation in the northern Santa Monica Mountains, just south of the San Fernando Valley of southern California. This formation consists of cobble to boulder conglomerates with intervening coarse sandstones, all of which were deposited in an ancient alluvial fan adjacent to the ocean (i.e. a fan delta environment) approximately 90 m.y. ago. The locale has a lot of small faults and fractures. These sheared cobbles are silent witnesses to the strong deformation that has occurred to these rocks.

Stringocephalus, an usually large brachiopod

This post concerns an unusual fossil specimen, which I used for many years (as an extra-credit specimen) in my lab final for my  geology major's class called “Fundamentals of Invertebrate Paleontology.” I had purchased the specimen many years ago, and its unusual size puzzled me for a long time, until I finally identified it as a very large brachiopod.

Some of you might know what a brachiopod is, but most people (including many  geologists who have never had a class in the fundamentals of paleontology) commonly confuse them with clams, even though these two groups belong to separate phyla. Brachiopods belong to Phylum Brachiopoda, and clams belong to Phylum Mollusca. Perhaps in a future post, I can illlustrate what the differences are in their shells. Brachiopods are extant (i.e., not extinct), but they were much more important in the past (especially the Paleozoic) than today.

Front view, Stringocephalus sp., Middle Devonian, GuangXi Province, China.
 Specimen is 12 cm high (about 4 3/4 inches).

Side view of same specimen shown above. Specimen is 9 cm wide (abut 3 1/4 inches). The oval shape with the "spike" is a fragment of another specimen of Stringocephalus. The"spike" is a median septum, which is a characteristic internal part of brachiopods.

The large articulate brachiopod under consideration here belongs to genus Stringocephalus, which comprises a small group of species that lived during the Middle Devonian (about 385 million years ago). Stringocephalus has an unusual shape and size for a brachiopod. It has a very inflated biconvex shape with a very prominent beak. Its shells can reach up to 12.5 cm (about 5 inches) in height, and their shell wall can be as much as 0.5 cm  (just less than a quarter of an inch) thick. It has been found in northern Europe (especially Poland), China, Western Australia, western North America (Brooks Range, Alaska; Nevada; and Sonora, Mexico). During the Devonian, the continents were in very different locations and positions that they are today, and the occurrences of Stringocephalus were within 30°N and 30° of the equator.

By the way, the name Stringocephalus is derived from the Greek words “strig,” meaning an owl and “cephal,” meaning a head. The latter word was perhaps applied to this fossil because it resembles a coiled nautiloid, which is a cephalopod (e.g., the group that includes squid and octopus).

I also want to take this opportunity to let you know that the 
"Clustrmap" that normally accompanies my blog has been inoperative for the past week. I notified the company, and, hopefully, they will soon fix the problem.