Rastellum, another unusual fossil oyster

Last November, I had a post concerning a cork-screw shaped fossil. I return to the subject that continues to intrigue me; namely, unusual shaped oysters (clams). This post is about a species of genus Rastellum, an extinct genus whose geologic time range is approximately 170 to 66 million years ago, during the Middle Jurassic to the end of the Cretaceous. This genus is characterized by small to large size; elongate-narrow to comma-shape; and the valves are almost equal in size and shape. The two valves have very distinctive radial ribs called plicae. Although the valves (left and right) look similar, only the left valve (the one that is attached to some foreign object) has a noticeable muscle scar. In the three pictures immediately below, the muscle scar is visible as a small depression.

The surface where the two valves of a clam join together is called the commissure. On many clams it is a simple, straight line. Rastellum has a "zig-zag" commissure, which enabled the feeding process. This "zig-zag" line is readily observable in the third picture below.

These images are  three views (left valve, right valve, and side view; in that order) of a closed-valved specimen Rastellum carinatum (Lamarck, 1819) of  Early Cretaceous (Albian Stage) age from Texas. The longest dimension is 2.25 inches (55 mm).

The last image is of another specimen of R. carinatum, but this one shows the interior of a left valve. The ligament area of oysters is very distinctive. The ligament is a groove where originally there was elastic material (now decomposed) which helped the oyster hold together and open its valves. The muscle scar of oysters is also very distinctive. It is commonly large and almond shaped. Its function was to firmly hold the valves together.

Interior of a left valve of Ratellum carinatum. The longest
dimension is 1.6 inches (40 mm).

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.

Skull and teeth of Dimetrodon

Inexpensive replicas of Dimetrodon, the so-called "sail-fin reptile" or "finback reptile," are very eye-catching because they show the  fin supported by elongated spines projecting from along the backbone. In terms of the skull and teeth of Dimetrodon, however, these replicas, are commonly not morphologically accurate, and this post will attempt to shed some light on these subjects. 

Dimetrodon lived during the Early Permian, approximately 295 to 275 million years ago. Most specimens have been found in red beds in Texas and Oklahoma. At least a dozen or so species have been described. Dimetrodon was a carnivore that had a maximum body length of approximately 2 to 5 m (5 to 15 ft.) and a maximum weight of 250 kg (500 lbs). Its fin was used for thermal regulation and certainly not for "sailing." 

In earlier (outdated) classification systems, Dimetrodon would have been called a "pelycosaur" or a "mammal-like reptile." Dimetrodon is now classified as a synapsid mammal because it has a temporal opening, located relatively low on the back of each side of the skull. Synapsids show remarkable transitions from reptile-like to dog-like forms, and Dimetrodon was an early synapsid that had reptilian-like features.



The side view of a skull of Dimetrodon shown below is morphologically correct. The temporal opening (also called the infratemporal fenestra), which was used to accommodate bulging jaw muscles, is an important evolutionary feature used to link Dimetrodon with mammals. Notice that the size of the teeth of Dimetrodon differ considerably. Also there are several teeth at the opening of the mouth that are located higher (i.e, the maxillary step) than are the other teeth. The name "Dimetrodon" means "two shapes of teeth." Its teeth show some differentiation, and this is a mammalian trait. 

Side (lateral) view of a replica of a Dimetrodon skull, length 16 inches, height 10.5 inches.

The ventral view, shown below, of the lower jaw clearly shows how the tooth size varies and how the teeth at the front of the jaw are constricted. The red arrow points to a row of throat teeth (pharyngeal teeth), which were used to pin struggling prey.

Dorsal view of the interior of the lower jaw of the same replica.

Pharyngeal teeth are rarely mentioned in articles dealing with Dimetrodon. An interesting and well-illustrated article about its throat teeth can be found on the web, and the link is HERE

The last picture (see below) shows the top view of the skull, which is solid (i.e., no openings) and unlike the skulls of dinosaurs. In a future post, I shall illustrate how a dinosaur skull differs from that of synapsids, like Dimetrodon.

Top (dorsal) view of the replica of the skull of Dimetrodon.