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).

Largest glacier calving event

I am deviating from my usual paleontologic format in order to share with you a geologic topic.

It concerns the largest glacier calving event ever recorded. I assure you that is a very memorable and mesmerizing video. Enjoy. Here

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.

Miocene fish scales

Macrofossils are not common in deep-marine mudstones of the Miocene Modelo Formation in southern California. A lucky find would be a partial or complete bony fish skeleton, but these are  rare. Fish scales, however, can be locally common, but a collector needs to know what they look like. Before enrolling in my paleontology class, most of my students would find a fish scale and not even know what they were looking at.

The fish scales shown below are of late Miocene age and from the Modelo Formation (commonly referred to as the Monterey Formation). Each scale is about 10- to 15-mm-wide and represents a paper-thin imprint on bedding planes of mudstone. Identification as to the family or genus of fish is not a trivial process and requires very specialized knowledge. At the very least, however, their presence indicates an aquatic environment.

Two late Miocene fish scales belonging to the same genus. There might be a third fish scale in
 the lower right-hand corner.

Another late Miocene fish scale, genus different from the two shown in the photo above. 

There have been only a few paleontologists that have published on Miocene fish scales from southern California. The principal workers were: D. S. Jordan (published between 1900 and 1920), Lore Rose David (published during the 1940s), and Richard Pierce (published during the 1950s). 

Fossil dolphin ear bones

Fossil-dolphin ear bones, which can somewhat resemble small pebbles, consist of very hard, dense bone that can be readily fossilized. Most examples of these fossils that are illustrated online these days are from rocks of Miocene to Pleistocene age from the Atlantic coast of the United States (e.g., Miocene rocks in Calvert Cliffs, Maryland; and Pliocene rocks at the Lee Creek Mine, North Carolina.

During life, the earbones were located in a cavity in the middle-ear area, located near the back of the dolphin skull. This cavity is filled with a dense foam that, along with ligaments, to support the ear bones. The bones consist of the tympanic and the periotic bones, and the entire structure is referred to as the “tympano-periotic complex.” The two bones are partially fused together with flanges, grooves, small holes, and a large bulbous (i.e., bowl-shaped area) region, called the bulla. The inner ear is located within the tympano-periotic complex.

Examples of fossil tympano-periotic complexes are shown below. They are from the shallow-marine Miocene Temblor Formation found at Sharktooth Hill in southern-central California.

Miocene tympano-perdiotic complex, greatest dimension 42 mm (approximately 1.75 inches).

Opposite side of same specimen shown above.

A different specimen, greatest dimension 43.5 mm.

Note: In one of my previous posts (8/15/2014) entitled “Fossil whale ear bone,” I illustrated a whale tympanic bulla.

Fossil yucca? plant

One of the rarest fossils I have collected from the Pico Formation south of Newhall, southern California is what I believe is a small base (trunk) of a yucca plant. It is from the upper part of this formation and is of late Pliocene age (about 3 million years old). 

The Pico Formation in this area was deposited in a marine-delta environment, and, as I mentioned in one of my earlier posts (8/15/2014), fossil pine cones can be found (rarely) in these beds. The pine cones were derived from pine trees that grew in the adjacent, ancient San Gabriel Mountains east of the delta. Some of the pine cones eventually floated down a braided river (full of coarse debris consisting of pebbles and cobbles) and were deposited in fine-grained sandstone near an ancient shoreline, and mixed with shallow-marine fossils (e.g., seashells and shark teeth). It seems likely that presumed yucca remains could have also floated into this marine-environment setting.

When I first saw the presumed yucca fossil, I thought it was a pine cone. Upon closer inspection, however, I realized the yucca? fossil is not like the pine cones from this formation. As I walked back to my car, I came across a modern yucca plant (see photo and comments below). I was immediately struck by the fact that the vertical-striations on some of the woody part of the trunk of both the modern and presumed fossil yucca are very similar. If you have knowledge of the bases of fossil yucca plants, please let me know if you think my identification is correct or not.

Late Pliocene yucca? base (trunk), height 12 cm (4.75 in.), width 7 cm (3 in.), from the upper Pico Formation near Newhall, California. The white fossil sticking out along the upper left side is a shallow-marine clam. Notice the cross-section of the high-spired, shallow-marine gastropod shell

of Turritella cooperi near the bottom. 

Modern-day base (trunk) and a few green leaves of a yucca [probably Hesperoyucca whipplei] height 24 cm (9.5 in.), width 15 cm (5 in.), from near Newhall, California. The non-green, hard, woody trunk is 14 cm height. Hesperoyucca whipplei is one of the most common yuccas of the chaparral and coastal-sage scrub plant communities living below 4000 feet in elevation in southern California. The leaves of this yucca are stiff and dagger like.