Desmostylians, the only order of mammals to have gone extinct

Desmostylians were marine mammals whose mode of life and evolutionary relationship with other similar mammal groups continue to be debated by paleontologists and evolutionary biologists.

Their geologic range is late Oligocene (about 25 m.y. ago) to late Miocene (about 4.5 m. y. ago), a span of about 20 million years. They lived in coastal waters of the northern Pacific from Japan to Mexico. Some complete specimens of these rare animals have been found in California.

The best modern analogy for what a desmostylian looked like in terms of shape and body mass is a hippopotamus. Desmostylians  were up to 1.8 m (6 feet) in length and weighed up to 200 kg (400 pounds). Six genera are currently known.

Some experts have reported that desmostylians probably walked along the shallow ocean floor, possibly ate oysters and other shelled animals, and could walk onto beaches. In recent years, other experts, however, have concluded that desmostylians might have been strictly aquatic and herbivores.

Their feet were very wide and long. It seems that their wide feet might have served as paddles, which certainly would have been effective for swimming.  In addition, levels of stable isotopes in their tooth enamel suggest an aquatic environment. They had inturned feet, and the feet could not be turned without rotating the whole leg. This would have been a severe limitation if they tried to walk on land.

They had short but stout tusks probably adapted for bottom feeding. The structure of their cheek teeth (molars) is unique among mammals. Their molars look like tubes welded together, with each peg-like tooth covered with thick enamel. This feature is how the name desmostylid was derived: desmos = linked, chain; stylus = pillar. Their molars are arranged in a cluster, which would have been ideal for crushing their food. It seems that their cheek teeth were replaced cyclically, as in elephants.

The above picture on the right shows a life-sized plaster replica of a cluster of teeth. The replica is 7 cm = 2.75 inches in width. 

                                      

The above picture on the left shows a 43-mm (1.75 in.) long tooth whose exterior enamel is gray. The other tooth (two views) is 45-mm long and has been cut in half, lengthwise. Its exterior enamel is black, and its interior, which has been polished, shows the root and core of the tooth.

Some experts believe that desmostylians are closely related to sirenians (manatees) and elephants, whereas other experts maintain desmostylians are closely related to horses and other perissodactyls. Both groups of experts have used cladistics techniques in their analyses. Thus, the subject of desmostylians is an on-going source of study and debate, even though complete specimens have been found.

My connection with the subject matter concerns my field work many years ago on the lower Miocene Vaqueros Formation of California. This formation contains many shallow-marine invertebrate fossils (oysters, turritellas, shark teeth, etc.). Among these remains are very rare individual teeth of desmostylians. 

Sedimentary rocks on Mars

This post is a time-lapse video (which you can control) showing geologic highlights that were "captured" by NASA's Curiosity Rover during a period of 28 months (833 Martian days or Sols) exploring a small part of Gale Crater. It found abundant evidence of sedimentary rocks. What next? Stromatolites on Mars? That would be sensational.

To view the really interesting video click here:  MARS VIDEO

The Natural History Museum of Los Angeles County Invertebrate Paleontology Collection

This post concerns a recent interview with various people at the Natural History Museum of Los Angeles County Invertebrate Paleontology (IP) Collection. I am providing the link so you can read the interview or actually listen to it (about 5 minutes long). IP is now working on obtaining a digital inventory of the Cenozoic invertebrate fossils in the collection. The work is funded by the NSF.  I am a Research Associate at IP, and their excellent collection has long served as my research base.  

 Click Here

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.