PLACENTICERAS

The focus of this post is an ammonite specimen with very complicated internal shell structure consisting of "sutures" or partitions. Please see the diagrams I provided in my previous blog on “Ammonite Sutures,” which I posted on Oct. 19, 2014.

Ammonites are extinct mollusks (cephalopods) that range in geologic age from the Devonian to the end of the Cretaceous. The following image shows a replica of the general appearance of many ammonites and is not the same genus as the one discussed below. The outer shell of an ammonite conceals the internal shell structures (called septa), which can be much more complex than what is visible on the outside of the shell.

The fossil ammonite discussed below is moderately large (12.5 cm tall, 11 cm wide). The first and third images illustrated below show its sides, and the second image shows its narrow, flat venter edge (2.75 cm thick). The locality of this ammonite is in the South Dakota region. It was collected nearly a century ago by an anonymous collector. The outer shell is missing on this specimen, but its internal shell details are well preserved.

The interior of an ammonite shells have many chambers, and the shelly chamber walls are called sutures. The sutures constitute lines which have have loops in them. If a loop points toward the aperture (the large opening of the shell where the main part of the animal was located), the loop is called a saddle. If a loop points in the opposite direction, it is called a lobe. Taken as a whole, each chamber wall consists of saddles and lobes. The more complicated the suture (i.e., the more it has squiggles or frills, the younger the geologic age of the ammonite). In the case of the ammonite figured here, it can be readily seen that the suture pattern has a suture line with all of its loops frilled. 

The suture pattern of the fossil ammonite shown here has very frilled sutures; so much so that they can be classified as ammonitic sutures. They help greatly in the identification of the family and the genus of an ammonite. Using its suture pattern, I was able to identify the family of this ammonite as Placenticeratidae and the genus as Placenticeras sp. Meek, 1876. This genus is Late Cretaceous in age and ranges in age from the late Cenomanian to the late middle Campanian. The genus is somewhat widespread in the world. In the United States, it has been found in the “Western Interior” region of Wyoming, South Dakota, Colorado, Utah, New Mexico, and Texas.

This paleogeographic map (http://jan.ucc.nau.edu) shows the distribution of the Western Interior Seaway (in light blue color), which existed during the time of Placenticeras.

It is important to mention that the outer shell of the above-illustrated specimen is mostly missing. In the identification of any specimen of Placenticeras it is critically important to know whether or not the outer shell had tubercles (where located on the shell and how many tubercles). On the specimen illustrate here, tThere are a few poorly preserved tubercules, but otherwise this information is not available. Identification of the species is, therefore, indeterminate. That is why it is always important to see as many specimens as possible when identifying a fossil. 

SOME EASTERN CALIFORNIA TRILOBITES

Trilobites belong to Phylum Arthropoda, whose members are characterized by a segmented body that has fusion and specialization of groups of segments into two or three major body regions (head, abdomen, and tail). The body consists of a rigid chitinous + calcium carbonate or calcium-phosphate exoskeleton that must be shed periodically in order for the animal to grow larger. This process thereby allows for the possibility of many fossils being created by a single animal.

Shown above is Olenellus fremonti, a trilobite of Early Cambrian age approximately 525 million years old. The head (cephalon) of this specimen is very obvious, as is the abdomen. The tail, however, is hard to distinguish from the abdomen. The specimen shown here is 7.8 cm in length, but its left side is partially missing. The rock is a slightly metamorphosed brittle siltstone, that approaches being slate. The eyes of this specimen are not very well preserved, but the left eye shows the best preservation. Regarding trilobite eyes, you might want to check out one of my early posts on this subject matter. 

The above plastic replica (three views: top, bottom, and right side) of Olenellus is the best replica of a trilobite that I have ever seen. It shows the exoskeleton (the outer surface = green color “created” by the manufacturer of the replica), which is underlain by the gills (white color), which is underlain by the numerous thin legs (red color). This replica also shows the two antennae, front and aft, which are found on some olenellids. Their preservation, however, is very rare.

Bristolia bristolensis (36 mm length) is also of Early Cambrian age and from the same rocks that contain Olenellus fremonti. This is mostly a complete specimen of B. bristolensis. The abdominal spines present on Bristolia were undoubtedly used for protection against predators. The exoskeletons of trilobites were flexible, thus this lateral spines could be flexed upward, like prongs.  

By comparing B. bristolensis with O. fremonti, you can see that the cephalon (head) of Bristolia is more rectangular.