Rocky Ridge sandstone member of the Pierre Formation
Fossil
 Cretaceous

The first fossil I remember finding was down the hill from my house by a canal. I was 10. It was a Baculite.

About a half mile from my house (Larimer county, Colorado) the Rocky Ridge member of the Pierre formation is exposed. I spent my youth collecting there and have been back many times.

The Pierre formation was deposited in the Western Interior Seaway during the Cretaceous and is exposed from Texas to Alberta.

Cephalopods

The Cretaceous was the apex of amminoid diversity and then they all died out with the dinosaurs. Here a few examples from north of Fort Collins, Colorado.

Baculite grandis. The large specimen is 12 inches long.
There are two species here. Baculite ovatus and B. compressus. The first piece still has the outside iridescent shell intact. The cross section at the bottom left shows the position of the tube (or siphuncle, the circle at the bottom of the image) that ran through the septa to equalize the pressure in the shell.
Two different coiled ammonnite species
View #1 of a Placenticeras species
View #2 of the specimen to the right and a second example of Plaacenticeras

The convoluted edge of the septa of the shell where it meets the outside shell of the animal is a distinguishing characteristic of ammonites.
The rounded convolutions show the direction of growth. In the example below, the direction of growth is to the left.
This image shows the interior of a shell where the septa meet the outside shell of a coiled ammonite.

Nautiloids are present but no where nearly as abundant as the amminoids. Of course, the nautiloids managed to survive into the Cenozoic and are represented in the Eocene as well as in the present—the Pearly Nautilus.

This speciment illustrates a basic difference between ammonoids and nautiloids: the siphuncle, the tube that runs from the body of the amimal through the septa, the chambers dividing the shell, is centered in nautiloids and is located at the edge of the shell in ammonids.

Contrast this specimen with the baculites and ammonites above.

Pelecypods/Bivalves

A common Cretaceous bivalve found in Western Seaway deposits is Inoceramus. Here are three different species.

These specimens are all casts. The Inoceramus shells did not preserve well in the Rocky Ridge member of the Pierre formation.

However, in the Niobrara formation below the Pierre, the Inoceramus shells are preserved and you can see the distinctive columns of calcite that made the shell.






Here are additional bivalves from the Pierre. This first group about one and a halt to two inches across.
This group are all about one inches in the longest dimension.
As with Inoceramus, the shells are delicate and not often preserved. However the casts may show significant detail of the interior of the shell. Note the muscle scar in the first specimen.
Details to com.

In contrast to the other bivalves, the shells of various oyster species are well preserved. This specimen is five inches long. (Two views)
This nondescript specimen is a colony of small oysters. The largest is one inch long.
Here are several additional oyster species.

Gastropods

There are snails and limpets but there isn't as much evidence of predation as compared to my New Jersey material.


Here is one species represented by a cast and a poorly preserved shell.
This and the small snails to the right are all less then an inch in their longest dimension.

Two limpet species.
An grouping of a Crepidula sp., slipper shells

Bryozoa

The is a common branching bryozoan (possibly a Ceriocava species) preserved at Rocky Ridge. It is most often encased in sandstone so that only the outside ends of the branches are visible but sometimes broken or eroded specimens show more of the structure.



This is an encrusting bryozoa growing on an ammonite shell. The blow up shows the bottom of the colony.

Brachiopods

Unlike the material from New Jersey where a brachiopod is common, there are few brachiopods at Rocky Ridge. Here are a two species.

Arthropods

Probably more abundant that my collection would indicate. Here is the only crab/lobster specimen I've found. (Two views)



Vertebrates

A fish scale and vertebrate
Bones from the flipper of a Pliosaurus
         

A Discussion: Ammonite septa and chambers.

Ammonites are distinquished by the pattern created by the internal septa connection to the inside of the shell. These septa created hollow chambers in the shell of ammonites. As the complexity of this connection increased, the strength of the bond and overall strength of the shell increased, allowing for a lighter and, therefore, less costly shell for the animal to make. Ammonite shell, in general, are thinner and lighter than shell of gastropods or bivalves of similiar size.

Rarely are chambers preserved empty but are usually filling with sediment, minerals such as calcite deposited after the shell is buried, or collapsed.

Here is one instance in which the chambers did survive empty with the shell breaking approximately in half when excavated. The second image is the same species seen from the side and from the top with a line indicating the approximate position of the break in the first image.

The fourth image is an enlargement of the first showing the edge of the mold formed by the external shell, the parallel ridges that decorated the external shell, and the edges of the septa where they meet the external shell.

The fifth image show the edge of the septa where they meet the shell in a baculite specimen with calcite filled chambers.

The last two images are a single filled chamber that separated from the rest of the shell. Not the small circle at one side of the oval. That is the siphuncle, a tube that ran through each septa and chamber allowing the animal the change the bouyancy of the shell.

Hollow chambers