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.

Specimens here from the Pierre include cephalopods, bivalves, brachiopods, arthropods,
bryozoa, gastropods, and vertebrates.

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.

This is Baculite grandis. The large piece 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 (The circle at the bottom of the image) that ran through the septa to equalize the pressure in the shell. Here are three coiled species with two views of one of the specimens.

 
 
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. Here is a close-up.
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

A common Cretaceous genus 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.

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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. (To the right)

 

 

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.

One last bivalve specimen: A group of three.

In contrast to the other bivalves, the shells of various oyster species, is well preserved. This specimen is five inches long. (Two views)

This nondescript specimen is a colony of small oysters. The larges 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.

These small snails are all less then an inch in their longest dimension.

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  Here two limpet species.

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.

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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 three.

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Arthropods

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

 

 
 

Vertebrates

Vertebrates are rare but here are three examples. A fish scale and vertebrate

And what appear to be bones from a marine reptile flipper.

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