PHYLUM MOLLUSCA, CLASS CEPHALOPODA
Cephalopods are those molluscs with an obvious head and eyes. The shelless types such as octopus and squid are the most obvious Recent examples, but there are a handful of Recent species that do have a shell. Fossil shelled cephalopods are very common. In some forms the shell is straight, or nearly so, but more commonly it is planispirally coiled (like a rope on the deck of a boat, or a watch spring). In addition, the shell has internal partitions ('septa') that separate the inside into chambers. The chambered nautilus from the Indo-Pacific region (Nautilus pompilius) is the most common modern species of this type. The first picture shows an example. The scale bar is 1cm. (Photo by B. Carter.)
The next photograph shows another specimen cut in half to reveal the internal structure. The coiling pattern is even more obvious in this example because each coil obscures all the earlier ones in the external view. More importantly the septa and chambering can be seen. Because each septum is added to the shell after the wall has grown beyond a point, there is a line along which the outer wall and the septum join. This line is called the "suture" between the two, and is outlined with pencil in the photograph. In nautiloid cephalopods the suture is a straight line or smooth curve like this one. the scale bar is 1cm. (Photo by B. Carter.)
Fossil cephalopods are usually preserved as internal molds because the shell is made of aragonite. The next picture is of a fossil nautiloid from Eocene rocks in Wadi Muela, Egypt. The obvious curved lines on the surface are the original sutures where the septa were attached to the outer wall, which is now gone. The scale bar is 1cm. (Photo by B. Carter.)
Another group of cephalopods is common in Paleozoic and Mesozoic rocks, but became extinct at the end of the Cretaceous. These are the ammonoid cephalopods, and they differ in the complexity of the sutures. The next photograph is of a fragment of an unidentified Lower Cretaceous ammonite from near Fredricksburg, TX. Only about a quarter of one whorl is preserved, and it is obviously an internal mold because the sutures are visible. Note that these are much more complex in shape than the nautiloid sutures shown above. The scale bar is 1cm. (Photo by B. Carter.)
The next picture is a more complete ammonoid mold (Metengonoceros sp.) from Cretaceous rocks near Ft. Worth, TX. Here also the internal mold reveals a very complicated suture pattern. The scale bar is 1cm. (Photo by B. Carter.)
