Texas Pennsylvanian Fossils

The Phylum Mollusca is a large and varied invertebrate group within the Animal Kingdom. They are characterized by a soft body with a strong muscular foot and usually (but not always) covered by a shell. The Cephalopods are a highly diversified, exclusively marine, Class of mollusks, some of which possess an external shell (nautiloids, goniatites and ammonites) and others which no longer have an external shell (squid, octopus) but all of which have tentacles which are a modification of the molluscan foot. The cephalopod shell is a series of chambers connected by a tube (siphuncle) that are added as the animal grows. The last chamber is the living chamber. Clearly recognizable cephalopods were first seen in the Ordovician and the shelled forms became a dominant group in the Paleozoic. Today, shelled forms are very limited in variety and distribution, but the non-shelled forms are more widely dispersed. 

During the Pennsylvanian, one group of cephalopods was the Subclass Coleoidea. This group represents the cephalopods without an external shell which are very common currently but in the Pennsylvanian were newly evolved and quite different from most modern representatives as they still carried a shell internally. The Pennsylvanian bactrids were probably early representatives of the belemnites which were common in the Mesozoic. 

The Finis contains nearly four dozen forms of cephalopods but only one species of coleoid is known. The genus Shimanskya is usually small, fragmentary and rare. 

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