Mollusca

Close up of Placenticeras, a large Cretaceous ammonite (Cephalopoda, Ammonoidea, Ammonitida, Hoplitaceae, Placenticeratidae) with a shell diameter of up to 50 cm. During the Mesozoic, ammonoids were extremely plentiful and diverse. In the middle background is the siphonal canal of a neogastropod shell. The carniverous neogastropods radiated during the Cretaceous, triggering an arms race among bivalves and other organisms on which they preyed.

The mollusks are a large and diverse group of soft-bodied unsegmented animals. Nearly 130,000 recent species are known, and some 35,000 fossil species (these latter are without doubt only a small fraction of those that have ever lived). They include many familiar animals, like snails, clams, squid, octopods, etc, as well as others not so well known. They range in size from microscopic forms to the giant squid Architeuthis), and have a long and venerable history appearing during earliest Cambrian time, if not before.

Evolutionary History

The mollusks first appeared during the earliest Cambrian (Tommotian), at the very start of radiation of coelomate animals. As the Cambrian period progressed, many types appeared that soon died out. Although previously included among the conventional classes of mollusks it is increasingly argued that these represent distinct classes, early experiments in molluscan evolution, or transition forms. However, the exact number of extinct classes remains debatable.

By the Ordovician period the three main classes - Gastropoda, Bivalvia, and Cephalopoda - had increased greatly in number and diversity. This was a period of major evolutionary diversity for the phylum, and they became common in most marine ecosystems.

The Devonian saw bivalves invade freshwater, and the first land snails appeared during the Carboniferous. During the Devonian also, the ammonoids took over from the nautiloids as the dominant cephalopod group, and these creatures continued to flourish throughout the entire Mesozoic, living alongside species of bivalves and gastropods not very different to modern forms. (Oysters for example were common during the Jurassic and Cretaceous, while scallops date back to the Paleozoic). By mid Cretaceous most mollusks, like most invertebrates and fish, were essentially like modern forms, except for the cephalopods which were still represented largely by the Mesozoic ammonites and belemnites. The ammonites were decimated by the K-T extinction event, as were the reef-building rudist bivalves (although these were on the decline for some time), but the coleoids (soft-bodied squids, octopods etc) continued to evolve quite happily.