Description
Ammonites (Ammonoidea) are an extinct subclass of the Cephalopoda (squid). They are marine animals that were abundant worldwide in the late Paleozoic and throughout the Mesozoic. They are found as fossils. Ammonites have a flat, spiral-shaped shell composed of several chambers. Each time the animal outgrows its current chamber, a new, larger outer chamber is formed. The animal lives in this outer chamber, using the other empty chambers as a means of vertical movement. The ammonite excretes gas into these chambers to regulate the upward force on the shell. Ammonites exist in hundreds of species and varieties. They almost all share the same basic plan: a flat, spiral-shaped shell. There are, however, a few exceptions: a few heteromorphic species that do not have a spiral shell. Ammonites range in size from less than a centimeter to over 2.5 meters in diameter. The closest living relatives are the Nautilidae (nautiluses). The name comes from the Egyptian god Amun, who was depicted as a man with the head of a ram. Ammonites resemble the curled ram's horns that Amun was depicted with. The Roman author Pliny the Elder describes fossils of these animals, which he called "ammonis cornu" (horn of Ammon). Ammonites first appeared in the late Silurian, survived the Permian-Triassic extinction with a remarkably small number of species, and truly flourished during the Mesozoic. At the end of this period (65 million years ago), most ammonites became extinct, along with the dinosaurs. However, fossils from the Netherlands and Denmark seem to indicate that the last ammonites persisted into the oldest strata of the Paleocene. The youngest known ammonite fossil comes from Denmark and belongs to Hoploscaphites constrictus, which persisted until approximately 65.3 million years ago, some 0.2 to 0.65 million years after the mass extinction, and possibly even for half a million years longer. However, these small populations were not enough to save the last ammonites from extinction. It is thought that the ammonites became extinct due to their reproductive strategy. The young were part of the plankton floating on the sea surface. Acid rain and the obscuration of the sun by dust clouds likely created poor conditions for the eggs and young to develop.
All about finding ammonites.
