Ammonites

Ammonites are one of the most easily recognised fossil that the amateur collector will come accros.

HABITAT

Only in extermly rare cases have any remains of their soft body parts been found preserved. What is generally believed is that they lived in open waters, rather than being bottom dwellers, their streamlined shells suggesting they would have been good swimmers, especially the Oxynoticeras. However, some of the less streamlined ammonites may indeed lived and hunted on the bottom. It is thought their prey was probably small fish, crustaceans and smaller creatures, while they in turn were probably predated by the larger marine reptiles, such as mosasaurs.

The ammonite shell was built up of walled-off segments. The animal lived in the largest segment at the mouth of the shell, walling off the smaller segments as it grew, filling the smaller segments with gas to regulate its bouyancy. The partf of an ammonite's shell which contains the chambered cells is called a phragmocone, which consists of progressively larger chambers to accommodate the animal as it grows, sealed off by thin walls called septa. The animal lived in only the last part of the chamber, known as the body chamber, adding newer and larger chambers to the open end of the coil as it grew. Running just inside the inner periphery of the shell was a living tube, the siphuncle, extending from the animal's body through the empty chambers beyond. This siphuncle was used by the ammonite to transfer water in and out of its shell to control bouyancy and allow it to rise or lower itself in the water column.

Unusual in the animal kingdom, the male shell was slightly smaller than the female, though the reason was probably that the female required a larger shell to to carry the eggs. Everyone think of ammonites as being like a flat coil, but some species are nearly straight (the baculites) while other species are coiled helically, and others are straight, then coiled, and again straight upon maturity. This characteristic started to develop around the early Cretaceous period in America and Japan.