Ammonites

Pervinquieria trinodosa, Duck Creek, Bryan, Oklahoma

Ammonites are of the extinct marine invertebrates of the subclass Ammonoidea within the class Cephalopoda. The name ammonite arose in 79 AD from Pliny the Elder. It is derived from the Egyptian god Ammon and the symbolic rams horns in his depictions. As seen in all fossils, ammonites are recognized for their spiral shape shells although there is variation between species.

Ammonite, Unknown

These predatory creatures lived in a shell that were constantly built as they grew. The size varied with fossils of many found to be over 1 m (3 ft) in diameter. Their tentacles extended from the shell to snare prey. A sharp, beak-like jaw awaited in the middle of the tentacles. Although not closely related, they moved like modern-day nautilus. Chambers within the shell were full of either fluid or gas and allowed movement within the water column. They also had a method of jet propulsion through a funnel opening within the shell for more precise horizontal movement.

Unknown, Arcadia Park Shale, West Dallas, Texas

Ammonites first appeared and evolved during the Devonian Period of the Paleozoic Era 419.2 million years ago, and they existed until the end of the Cretaceous Period of the Mesozoic Era 66.0 million years ago, disappearing with the dinosaurs. Fossils of these creatures play an important role as size, shape, and shells of ammonites can be used to date geological layers. The septa, or dividing walls of the shell, can be used to classify order, and therefore time period, of the specimen.

From the newest fossil collection at the Heard Natural Science Museum and Wildlife Sanctuary.