Ammonite & Goniatite

Ammonites

in the broad sense (Ammonoidea) form an extinct subclass of cephalopod molluscs. They appear in the fossil record during the Devonian and disappear shortly after the Cretaceous-Paleocene crisis. They were characterized by a more or less coiled univalve shell of which only the last chamber was occupied by the animal, the other chambers serve to control its flotation. Their fossils are considered excellent chronological markers. Their size ranges from a few millimeters to more than 2 meters in diameter.

Goniatites

Are an order of the subclass Ammonoidea (which also includes the Ceratitida and Ammonitida or « true » ammonites of the Mesozoic). Goniatites appeared at the beginning of the Devonian (about 400 million years ago). They reached their maximum development in the Carboniferous and Permian periods and disappeared at the end of this period, 251 million years ago (Permian Extinction).

Goniatites had a characteristic shell, the chambers of which were separated by zigzag sutures. The animal lived in the largest chamber, and the others, filled with gas, had a hydrostatic role: Like ammonites, goniatites swam in open water. They had two well-developed eyes and tentacles.

Goniatites are small in size, rarely exceeding 15 cm in diameter, and often less than 5 cm. Their shell is always rolled up in a single plane; its shape suggests that most of them must have been poor swimmers.