Interesting spiral creation of ammonites

Ammonites were predatory mollusks that resembled squid. They are more closely related to a living octopus, though the shells resemble that of a nautilus. Suture patterns and whorl characteristics are used to identify species. 

Anatomy

Many Ammonites floated or swam above seafloors that lacked oxygen to support animal communities. When they died, they drifted to the bottom and bacterial decomposition altered the redox chemistry of the water- making minerals less soluble and thereby concreting around the ammonite. 

This scenario promoted excellent preservation of shell material. Though soft tissues are poorly preserved, some ink sacs and beak-like jaws have been found. The rapid evolution of distinguished genera, their subsequent extinction after a few million years, and the excellent preservation of unique shell features make ammonites an excellent index fossil.

Evolution and Taxonomy

Ammonoids survived several major extinction events- though in each instance, only a few species survived to diversify before the next event. Their extinction and the survival of the nautiloid may be tied to different reproductive strategies. About 8 species of Ammonites from two Families survived to the end of the Cretaceous.

Suture Patterns

Suture patterns and whorl character are used to identify species. There are three major types of suture patterns are found in the Ammonoidea: Goniantitic sutures are typically evident as 8 lobes on the conch. It is found on Paleozoic ammonoids and the lobes and saddles have many, undivided lobes and saddles. 

Ceratitic sutures have rounded, undivided saddles. The lobes have a saw-toothed appearance because of their subdivided nature. This suture pattern appears twice in the fossil record.

Ammonitic sutures are indicated by saddles and lobes which are fluted, usually rounded, and highly subdivided. These sutures are found in specimens from the Permian, Jurassic, and Cretaceous, though they are more common to the Jurassic and Cretaceous. These characteristics make species of this type the best for biostratigraphy.

Buoyancy

Ammonites controlled their depth underwater with a thin, tubular structure called a siphuncle. The siphuncle, which ran along the outer rim, connected the gas chambers of the phragmocone to the body chamber. The gas chambers, called camerae, were separated by septa. The ammonoid suture line, where the septum meets the outer shell, has characteristic folding which creates saddles and lobes- features that distinguish it from the gently curving sutures which appear on nautiloids.

Ammonoids reproduced toward the end of their life by large quantities of eggs in a single batch. It is thought that along with juveniles, the eggs floated with plankton at the surface of the ocean. If the effects of an impact killed off the plankton, then the ammonites would have also met their demise. The fossil record supports that at least one species of ammonite ate plankton, as well.

Check out the ammonite collection by limany HERE