Ancyloceratina

The Ancyloceratina were a diverse suborder of ammonite most closely related to the ammonites of order Lytoceratina. They evolved during the Late Jurassic but were not very common until the Cretaceous period, when they rapidly diversified and became one of the most distinctive components of Cretaceous marine faunas. They have been recorded from every continent and many are used as zonal or index fossils. The most distinctive feature of the majority of the Ancyloceratina is the tendency for most of them to have shells that are not regular spirals like most other ammonites. These irregularly-coiled ammonites are called heteromorph ammonites, in contrast to regularly coiled ammonites, which are called homomorph ammonites.

Biology

The biology of the heteromorph ammonites is not clear, but one certainty is that their uncoiled shells would have made these forms very poor swimmers. Open shells, particularly ones with spines and ribs, create a lot of drag; but more importantly, the orientation of the shell, with the body hanging below the buoyant part of the shell, would have created a serious impediment to efficient swimming. It's more likely these ammonites either drifted in the plankton, collecting small animals on long tentacles like modern jellyfish, or else they crawled along the sea floor feeding on sessile or slow-moving animals such as clams.

Morphology

Ancyloceratida varied widely in size, ranging from diminutive Ptychoceras, which was barely 3 cm (1.2 in) long, to Baculites and Diplomoceras, which could grow to 1–2 m (3.3–6.6 ft) in length. Some species were very widely distributed, for example some species of Hamites can be found in Eurasia, South America, Australia, and Antarctica.