Irregularly-coiled ammonites are called heteromorph, oppose to the regular more common coiled Ammonites.

The Late Cretaceous Period had the widest range of Heteromorphs, some of which include the straight-shelled Baculites and Sciponoceras, the helically coiled Turrilites, and with some forms combined different coiling modes. For example, Nostoceras started off with a helix like a Turrilites, but had a planar hook hanging underneath.

Why these Heteromorph Ammonites developed is this way is not clear but it is considered the uncoiled shells would have made these Ammonites very poor swimmers. These open shells would create a lot of drag, in particularly ones with spines and ribs. It felt these Ammonites would ether drift, rather then swim and fed off, using long tentacles like modern jellyfish to collect small animals or crawled along the sea floor feeding on slow-moving animals such as clams.