Baculite

In this Late Cretaceous genus, only the very earliest part of the shell remained coiled; the latter part grew into a straight shaft. Baculites can occur in vast numbers at some localities, often to the virtual exclusion of the other species, and could grow to over a metre in length.

The mode of life of these straight ammonites is controversial: some palaeontologists believe that they lived upright in the water, with the tentacles on the sea bed foraging for food; others that they had horizontal orientation and lived nearer the surface of the sea.