Colour pattern polymorphism in Silurian nautiloid Phragmoceras Broderip, 1839

Abstract

Flamboyant colour pattern polymorphism documented in Silurian nautiloid Phragmoceras has not been observed in any other nautiloid genus. Two specimens of P. imbricatum Barrande, one from the early Ludlow of Bohemia and the second from the late Wenlock of England display quite different shell coloration: narrow, densely spaced longitudinal bands subparallel to the shell axis, combined with narrower transversal bands versus transverse bands running laterally obliquely to growth lamellae. Two additional types of colouration have been observed in several specimens of Phragmoceras from the early Wenlock and late Ludlow of Gotland. The colour pattern in P. eurystoma flexibile Hedström consists of zigzags bands around the whole circumference of the shell. In contrast, distinct colour bands following growth lines along the whole circumference of the shell, like that in the Cretaceous nautilid Eutrephoceras, are characteristic in P. dubium Hedström. If pigmentation pattern served as a form of crypsis, then the quite different types of coloration in Phragmoceras indicate different solutions of this problem. Colour pattern has been regarded as a useful tool also for taxonomic purposes in nautiloid cephalopods. Nevertheless, the colour polymorphism in Phragmoceras suggests the limited significance of this feature for the taxonomy of nautiloids. Some other remarkable cases of colour variations in Silurian nautiloids are discussed. Changes in colour pattern most probably correspond with depth inhabited; nautiloids that occupied shallow water environments display rather light coloured shells in comparison with forms inhabiting a deeper water setting.