IRIDESCENT COLOR PATTERNS IN MESOZOIC AMMONOID CEPHALOPODS

Color patterns in molluscs are sometimes exhibited on the exterior shell surface as shades of black, gray, brown, and red. A wide variety of shapes and designs are known including zigzag, chevron and wavy patterns, longitudinal and transverse bands, and spots. In the Ammonoidea (Early Devonian to Late Cretaceous), the oldest known color patterns are transverse stripes and longitudinal bands from the early Triassic in Nevada, USA. It is unknown why ammonoids from rock units older than the Triassic have not yielded specimens with color patterns, especially in places where Paleozoic orthoconic nautiloids with color patterns co-occur with ammonoids.

In the last decade, hundreds of ammonite specimens with undescribed color patterns have been discovered. These new patterns are iridescent longitudinal bands on the external surface (the outer prismatic layer) of the shell. These bands are probably the result of the prismatic shell ultrastructure selectively breaking down light into different wavelengths (i.e., different colors). Ammonoid sites that preserve an iridescent color pattern have been collected from around the world including: the Upper Jurassic Callovian Stage of Saratov, Russia and Łuków, Poland; the Early Cretaceous Albian Stage of Mahajanga Province, Madagascar and Normandy, France; and the Late Cretaceous Campanian Stage from Coon Creek, Tennessee, USA and the Maastrichtian Stage of north-central South Dakota, USA.

These sites have incredibly good preservation with seemingly unaltered aragonite shell. The functions of these iridescent markings are probably similar to those of opaque color patterns produced by modern Mollusca shells including camouflage, sexual attraction and light screening. In order to utilize these patterns these animals lived in the photic zone (shallow water) as part of the nektonic food web. Because we are not aware of any modern cephalopod (or Mollusca) that has this type of color pattern, it appears that this pattern became extinct with the demise of the Ammonoidea at the K/T extinction event.