Discovery of two coloured Phylloceratidae belonging to the genus Adabofoloceras Joly, 1977. Hypotheses on the relationships between colour patterns of the shell and mode of life

Abstract

Abstract Until now the references of coloured specimens in the scientific publications about Juraphyllitidae, Phylloceratidae and Neophylloceratidae are not very numerous. The recent discovery of Adabofoloceras ardechicum (Munier-Chalmas, in Collot, 1880) and Adabofoloceras chantrei (Munier-Chalmas, in Collot, 1880) fills a part of this gap. The new findings allow to compare with coloured specimens published before, above all with the Calliphylloceras of the Kutch and with a Calliphylloceras demidoffi (Rousseau, 1842) of Upper Bathonian of Malagasy. Like every coloured specimen published before, the dark biochromes of the specimens studied in this note belong to the group of melanins. The other observable colours of both species of genus Adabofoloceras are in fact a schemochromy, that is an interferential phenomenon in the thin calcium carbonate layers; it does not imply the presence of biochromes. The exposure of the three studied specimens to UV shows the lack of biochromes such as porphyrins in these ammonites. The only biochromes found in these shells are probably melanins according to the works published earlier. In both Adabofoloceras, the colours are placed in radial stripes like “flames”, as in modern Nautilus. The Malagasy Calliphylloceras demidoffi shows that coloured longitudinal stripes can’t be excluded in the Phylloceratidae. It seams apropriate to compare the colours described in this note with those published before and with those of modern Nautilus in order to try to understand their biological function. The hypotheses proposed on the biological function of the ammonites shells colours are briefly explained and discussed. It is the same for Adabofoloceras ardechicum, the best example of coloured shells among Phylloceratidae known nowadays. We can think that these colours permitted a good protection against the predators by camouflage of the shell. It is possible to think that the biochromes could be made of excreted matter. It is more difficult to explain Calliphylloceras demidoffi because its colour is bad. Photos with IR films show biochromes in radial stripes and perhaps coloured longitudinal stripes. With most of the authors we can think that the biochromes were located in the most external layers of the test. Probably, first, they were excreted matter and secondly, they protected the animals against the predators.