Article: The oldest ammonoid 'colour' patterns: description, comparison with Nautilus, and implications

Twelve Lower Triassic ammonoid specimens that retain four different 'colour' patterns are described. These 'colour' patterns are the oldest known for ammonoids; three genera (Dieneroceras, Prosphingites, and Owenites) are represented. Four factors contribute to the conclusion that these 'colour' patterns were deposited at the time of growth: (1) the transverse bands are bilaterally symmetrical; (2) the coloration is confined to the outer layer of the test; (3) the pattern is disrupted by sublethal damage to the conch; and (4) the 'colour' is observable through the Runzelschicht and is present under the dorsal shell. The ammonoid 'colour' patterns differ from modern Nautilus by being present through the terminal growth stage and by being concordant with the growth lines; Nautilus loses the colour banding at maturity and has a discordant relationship of colour patterns with growth lines. The transverse 'colour' patterns on ammonoids appear to be less sophisticated than those observed in Nautilus', this suggests that the colour patterns on ammonoids may have served a different primary function than that of camouflage.