A Silurian oncocerid with preserved colour pattern and muscle scars (Nautiloidea)

A new genus Euryrizocerina (Nautiloidea, Oncocerida, Oonoceratidae) from the late Silurian strata of Bohemia (Prague Basin) is established. Two species are included within the genus: the type species E. normata (Barrande, 1877) and E. pulchra sp. nov. The genus is striking for two features – the preservation of shell colouration and muscle attachment scars in the type species. These features are reliable for taxonomic and palaeoecological analysis of nautiloids, but due to their only occasional preservation in fossils, the evolutionary trends of these characters remain poorly understood. The colour pattern of Euryrizocerina consists of irregular pulsed transverse bands that clearly differ from the colour patterns known in other oncocerids where rather regular wave-like or zig-zag bands have been observed. It is questionable whether the new type of colouration represents a taxonomic or morphotypic feature, as Euryrizocerina is the first oncocerid to be recognised with a highly curved shell that retains colouration. Oncocerids exhibit multiple paired muscle scars and there is uncertainty whether these represent an original metamery, inherited from a cephalopod ancestor, or a derived feature. Euryrizocerina possesses four pairs of retractor muscle scars, but its ancestor " Oonoceras " fraternum and allied forms, possess multiple pairs of muscle scars around the base of the body-chamber, as do the majority of oncocerids. A similar reduction in the number of muscle scar pairs has been documented in Devonian rutoceratoids. The high evolutionary plasticity of muscle attachments patterns, as indicated by these examples, suggests that the muscle complex and its attachment to the shell was under adaptive control and is more likely to reflect the form of the shell and the mode of life of the organism. It seems likely that shell colouration and the pattern of attachment of the retractor muscles have a much more complex evolutionary history (including several convergences) than was previously supposed.