Ontogeny, muscle scars, colour pattern and predation marks in a Silurian orthoceratid cephalopod

Muscle scars, embryonic shell, colour pattern and intraspecific variation are known together in a very limited numberof straight shelled cephalopods, leading to uncertainty in the classification of the subclass Orthoceratoidea. Thenewly described genus Lepidoceras from the Silurian of Bohemia, interpreted as a demersal orthoceratid similar to Kionoceras, shows all relevant diagnostic characters. The earliest late Wenlock populations of Lepidoceras have aweakly curved endogastric shell, but the shell becomes increasingly more curved throughout the Ludlow. The sculptureis characterized by distinctive longitudinal lirae or ridges, the number of which is variable and maintained duringontogeny. The embryonic shell of Lepidoceras is one of the largest in orthoceratids; it is conical or weakly curved inshape beginning with a smooth non-accretion initial shell. Hatching is manifested by growth walls in some specimens;hatchlings were apparently demersal. Adults are usually characterized by mature modifications of the shell: septalcrowding and shell wall thickening. A high proportion of adult specimens indicates low mortality in palaeopopulations.Healed repairs indicate a lower predation pressure, which decreases after the juvenile stage. Extensive shell repair injuveniles demonstrates high regenerative capacity. Orthoceratomorph muscle scars found only in adults show a pairof dorsal retractor imprints that gradually expanded laterally. The colour pattern consists of irregular transverse bands,which have never been detected in orthoceratids before. The pattern, which is similar to that in curved oncocerids, isprobably an expression of adaptive convergence of this feature in both groups. Shell pathologies, which have beenobserved in juvenile orthoceratids for the first time, are rare; an extensive pathology in a mature specimen illustrateslimit of survival in orthoceratids. Lepidoceras is an example of a demersal orthoceratid entering a niche inhabitedpredominantly by multiceratoids.