The ichnogenus Ophiomorpha : Taxonomy and environmental distribution

The classic trace fossil Ophiomorpha ranks among the most common biogenic sedimentary structures on earth and has been widely reported from Mesozoic and Cenozoic strata. Originally thought to be restricted to shallow-marine environments, subsequently Ophiomorpha was also recognised in marginal-marine and deep-marine deposits. A proper application for palaeoenvironmental reconstructions is hindered by inadequately defined ichnospecies and gradual transitions among them. This review reveals five ichnospecies as valid, each of which occurs in a particular sedimentary environment. Ophiomorpha saxonica and O. nodosa are common in shallow-marine and nearshore environments. Ophiomorpha nodosa may cooccur and intergrade with O. irregulairis, which reaches far into marginal-marine environments with brackish-water conditions. Ophiomorpha rudis and O. annulata are deep-marine forms, the latter probably reaching up to the upper continental slope. Care must be taken assigning burrows with a knobby lining from continental deposits to Ophiomorpha, which might be more conformable with other, similar ichnotaxa. Based on modern analogues and body fossils within their burrows, Ophiomorpha nodosa and partly O. saxonica are believed to result from the burrowing activity of callianassid shrimp. Decapod crustaceans belonging to Axiidea (mud shrimp, ghost shrimp and burrowing shrimp) and Gebiidea (mud lobsters and mud shrimp) are the likely producers of the other ichnospecies. A combined deposit and suspension-feeding can be inferred for the tracemaker. Intimate association of Ophiomorpha with minute mud-lined burrows suggests the occurrence of brooding chambers and emergence of juvenile crustaceans.