Siphonichnidae (new ichnofamily) attributed to the burrowing activity of bivalves: Ichnotaxonomy, behaviour and palaeoenvironmental implications

The ichnofamily Siphonichnidae nov. is established for encompassing burrows of varying morphology and consisting of one or more sub-vertical tube(s) with core and surrounded mantle or lining. It includes the ichnogenera (in order of their introduction) Laevicyclus Quenstedt, 1879; Parahaentzschelinia Chamberlain, 1971; Siphonichnus Stanistreet, le Blanc Smith and Cadle, 1980; Scalichnus Hanken, Bromley and Thomsen, 2001; and Hillichnus Bromley, Uchman, Gregory and Martin, 2003, all of which were critically reviewed. The lined or mantled burrow Skolithos linearis Haldeman, 1840, may be also included as potential senior synonym of Laevicyclus. The eponym of the ichnofamily, the trace fossil Siphonichnus, is a common ichnogenus which has been frequently reported from the Devonian to modern deposits around the world, particularly in connection with coal mining and from well cores in the oil and gas industry. Although already known as burrows with an eye-shaped cross-section and as Planolites ophthalmoides to the coal miners in Germany since the 1930s, Siphonichnus became formally introduced not until 1980. Moreover, some characteristic features (e.g., variable orientation) were not included in the original ichnogeneric diagnosis but are important for its recognition. The clarification of its ichnotaxonomical status becomes necessary, for instance in order to use this trace fossil as palaeoenvironmental indicator. Consequently, Siphonichnus ophthalmoides (Jessen, 1950) is the valid type ichnospecies of the ichnogenus Siphonichnus. By means of modern analogues, Siphonichnus can be attributed to the activity of endobenthic bivalves such as tellinids and nuculids, which include siphon-bearing species with a predominantly deposit-feeding behaviour. Previous interpretations have regarded Siphonichnus as the dwelling trace (domichnion) of bivalves feeding on organic material from the sediment surface or above it, which are able to adjust their position with respect to erosion and deposition (equilibrichnion). However, the characteristics of the emended diagnosis also indicate the activity of a (temporarily?) vagile endobenthic organism for the purpose of locomotion and partly deposit-feeding. A review of reported occurrences of S. ophthalmoides (and its synonyms) shows it as a clear indicator of shallow-marine and marginal-marine environments, often related to fluctuating salinities and freshwater influx. Its distribution ranges from proximal offshore and shoreface to deltaic, estuarine and lagoonal environments, and rarely into the deep sea.