PHYCOSIPHON INCERTUM REVISITED - ANCONICHNUS HORIZONTALIS IS ITS JUNIOR SUBJECTIVE SYNONYM

Comparisons between Phycosiphon incertum Fischer-Ooster (1858) and Anc onichnus horizontalis Kern (1978) show that these two monotypic ichnog enera do not differ significantly in geometry, size, sediment fill, an d environmental occurrence. Therefore, taxonomic priority has to be gi ven to Phycosiphon and Anconichnus becomes a subjective junior synonym . The trace fossil is a small, highly lobed spreite structure produced by a zoned backfill consisting of a pale mantle and dark core. In a f ew cases, a meniscate structure is visible in the core. The spreite se diment is an extension of the mantle material; it commonly is poorly v isible. The spreite-producing behavior may be locally interrupted, lea ving a single tube of backfill. The spatial arrangement of the trace f ossils seems to depend on the host sediment; in muddy and homogeneous material the lobes tend to be oriented randomly, including vertically, whereas in laminated sand and silty sediments they tend to lie parall el to bedding. This latter orientation is exaggerated by compaction. T extural differences between host sediment and spreite or mantle may be subtle and not always evident. Observations in modern sediments are i n agreement with the type material of both ichnogenera.