Phycosiphon incertum revisited: Anconichnus horizontalis is its junior subjective synonym

Abstract

Comparisons between Phycosiphon incertum Fischer-Ooster (1858) and Anconichnus horizontalis Kern (1978) show that these two monotypic ichnogenera do not differ significantly in geometry, size, sediment fill, and environmental occurrence. Therefore, taxonomic priority has to be given 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 few cases, a meniscate structure is visible in the core. The spreite sediment is an extension of the mantle material; it commonly is poorly visible. The spreite-producing behavior may be locally interrupted, leaving a single tube of backfill. The spatial arrangement of the trace fossils 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 parallel to bedding. This latter orientation is exaggerated by compaction. Textural differences between host sediment and spreite or mantle may be subtle and not always evident. Observations in modern sediments are in agreement with the type material of both ichnogenera.