Phycosiphoniform burrows in marine ichnofabrics

Phycosiphoniform trace fossils are an important component of most post-Palaeozoic shallow marine ichnological assemblages, particularly those with a mixture of clay and silt grade material (Goldring et al. 1991; Fu 1991). The recognition of Phycosiphon incertum has been greatly encouraged by publication of a series of representative hypothetical cross sections based on bedding-parallel specimens (Bromley 1996). We consider it likely that all phycosiphoniform burrows result from deposit feeding by organisms that selectively ingest clay grade material in order to process microbial biomass, dissolved organic matter (DOM), bio-films on sediment grains and the associated meiofaunal/interstitial biomass. The clay grade material ingested is concentrated into a faecal strand, surrounded by a zone of biologically processed sediment (silt to very fine-grained sand) that has been cleaned of clay-grade material. Published occurrences of Phycosiphon are commonly taken to include older literature mentioning the trace fossil Anconichnus horizontalis, which was described exclusively from vertical and horizontal cross sections in slabbed material (Kern 1978). The synonymization of A. horizontalis with Phycosiphon incertum (Wetzel and Bromley 1994), based on revision of the type material of A. horizontalis, and emendation of the original diagnosis of P. incertum to include non-bedding-parallel specimens, has been widely adopted. As a result, Anconichnus is seldom referred to in modern literature.

In most cases, phycosiphoniform burrows are found as part of diverse ichnofabrics developed in shallow marine depositional environments (Goldring et al. 1991; Bockelie 1991; MacEachern et al. 2007c). In ichnotaxonomically diverse shallow marine ichnofabrics, Phycosiphon incertum is generally a late-stage component of the ichnofabric, cross-cutting and reworking earlier burrow fills (e.g., Goldring et al. 1991; McIlroy 2007). Modern Phycosiphon incertum are common in deep marine settings (Wetzel 2008), though the trace maker is not as yet identified. Where phycosiphoniform burrows are found in mono-taxic assemblages, the depositional environment is typically inferred to have been stressed. Examples of stressful depositional environments with mono-taxic assemblages of phycosiphoniform trace fossils include tide dominated deltaic deposits, in association with fluid mud deposits (McIlroy 2004b), and dysoxic mudstones (Bromley and Ekdale 1984, 1986; Ekdale and Mason 1988; Savrda 2001).