Palaeobiology of the Phycosiphon trace-maker

Style of Feeding

The Phycosiphon-making organism was sensitive to grain size variability of the host sediment, and is not found in sediments coarser than fine-grained sandstone (Ekdale and Lewis 1991, in reference to Anconichnus). The trace maker is considered to selectively ingest the clay-grade material from the sediment, leaving clean, coarser grained spreite or halos, and depositing behind it a continuous clay-rich fecal string. This is perhaps analogous to the selective deposit feeding behaviour of Euzonus mucromata, which is known to produce Macaronichnus-like burrows (cf. Gingras et al. 2002a, 2002b). The depth to which Phycosiphon is thought to bioturbate is up to 15 cm below the sediment-water interface in a wide range of bathymetric conditions from shallow marine to bathyal and perhaps even abyssal depths (Wetzel and Bromley 1994).

The presence of a meniscate backfill in the marginal tube strongly supports its origin as a fecal string (Ekdale and Lewis 1991 in reference to Anconichnus [= Phycosiphon]). The trace maker was probably a vermiform organism that produced a series of closely spaced feeding probes lateral to the marginal tube (in the centre of what is eventually a feeding loop Figure 4.1). Each probing, feeding activity leaves a tubular zone of manipulated sediment that is cleaned of clay-grade material (upon which the trace maker feeds). Successive probes are made until the organism has produced a marginal tube the length of its body (Figure 4.2). The trace maker is then inferred to burrow along the outer margin of the earlier probes, to produce the outer margin of a loop (Figure 4.3). When the body of the trace maker is once again straight, either lateral probes are produced at the start of a second loop (Figure 4.4) or the organism abandons the region and moves in search of a new food-rich region (based upon Wetzel and Bromley 1994; Bromley 1996; Seilacher 2007). When considered together these multiple phases of burrowing can be seen to leave behind a Phycosiphoniform trace fossil (Figure 4.5 - animation).