Marine Invertebrate Neoichnology - Trace Fossils as Indicators of Sedimentary Environments - page 270

and harvest bacteria ( Aller and Yingst, 1978; Bromley, 1990 ). A last variant of
sessile vermiform animals is the development of spiral morphologies ( Fig. 2 Band
C). In neoichnological studies of marine vermiform animals, true spiral morphol-
ogies have been attributed to capitellid polychaetes ( Howard et al., 1975; Powell,
1977 ) and paraonid polychaetes ( Risk and Tunnicliffe, 1978 ). A vertical to hori-
zontal helical morphology is also observed at the base of hemichordate worm
burrows, mainly those produced by Saccoglossus sp. ( Figs. 1 and 2 B, C; Gingras
et al., 1999; Hauck et al., 2009 ). Variations in the morphology of motile worm
burrows are also noted, and include vertical readjustment of burrows ( Figs. 1
and 2 I), variability in two-dimensional and three-dimensional burrow networks,
and backfilling of burrows. Gingras et al. (2008a) discuss at length variations in
motile vermiform burrows.
Although deep-water settings are not particularly well studied, oceano-
graphic research has yielded a range of trace-fossil analogs (e.g., Heezen and
Hollister, 1971 ). In these settings, tracks and burrows are reported from photo-
graphic images and from box cores, but the tracemakers commonly remain
unseen. For example, Hersey (1968) shows a range of modern traces that might
be comparable to Planolites and Neonereites , but the tracemakers are com-
monly out of view. Several studies document the ichnology of Holocene
deep-sea sediments from box cores, and a range of recent burrows are reported,
including Zoophycos and Spirophyton (e.g., L¨wemark and Sch¨fer, 2003;
Wetzel, 2008; Wetzel and Werner, 1981 ) and Asterosoma ( Ekdale and
Lewis, 1991 ). Although it is generally accepted that a vermiform morphology
is suited to the generation of these traces—echiurans and priapulids may be
ascribed to such traces—the tracemakers have not been observed in clear asso-
ciation with the biogenic structure. On the other hand, Vannier et al. (2010) used
aquarium experiments to show that priapulids were a feasible tracemaker for the
commonly deep-water-associated Treptichnus .

2.2 Bivalves
Traces produced by deeply burrowing bivalves (burrow depth 5 cm) have a

higher preservation potential and, therefore, are ichnologically more important
than traces produced by shallowly burrowing bivalves. Burrows and trails made
by gastropods and shallow-burrowing bivalves (e.g., Clinocardium sp.) are
abundant but have a much lower preservation potential due to their near-surface

>

Cylindrichnus and Rosselia (if vertical) and Asterosoma (if horizontal). (F) Thick mud lining on the
wall of a nereid polychaete burrow constructed in a mixed sand and fine-gravel substrate. This bur-
row morphology is best described as Palaeophycus -like (Pa). (G) Lined Skolithos (Sk) traces made
by the head-down deposit feeder Clymenella torquata (a maldanid polychaete). (H) Trace assem-
blage including Skolithos and Thalassinoides (Th) generated by the polychaete Nereis sp. Also evi-
dent in the radiograph is a crustacean-generated Thalassinoides burrow (lower left). (I) Teichichnus
(Te) and Skolithos (Sk) traces generated by the polychaete Nereis sp.