Marine Invertebrate Neoichnology - Trace Fossils as Indicators of Sedimentary Environments - page 267

The focus of this chapter is on the neoichnology of siliciclastic, marginal-
marine to marine depositional environments. The chapter is divided into two
parts. Part one is a review of the major tracemaking groups, and the similarities
and diversity in their burrow morphologies. Part two is a discussion of the
impact of various environmental stresses on modern trace distributions and
draws extensively on published examples.
2. MAJOR INFAUNAL GROUPS AND THEIR COMMON TRACES
A majority of burrowing marine macrofauna are invertebrates. Of the major
invertebrate groups, vermiform animals (e.g., annelids, nemerteans, and hemi-
chordates) are the dominant burrowers, followed by, in no particular order,
bivalves, crustaceans, echinoderms, and cnidarians.
2.1 Vermiform Animals: Annelids, Hemichordates, and
Nemerteans
Vermiform animals produce smooth-walled burrows that are usually mucous
lined ( Fig. 1 ). Preservation of the lining is dependent upon the incorporation
of sediment with the mucous, and the geochemical microenvironment in and
near the burrow. Sessile vermiform organisms construct mainly unbranching,
vertical ( Figs. 1 and 2 G; e.g., Clymenella spp., Pectinaria spp.) to U-shaped
( Figs. 1 and 2 A) burrows. Motile vermiform animals (e.g., Nereis sp., Nephtys
sp., Cerebratulus sp.) commonly produce three-dimensional burrow networks
that are either branching (if the burrow network acts as both a dwelling and a
feeding structure; Figs. 1 and 2 H, I) or unbranching where the burrow is a tem-
porary structure mainly used for locomotion (i.e., sediment intrusion or swim-
ming). In the case of both sessile and motile polychaetes, the diameter of the
tube is typically uniform along the length of the burrow, enabling identification
of these structures as vermiform-generated traces.
Variations from the morphological generalities described above are common
in both sessile and motile vermiform burrows. For sessile vermiform burrows,
variations include vertical readjustments of burrows, thin and thick burrow lin-
ings, and spiral morphologies ( Gingras et al., 2008a ). Vertical readjustment of
U-shaped burrows produces spreiten either below or above the occupied burrow
(e.g., Polydora ligni , Fig. 1 ; Gingras et al., 2000b, 2001; Ginsburg et al., 1966;
Howard and Frey, 1975; Pemberton and Frey, 1985; Rijken, 1979; Sch¨fer,
1972 ). Burrow linings are commonly preserved when sediment is incorporated
into the lining. For example, pectinariids (e.g., Pectinaria gouldii ; Fig. 1 )con-
struct brick-layer style sand tubes that surround the animal as it moves through
the sediment ( Hauck et al., 2009; Rhoads, 1967 ). Thin, sandy burrow linings are
employed by vertical deposit feeders, in both sand- and gravel-dominated sedi-
ments ( Fig. 2 D, E, and G; Bromley, 1990; Dashtgard et al., 2008; Rhoads, 1967;
Sch¨fer, 1972 ). Thick burrow linings, composed mainly of mud, are common for