Marine Invertebrate Neoichnology - Trace Fossils as Indicators of Sedimentary Environments - page 276

employed in settings where high population densities restrict the lateral extent
of individual burrows.
Other burrowing decapods include crabs and lobsters ( Fig. 3 ). Modern crab
burrows have many characteristics that generally are not shared by other
crustaceans. These include comparatively simple architectures (U-, Y-, and
J-shaped), open apertures and irregular ovoid cross-sections ( Figs. 3 and 5 ).
Crab burrows are normally unlined ( Atkinson, 1974; Curran and White,
1991; Farrow, 1971 ) and are best described as Psilonichnus ( Frey and
Pemberton, 1987; Gingras et al., 2000a; Pemberton et al., 2001 ). Moreover, crab
burrows are domiciles, as opposed to combined feeding structures, and lack
complex basal geometries. The architectures observed in crab traces are similar
to those of lobster traces, although lobster burrows normally display more
shallow, subapertural branching ( Fig. 5 B; Farmer, 1974; Rice and Chapman,
1971 ) and possess more irregular burrow diameters ( Fig. 3 ). No widely used
ichnogenus resembles the irregular burrows constructed by lobsters; however,
these traces are most similar to Psilonichnus and Thalassinoides .
2.4 Echinoderms
Mobile echinoderms include starfish, brittle stars, irregular and regular echi-
noids, and sea cucumbers. All these animals are known to leave distinctive
traces in modern sediments, although only spatangoid (irregular) echinoids
and some groups of sea cucumbers burrow deeply into soft substrates ( Fig. 6 ).
Spatangoid echinoids burrow in both sandy and muddy substrates. These
animals move through the sediment by passing grains around their bodies using
their tube feet ( Bromley, 1990; Bromley and Asgaard, 1975; Kanazawa, 1992;
Nichols, 1959 ). In muddy sediments, spatangoid echinoids burrow only a few
centimeters below the surface. In sand, they burrow deeper, up to 20 cm in the
case of Echinocardium cordatum ( Fig. 6 ). The back-filled echinoid tunnels can
be broadly classified as Scolicia or Beaconites ( Fu and Werner, 2000 ), although
Scolicia should show a preserved axial canal near the center of the trace fossil
( Plaziat and Mahmoudi, 1988; Smith and Crimes, 1983 ). Interfacial preserva-
tion of Scolicia leads to the production of Taphrhelminthopsis , which has
recently been observed on submarine videos of the Fraser Delta front, British
Columbia, Canada. Sand dollars are also known to tilt into the sediment
(in the same manner as brittle stars), such that part of their body is buried
and part extends above the sediment/water interface ( O'Neill, 1978 ). This
behavior is likely to produce small chevron-shaped disruptions that reflect
the orientation of the brittle stars and sand dollars.
Sea cucumbers probably produce a broader range of trace fossils than are
presently ascribed to them in the ichnological literature. They are observed
to make crude, sinuous trails at the sediment/water interface and are known
tomake large, if crude, bow-shaped Arenicolites and Diplocraterion in intertidal
settings ( Howard, 1968; Pearse, 1908 ). In deeper water settings, sea cucumbers