Marine Invertebrate Neoichnology - Trace Fossils as Indicators of Sedimentary Environments - page 278

2.5 Sponges and Sea Anemones
In soft sediments, sponge traces are mostly limited to cup-shaped resting marks
that are best described as Bergaueria -like traces. These structures range in size
from centimeter- to decimeter-scale and generally do not exceed a few centime-
ters in depth. Sponges may also be responsible for generating the deep-water
graphoglyptid Paleodictyon . Paleodictyon was previously attributed to the
activity of worms ( Ekdale, 1980; Seilacher, 1977 ) or xenophyophores
( Swinbanks, 1982 ); however, computer modeling and analysis of modern
Paleodictyon suggest that this trace may, in fact, be a body fossil of a hexacti-
nellid sponge adapted to living in soft sediments ( Rona and Merrill, 1978; Rona
et al., 2009 ).
Some sea anemones of the order Ceriantharia can produce large, penetrative
biogenic structures ( Bromley, 1996; Gingras et al., 2008a; Ricketts et al., 1985;
Sch ¨ fer, 1972 ). The burrowing anemone, Cerianthus , for example, occurs on
both the Pacific and Atlantic coasts of North America. The animal may reach
70 cm in length and burrows deeply below the sediment/water interface
( Hargitt, 1907 ). The animal survives by extending its tentacles and mouth above
the sediment/water interface to feed and retracts its body to nearly the full depth
of the burrow to take shelter. The iterative probing up through the sediment by
burrowing anemones produces a collapsed, chevron-shaped trace fossil that is
conical in cross-section (e.g., Pachycerianthus fimbriatus , Fig. 6 ). Cerianthus is
also very capable of reestablishing its position with respect to the sediment/
water interface, such that its dwelling trace can show notable aggradation in
response to sedimentation events ( Shinn, 1968 ). Urticina coriacea is another
burrowing anemone common on the west coast of North America and Europe
( Lamb and Hanby, 2005 ) in permanently subtidal (to 50 m deep) and euhaline
waters. It is generally smaller than Cerianthus , up to 10 cm in diameter and
15 cm long, and lives in coarse sand and gravel ( Fig. 6 ; McDonald, 2010 ).
Unlike Cerianthus ,however, U . coriacea attaches itself to gravel below the sedi-
ment surface, likely resulting in limited vertical shifting of its burrow ( Fig. 6 ).
3. ENVIRONMENTAL STRESSES AND THEIR IMPACT
OF NEOICHNOLOGY
Studies in neoichnology aid in establishing the association between environ-
mental stresses and the resultant bioturbate texture. Indeed, neoichnological
studies led to the recognition of ichnological models for oxygenation (e.g.,
Rhoads and Morse, 1971 )andforsalinity(e.g., Howard and Frey, 1975 ). The
interpretation of archetypal ichnofacies, too, was profoundly influenced by the
recognition that ichnofacies are recurrent in modern sedimentary environments
(e.g., Ekdale, 1980; Frey and Dashtgard, 2011; Fu and Werner, 2000; Pemberton
and Frey, 1985; Rona et al., 2009; Wetzel and Werner, 1981 ). This recurrence
enabled parameterization of associated environmental characteristics.