Marine Invertebrate Neoichnology - Trace Fossils as Indicators of Sedimentary Environments - page 271

position and likely erosion. Deep-burrowing bivalve structures are mainly
generated by filter feeders, where the burrows—collectively referred to as
Siphonichnus ( Stanistreet et al., 1980 )—comprise two parts: a lower disruption
associated with the bivalve and an upper elongate, vertical disruption made by
the bivalve's siphon.
The lower part of Siphonichnus includes the shell cavity and any retrusive
spreite below the shell (e.g., Fig. 3 ; Reineck and Singh, 1980; Sch¨fer, 1972 ).
The type of bivalve and its size, the rate and duration of sediment accumulation
on a substrate, and the type of substrate burrowed into contribute to the substantial
variability observed in the burrows produced. For most bivalves burrowing in soft
substrates, the animals vertically readjust their burrow in response to sediment
deposition and erosion. The ability to readjust within the sediment tends to
decrease for deep-burrowing large bivalves, such as Tresus nuttalli ( Fig. 4 A)
and Mya arenaria ( Figs. 3 and 4 B-D) that cannot substantially readjust their bur-
row once they reach adult size. The resultant structure comprises a mold or empty
cavity of the bivalve with minimal sediment disruption above or below the shell
( Fig. 4 A). If there is no sediment disruption around the bivalve, the basal resting
trace is called Lockeia ( Fig. 4 B). Small- and mid-sized bivalves, such as Macoma
sp. ( Fig. 3 )and Nuttallia obscurata ( Fig. 4 E), are capable of moving short vertical
and horizontal distances within the sediment (up to 40 cm horizontally in the case
of Macoma spp.; Powilleit et al., 2009 ). In stiffgrounds, firmgrounds, and hard-
grounds, pholadid bivalves produce clavate structures that widen with depth due
to the growth of the animal ( Fig. 4 F). These burrows are permanent and cannot be
readjusted upward. Consequently, burrowing in these substrates only occurs in
settings characterized by very low sedimentation rates.
The upper part of bivalve-generated burrows includes the siphon trace (one
or two siphons) that extends from the body cavity to the sediment/water inter-
face, and any protrusive spreiten above the shell. For deep-burrowing bivalves,
the siphon trace (or traces) is (are) relatively large and straight. Adjacent to the
siphon trace of large- and mid-sized, deep-burrowing bivalves, it is common to
observe downward-deflected sediment lamination (e.g., M . arenaria ; Figs. 3
and 4 D). These laminae record the position of the funnel that developed around
the siphon hole at the sediment/water interface ( Fig. 4 C) or form as sediment
caves into the siphon cavity as the animal makes small adjustments to its posi-
tion in the subsurface. Small bivalves also have siphons, although the siphon
traces are temporary structures and are unlikely to produce downward deflect-
ing lamination. For small- and mid-sized bivalves ( Fig. 4 E), it is also possible to
produce convex spreiten that are cross-cut by the siphon trace. These spreiten
record the downward movement of the bivalve to maintain its position relative
to the sediment/water interface ( Stanistreet et al., 1980 ).
Biogenic structures produced by the lateral movement of shallowly burrow-
ing bivalves and gastropods are also common (e.g., Mercenaria mercenaria ,
Fig. 3 ). The horizontal movement of bivalves is recorded either as a furrow,
as the bivalve uses its foot to pull itself through the sediment, or as a series