Behavioral plasticity of modern and Cenozoic burrowing thalassinidean shrimp

The use of trace fossils as paleoenvironmental indicators is based on empir ically-derived and tested links between environmental conditions, behavior, and trace fossil morphology. Four approaches were used to assess how faith fully and at what resolution trace fossils, as mirrors of behavior, reflect environmental change: (1) comparing the abundance and morphology of Ophiom orpha nodosa in tidal channel-margin and tidal channel-axis facies (Miocene , Delaware); (2) determining the range of morphology of O. nodosa produced under unchanging environmental conditions (within the channel-margin facies ); (3) evaluating the behavioral response of the modern burrowing thalassin idean, Neotrypaea californiensis, in Mugu Lagoon, California to an environm ental perturbation, namely the burial of layers of glass plates; and (4) as sessing how ancient producers of Ophiomorpha dealt with obstacles presented by dense shell and coral accumulations (Miocene, Maryland; Pleistocene, Ba hamas). Comparison of Ophiomorpha nodosa in channel-margin versus channel-a xis facies indicated that O. nodosa was significantly more abundant in the channel-margin facies. However, there was no significant difference in burr ow characteristics (exterior or interior diameter, wall thickness) between the facies. As recorded by O. nodosa, its thalassinidean producer did not m odify its behavior in response to conditions in the tidal channel axis. Rat her, the tracemakers tended to avoid the channel axis, as indicated by the reduced abundance of O. nodosa. In contrast, O. nodosa within the channel-m argin facies was highly variable in degree of pelletization of the burrow w all, in burrow fill and definition of the burrow margin, and in architectur e of the burrow system. Variation in O, nodosa found within the channel-mar gin facies reflected behavioral flexibility in the absence of environmental change. Modern burrowing shrimp adapt to barriers (layers of glass plates) implanted within the substrate. They alter the geometry and depth of their burrow systems; they may even share shafts that penetrate the barrier. Met ers thick Miocene shell beds (Maryland) in which the shells are not densely packed contain Ophiomorpha and Thalassinoides; the producing thalassinidea ns were able to penetrate and move through the shell bed. Similarly, Pleist ocene thalassinideans maneuvered around coral rubble in Bahamian fossil cor al reefs. However, Miocene decimeter-thick shell beds that are densely pack ed and well sorted are not penetrated by thalassinidean burrows, implying t hat thalassinidean behavioral flexibility was not sufficient to penetrate d ensely packed shell beds. Likewise, in the Bahamian reefal settings, Ophiom orpha producers formed extensive maze systems immediately above impenetrabl e lithified surfaces in the reefal sequence. Behavior of thalassinidean shr imp is neither tightly constrained nor highly programmed, and there is no i ndication that this has changed since Miocene time. Small changes in morpho logy of traces produced by thalassinideans cannot be used to identify subtl e changes in environmental conditions. Shrimp vary behavior apparently "whi msically", as well as in response to environmental change. If this is true of animals other than thalassinideans, the challenge to the ichnologist is to distinguish between "background" and "environmentally triggered" behavio ral variation as recorded in the trace fossil record. (C) 2001 Elsevier Sci ence Ltd All rights reserved.