Behavioral plasticity of modern and Cenozoic burrowing thalassinidean shrimp

Abstract

The use of trace fossils as paleoenvironmental indicators is based on empirically-derived and tested links between environmental conditions, behavior, and trace fossil morphology. Four approaches were used to assess how faithfully and at what resolution trace fossils, as mirrors of behavior, reflect environmental change: (1) comparing the abundance and morphology of Ophiomorpha 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 thalassinidean, Neotrypaea californiensis, in Mugu Lagoon, California to an environmental perturbation, namely the burial of layers of glass plates; and (4) assessing how ancient producers of Ophiomorpha dealt with obstacles presented by dense shell and coral accumulations (Miocene, Maryland; Pleistocene, Bahamas). Comparison of Ophiomorpha nodosa in channel-margin versus channel-axis facies indicated that O. nodosa was significantly more abundant in the channel-margin facies. However, there was no significant difference in burrow characteristics (exterior or interior diameter, wall thickness) between the facies. As recorded by O. nodosa, its thalassinidean producer did not modify its behavior in response to conditions in the tidal channel axis. Rather, the tracemakers tended to avoid the channel axis, as indicated by the reduced abundance of O. nodosa. In contrast, O. nodosa within the channel-margin facies was highly variable in degree of pelletization of the burrow wall, in burrow fill and definition of the burrow margin, and in architecture of the burrow system. Variation in O. nodosa found within the channel-margin 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. Meters thick Miocene shell beds (Maryland) in which the shells are not densely packed contain Ophiomorpha and Thalassinoides; the producing thalassinideans were able to penetrate and move through the shell bed. Similarly, Pleistocene thalassinideans maneuvered around coral rubble in Bahamian fossil coral reefs. However, Miocene decimeter-thick shell beds that are densely packed and well sorted are not penetrated by thalassinidean burrows, implying that thalassinidean behavioral flexibility was not sufficient to penetrate densely packed shell beds. Likewise, in the Bahamian reefal settings, Ophiomorpha producers formed extensive maze systems immediately above impenetrable lithified surfaces in the reefal sequence. Behavior of thalassinidean shrimp is neither tightly constrained nor highly programmed, and there is no indication that this has changed since Miocene time. Small changes in morphology of traces produced by thalassinideans cannot be used to identify subtle changes in environmental conditions. Shrimp vary behavior apparently “whimsically”, 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” behavioral variation as recorded in the trace fossil record.