Testing bioturbational ammensalism. A suite of qualitative and quantitative techniques for evaluating potential infaunal-epifaunal interactions and their application to the lower Devonian of New York State
Bioturbational Ammensalism (BAM) occurs when organisms burrowing in the sediment (bioturbators) disrupt primary sedimentary fabric and in the process have significant adverse effects on immobile, free-living, benthic organisms. The concept of BAM is very similar to the Bulldozer Hypothesis (Thayer, 1979; 1983) which suggested that this ammensal relationship resulted in a significant decrease in free living sessile epibenthic organisms and ultimately a “race” for hard substrates.
A suite of methods are developed and applied here in order to rigorously test Bioturbational Ammensalism. These methods include: (1) Ichnology, identification of the bioturbators, and their ammensal potential; (2) Faunal Evaluation, quantitative measurement of faunal elements, identification, and trophic group assignment; (3) Taphonomy, the condition and apparent history of the fossils identified; (4) Bed Thickness, number and thickness of distinct lithologic units; and (5) Percent Bioturbation, a quantitative measurement of the amount of bioturbation in hand specimens. The methods are applied to formations within the Lower Devonian Helderberg and Tristates Groups of eastern New York State, which represent Inner, Middle, and Outer Shelf environments. These formations were chosen because their paleoenvironments are well understood, they were deposited in a single basin of deposition, and the same paleoenvironments recurred after ∼7 million years.
Results show that the Inner Shelf environment had no trace fossils and thus no BAM could be detected. The Middle Shelf showed a slight increase in bedding thickness coupled with an overall statistically significant increase in bioturbation, and a significant decrease in faunal abundance. Similarly, overtime, the Outer Shelf showed an increase in bedding thickness, an increase in bioturbation, and a decrease in faunal abundance. However, changes in bed thickness and Percent Bioturbation were not significant between most Outer Shelf formations of different ages. These results suggest that BAM was occurring in the Middle Shelf environment and potentially was occurring in the Outer Shelf environment.