Paleozoic brachiopod symbioses: testing the limits of modern analogues in paleoecology

Paleoecologic interpretations involving symbiotic interactions begin with a determination that the partners were simultaneously alive. Criteria for recognizing live-live interactions among benthic suspension feeders include: (1) preferred locations of symbionts on (encrusting) or in (boring or embedding) the host; (2) preferred orientations of elongate symbionts or clustering of symbiont apertures and feeding structures in relation to host-induced currents; and (3) growth abnormalities and/or distortions of the host, the symbiont, or both.

Interpretations of the nutritional aspects of ancient symbioses involve establishment of feeding locations and methods and the diets of Holocene descendants. However, use of living analogues and, hence, their value in paleoecology, commonly invoke the unwarranted assumption that the host-symbiont interaction and the foods available to the partners have not evolved in concert with their skeletons. These nutritional uncertainties are compounded further in symbioses involving fossils of unknown biological affinities or taxa that have evolved by different rules.

Analyses of Paleozoic brachiopod symbioses spanning the gamut from symbionts of undoubted ancestor-descendent relations (e.g., Spirorbis) to trace fossils (e.g., Trypanites) exemplify these nutritional uncertainties. Most of these symbionts suggest that in life, spiriferaceans were attached to the substrate by a thick, short pedicle and that the hinge line and/or umbo of the brachial valve acted as a fulcrum to elevate the commissure into the water column for efficient suspension feeding. Conversely, the same symbionts provide equivocal results when used to infer the circulation pattern of spiriferacean mantle cavity currents. These analyses also exemplify the great limitations of uniformitarian concepts in evolving biological systems.