MICROCONCHID TUBEWORMS: A SHORT STORY ABOUT THEIR LONG EVOLUTIONARY HISTORY

Microconchids were sessile, suspension-feeding tentaculitoids, similar to the extant polychaete genus Spirorbis with respect to morphology and inferred mode of life. They appeared in the Late Ordovician and lasted until the end of the Middle Jurassic (Bathonian). Current data indicate that they initially occupied only normal marine environments. However, at the beginning of the Early Devonian, they began also to colonize brackish and freshwater habitats, where they often encrusted land plants. The reasons why they entered such habitats are currently not well understood, but it is probable that they benefited from the vast amount of suspended organic matter from the burgeoning terrestrial plant biomass. They thrived in these brackish and freshwater habitats until the Late Triassic.

Microconchids were able to form small bioherms in the Early Devonian, and from the Middle Devonian they became common constituents of stromatolite build-ups, often in hypersaline settings. Such microconchid-associated stromatolites are especially well-known from the Lower Carboniferous and Lower Permian, and microconchid build-ups continue to the Middle Triassic.

In open marine, marginal marine brackish and hypersaline settings, as well as in freshwater habitats, microconchids were characterized by encrusting, spirally coiled and/or helically uncoiled tubes, the latter morphology adapted to keep pace with rapid sedimentation. During the Early Permian a bioherm-forming clade of microconchids developed budding but this was apparently a short-lived evolutionary experiment.

Microconchids survived many major and several minor extinction events through their evolutionary history. They were opportunists that recovered rapidly and were present in great abundance, dominating the encrusting communities in the aftermath of at least some major extinctions. Their final disappearance in the latest Bathonian is an enigma as it did not coincide with any major extinction events but followed a period when they were rare in encrusting communities.