Invasion of freshwater and variable marginal marine habitats by microconchid tubeworms – an evolutionary perspective

Abstract

Microconchids are an extinct group of Spirorbis-like tentaculitoid tubeworms that dwelled in a variety of aquatic environments, ranging from normal marine, through brackish and hypersaline, to freshwater. An analysis of published microconchid occurrences focusing on their ecology and palaeoenvironmental distribution through geological time is conducted in order to establish the timing of microconchid colonization of freshwater and marginal marine habitats. Microconchids originated during the Late Ordovician in shallow shelf, normal marine environments where they thrived until their extinction at the end of the Middle Jurassic (latest Bathonian). Microconchid colonization of marginal marine brackish habitats seems to have started already by the Early Silurian (Wenlock). The freshwater habitats were invaded by microconchids in the Early Devonian, nearly simultaneously in several regions (Germany, Spitsbergen, USA). Since shallow marginal marine and freshwater habitats are more unstable, especially in terms of temperature and salinity fluctuations, as well as prone to desiccation, than normal marine, shelf environments, the drivers of the colonization of these habitats by microconchids are currently incompletely understood. We hypothesize that by colonizing such environments, microconchids gained access to abundant food resources in the form of suspended organic matter delivered from the land by rivers and streams. These, combined with their biology, enabled microconchids to reproduce fast and in large numbers. Microconchids are considered to have gone extinct by the end of the Middle Jurassic (Late Bathonian). Their youngest occurrence in freshwater environments is known from the Late Triassic and it is currently not known whether microconchids continued to occupy such habitats later on in the Jurassic. All the Middle Jurassic records of microconchids come from marine settings. Thus, more focused research on Jurassic brackish and freshwater deposits worldwide is needed to check whether they may have thrived in such environments at some locations, until their hypothesized extinction.