Bryozoans in transition: The depauperate and patchy Jurassic biota

摘要

Bryozoans were profoundly affected by the end Permian and probably also by the end Triassic mass extinction events. Their recovery in terms of diversity and disparity during the Jurassic was slow and geographically patchy. Critical compilation of data from the published literature reveals only 172 valid species – 160 cyclostomes, 10 ctenostomes and 2 cheilostomes – some as yet not formally named. However, the rate of description of new Jurassic bryozoan species during the last two hundred years implies that many more species remain to be discovered and described. The Jurassic is the only geological period during which cyclostomes were the dominant order and is also important in having the oldest cheilostome bryozoan, the first representative of the order that dominates the modern bryozoan biota. Species diversity peaked in the Bathonian where local assemblages may contain up to 33 species. Encrusters account for 124 (73%) of the 172 Jurassic species, among which small sheet-like forms, especially those assigned to the form-genus 'Berenicea', are the most numerous. The overwhelming predominance in the Jurassic of ‘weeds’ with runner, ribbon or sheet colony-forms is striking. Unlike the Triassic and Cretaceous, large, tree-like or frondose erect colonies are uncommon, and species with fenestrate, articulated and free-living colonies are unknown. Of 92 geographically categorized entries for Jurassic bryozoans in the Zoological Record (1937–2003), 80 (87%) are European records. Outside Europe, diversity appears to have remained low throughout the Jurassic, and nonweedy species are particularly scarce. Compared with the Cretaceous, Jurassic bryozoans show no adaptations that can be interpreted as related to the increased predation pressure associated with the Mesozoic Marine Revolution. The Jurassic bryozoan biota shares characteristics with both the stenolaemate-dominated Palaeozoic–Triassic biota and the cheilostome-dominated Cretaceous–Recent biotas.