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FOSSIL INVERTEBRATES/Bryozoans 317 Avicularian polymorphs are widespread among cheilostomes Stenolaemates Stenolaemates were the dominant bryozoans throughout the Palaeozoic, where they are represented by five orders – Trepostomata, Cystoporata, Cryptostomata, Fenestrata, and Cyclostomata – of which only the cyclostomes are extant The oldest stenolaemates are trepostomes reported from the Tremadoc of China By the Arenig, representatives of the other four orders had appeared, initiating a phase of exponential diversification that continued into the Caradoc The origin of the mineralized skeleton from a ctenostome-grade ancestor may have been the primary trigger for stenolaemate radiation, opening the way for the evolution of a much greater variety of colony forms A plateau of stenolaemate family diversity characterizes the remainder of the Palaeozoic, before the catastrophic decline in diversity marking the end-Permian mass extinction (Figure 6) Although only one order (Fenestrata) is thought to have become extinct at this time, stenolaemates were rare in the Triassic, and the Palaeozoic ‘holdover’ orders (trepostomes, cystoporates and cryptostomes) did not survive beyond this period Bryozoan faunas of Jurassic and Early Cretaceous age, before the rise of the cheilostomes, consist predominantly of cyclostomes Trepostomes Trepostomes (approximately 200 genera) are the ‘stony bryozoans’ that abound in Palaeozoic deposits such as the Upper Ordovician around Cincinnati in the USA A few trepostome species have sheet-like encrusting or frondose erect colonies, but most have dome-shaped colonies (Figure 2C) or, more commonly, robust tree-like colonies, which are typically broken into individual branches during fossilization Trepostome zooids are characteristically long and tubular, terminating in a polygonal aperture (Figure 5A) Many species have Figure Fossil stenolaemate bryozoans (scanning electron micrographs) (A) Unidentified trepostome with polygonal zooidal apertures surrounding a monticule (Ordovician, Utah, USA, magnification Â30) (B) Diamond shaped zooidal apertures with lunaria in the cystoporate Favositella (Silurian, England, magnification Â30) (C) Branch of the rhabdomesine cryptostome Rhabdomeson with styles protruding as blunt spines (Carboniferous, Scotland, magnification Â50) (D) Flattened frond of the ptilodictyine cryptostome Phaenopora showing autozooids arranged in well defined rows and some smaller polymorphs (Silurian, Sweden, magnification Â15) (E) Fragment of a colony of the fenestrate bryozoan Fenestella s.1 with branches bearing two rows of apertures, short dissepiments and fenestrules (Carboniferous, Scotland, magnification Â15) (F) Autozooids and a gonozooid (centre) in the cyclostome Hyporosopora (Jurassic, England, magnification Â25)
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