lilies and an increase in more complex mobile species such as snails, sea urchins and crabs. Before the Permian mass extinction event, both complex and simple marine ecosystems were equally common. After the recovery from the mass extinction, the complex communities outnumbered the simple communities by nearly three to one, and the increase in predation pressure led to the Mesozoic Marine Revolution. Bivalves were fairly rare before the P–Tr extinction but became numerous and diverse in the Triassic, taking over niches that were filled primarily by brachiopods before the mass extinction event, and one group, the rudist clams, became the Mesozoic's main reef-builders. Some researchers think the change was attributable not only to the end-Permian extinction but also the ecological restructuring that began as a result of the Capitanian extinction. Linguliform brachiopods were commonplace immediately after the extinction event, their abundance having been essentially unaffected by the crisis. Adaptations for oxygen-poor and warm environments, such as increased lophophoral cavity surface, shell width/length ratio, and shell miniaturisation, are observed in post-extinction linguliforms. Crinoids ("sea lilies") suffered a selective extinction, resulting in a decrease in the variety of their forms. Their ensuing adaptive radiation was brisk, and resulted in forms possessing flexible arms becoming widespread; motility, predominantly a response to predation pressure, also became far more prevalent. Microbial-metazoan reefs dominated surviving communities in the immediate wake of the mass extinction. Metazoan-built reefs reemerged during the Olenekian, mainly being composed of sponge biostrome and bivalve builups. "Tubiphytes"-dominated reefs appeared at the end of the Olenekian, representing the earliest platform-margin reefs of the Triassic, though they did not become abundant until the late Anisian, when reefs' species richness increased. The first scleractinian corals appear in the late Anisian as well, although they would not become the dominant reef builders until the end of the Triassic period. Metazoan reefs became common again during the Anisian because the oceans cooled down then from their overheated state during the Early Triassic. Ichnocoenoses show that marine ecosystems recovered to pre-extinction levels of ecological complexity by the late Olenekian. Anisian ichnocoenoses show slightly lower diversity than Spathian ichnocoenoses, although this was likely a taphonomic consequence of increased and deeper bioturbation erasing evidence of shallower bioturbation. Ichnological evidence suggests that recovery and recolonisation of marine environments may have taken place by way of outward dispersal from refugia that suffered relatively mild perturbations and whose local biotas were less strongly affected