LOWER CAMBRIAN REEFAL CRYPTIC COMMUNITIES

Phanerozoic reefs were differentiated into distinctive open surface an d cryptic communities from their first appearance. During the Lower Ca mbrian, cryptic communities were surprisingly diverse with small, soli tary chambered archaeocyath sponges, calcified cyanobacteria and a mic roburrowing (?)metazoan being the most ubiquitous and abundant element s. Putative primitive cnidarians, spiculate sponges and various proble matica were also common crypt dwellers. Several species of archaeocyat h sponge, as well as cribricyaths, the calcified cyanobacteria Chabako via spp. and possibly boring sponges, were obligate cryptobionts. Lowe r Cambrian crypts offered a habitat of reduced environmental stress, a nd they housed a substantial proportion of the total biotic diversity of early reefs. Cryptic communities were composed of solitary, pioneer ing organisms and displayed no succession. Lower Cambrian crypts were small, short-lived structures compared with most modern reefal crypts, and were sites of extensive syn-sedimentary cementation supporting th e conjecture that crypts did not remain open for long before partial o r total occlusion. There is ample evidence, however, of a soft-bodied cryptos and of intense competition for space, as organisms commonly fo rm multiple overgrowths or chains of individuals. On a sub-zonal scale , the vast majority of archaeocyath species appear simultaneously in b oth open surface and cryptic niches, suggesting that Lower Cambrian cr ypts did not serve either as 'safe-havens' harbouring formerly open su rface inhabitants or as 'brood-pouches' of evolutionary innovation.