Lower Cambrian reefal cryptic communities

Phanerozoic reefs were differentiated into distinctive open surface and cryptic communities from their first appearance. During the Lower Cambrian, cryptic communities were surprisingly diverse with small, solitary chambered archaeocyath sponges, calcified cyanobacteria and a microburrowing (?)metazoan being the most ubiquitous and abundant elements. Putative primitive cnidarians, spiculate sponges and various problematic were also common crypt dwellers. Several species of archaeocyath sponge, as well as cribricyaths, the calcified cyanobacteria Chabakovia spp. and possibly boring sponges, were obligate cryptobionts. Lower Cambrian crypts offered a habitat of reduced environmental stress, and they housed a substantial proportion of the total biotic diversity of early reefs. Cryptic communities were composed of solitary, pioneering 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 the conjecture that crypts did not remain open for long before partial or total occlusion. There is ample evidence, however, of a soft-bodied cryptos and of intense competition for space, as organisms commonly form multiple overgrowths or chains of individuals. On a sub-zonal scale, the vast majority of archaeocyath species appear simultaneously in both open surface and cryptic niches, suggesting that Lower Cambrian crypts did not serve either as 'safe-havens' harbouring formerly open surface inhabitants or as 'brood-pouches' of evolutionary innovation.