Microconchids and the rise of modern encrusting communities

Abstract

Encrusting organisms are diverse and widespread in Recent marineenvironments. They are represented by various phyla that adhereto firm or hard substrata and range from tiny foraminiferans tolarge oysters. They may be represented by both solitary (e.g. poly-chaete worm tubes, bivalves, brachiopods) and colonial animals(e.g. corals, bryozoans). The ecological niches they occupy are alsodiverse; from tiny pebbles, through floating kelp, to coral reefs –where they inhabit both exposed and cryptic spaces. Since mostencrusters possess hard skeletons, they easily fossilize, leaving arich, albeit patchy, fossil record extending in time as early as theCambrian (Taylor & Wilson 2003). As with other invertebrate andvertebrate animals, encrusters also were a characteristic componentof each of the Sepkoski faunas (Sepkoski 1981), so generally speak-ing we can say that each era had its own characteristic encrustingcommunities.This paper concentrates on the question of when the modernencrusting communities originated. Looking at the fossil record, wefind many encrusting groups that look similar to their moderncounterparts. For example, calcareous tube-dwelling polychaetesevolved nearly at the beginning of the Mesozoic and are very wide-spread in today’s ecosystems, even sharing the same genera. We cansay the same about the cyclostome bryozoans, the post-Palaeozoiccrown-group of which originated in the Triassic and diversified inthe Jurassic. Thus, if we have a look at the particular encrustinggroup separately, we’ll see many modern counterparts in the fossilrecord. Therefore, if we look for the genuine modern encrustingassemblages we must focus at the community level as a whole.It is known from the literature (e.g. Palmer 1982; Taylor & Wil-son 2003) that the modern encrusting communities evolved duringthe Jurassic period. However, a closer look at Jurassic encrustingcommunities reveals that at the ordinal level the communitiesshared an old ‘Palaeozoic’ group that at the end of the MiddleJurassic vanished completely. This group were microconchids(Order Microconchida Weedon 1991).