Organism-sediment Relationships I N S I L U R I a N M a R I N E Environments

Organisms pIayed major roles as producers and modifiers of sediments in many Silurian marine env~ronments. Pelrnarozoan skeietal debris famed extensive shoals and banks in many high-enerw, tropical environments : algae. bvozoans. tabulate corals. and strematoporoids were the major skeieral contributors to reeial buridups common in mid and late SiIunan subtropical shelves. ArtlcuIare brachiopods and Lesser numben oiosrracodes, trilobites, and molluscs formed extensive. although reIatively !Em (5-15 mi. shell beds in shaIloiv shelf fac~es: pefagc. cephalopod limestones characrerized the circum-Gondwana region. Skeletd hard substrates and hardgrounds supported relarively complex eprfaunal suspenaan-feeding communities. The apparent scarclry of encollthic sponges and bivalves indicates thac rates of hard subsrrare bioerosron were far below those of post-Palaeozolc times. Substra~e selection. spatial compeotion. anc porarlry of c:Tpnc versus exposed subcommun~ties are clearly evidenr. for the first rzme. in Ordovician to latesr Siiurian hard substrate cornrnuniries. Unconsoi~dated sediments displayed a wide variety of characrerisrtc trace and body fossd assoclatlons that were. to some degree. subsrrare conrrolled. Sand and silt subsrrates mpporred a low die'ersltv oi shelly organisms bur preserved abundant rraccs, nngmg from fkoiirhas ana .-~rthropir~cus-aomina~ed nensshore sed~ments ro offshore diverse ichnolicles. Some Silur~an nearshore sediments display b~orurbat~on up to Im deep and contaln very Fen epifaunal organisms. susgesting destabiiizat~on of substrares. However. b1ogentc disturbance of offsfshore, soft substrates rematned relat~vely resrricted In the Sl lur~an wnh tnfauna extenarng onlv abour 1C-15 cm tnro the sediment. Silunan epthunai organlrrns dispIayea varled morpnolo_eical ad~pta t ions rs soft substrates. inciua~ng 'snowshoe' and 'iceberg' strategies. as %ell as attacnmenc to other or,nanrsms. Taohonomic feedback. med~ated by storm depos~tionaf processes, led to variably cornpiex shelly communltles rn oiTsfshore. levei-bottom areas. Low. inrermedi~te. and h~gh-level suspension feeding r:ers exlstcd. w i rh some pelmarozoans artaimng levels of over Im above the substrate. In shallow carbonate shelves. sediment baffiing and brnaing were induced by thickets of pelmarozoans. ramose bryozoans. snd corals. S~rornarolrtes were confined largely to hypersaline or low-ouygen mner shelrenvironments. presumably due ro d ~ s r u p u o n of algal mats by grazers In normal manne settings. However. many ofshore bioherms are partrally thrornbolrrlc. Hencc, algal bindlnp iacll~tated rhe developmenr of coral-strornatoporotd brohems in many hlgh energy shelf seltlngs. B~oherms, In turn. provided niches lor numerous secondap inhabitants. Subsrnte appears to have been a cntical determinant oilocai patch communtty development wthirr generalized Silunan habltars. However, at the broader level of b~ofacres. bathyrnetric and sedimentolog~cal (deposrwn rates. turb~diry) features plaked more domlnanr roles in conrrolting distnbur~on pattens. O N E of the most exciting and challenging areas of pa1aeoecoIogy is the elucidation of interactions between physical environments and modes of life in ancient organisms. The selarionship of organ~sms and sediments is a complex one involving numerous feedback !oops. On rhe one hand. orpnisms are significant producers and modifiers of sediments. and on he other hand. sedimenrs compslse the substrares upon and within which benthic organisms dwell. In some Silurian tnvlronments, parrrcuIarIy carbonate platforms. organisms were the pnrnary producers of sediments. both as bioioeically standardized sedimentary particles and as organic buildups such as reefs. brostrornes. and shell beds. Marine animals aIso played a criticai role in modifying sediments by producrng faecal pellets. aeglutinaung fine-gralned sediments into silt and fine sand-size parnctes. and by the sortrng and churning action associated w~rh bioturbarion. Cyanobactena. algae. and certain an~mals (corals. sponges. biyazoans. pelmatozoans) baffled or bound fine-grained sediments In place. greatiy changlng the local properties of the sea floor. Other organisms produced fine-grained sediments out of larger grains by their bioerosive functions. (Special Papers in Pzlnmntohgy No. U 11991). pp. 301-Y( E The Palwontdgeai Associatrm