PI<XItPRILL AND BREN'CIILKY: ORDOVICIAN MARINE COI«IMUNITIES 2SS

The degree to which communities intergrade appears to be related to the environ- raental gradients within a region. %here, RS in NOrth %ates, the basin succession is thick and environmental gradients are tow, both laterally and vertimlty, there is considerable illtergradatIGQ betweeB ecommututles, and 'Buxed Rskealbtages Rre found. The variable composition of these assemblages implies R high rate of local immigra- tlOB Rnd eXtlnctIOB, Rnd cOnsellueatty 8: high eqtuhbrlum Bunlber for eRCh coInmuiuty (e;g. Bretsky and Bretsky 1.976) IB contrast, where 8 shelf succession is relatively thin, aad. lateral and vertical environmental gradients are sharp (e,g. east Shropshire) the commumties are more clearly partitioned into distinct hthotopes, aad the com- positionk of the communities are less variable. Such cOmmunities lead tO show 8 lovver equilibriuxn number Rnd therefore resemble those.snlatt, ikohted geographicat areas such as oceamc islands (MacArthur 1972). Clearly a recognition of the local tectonic and sedimeatotogicat framework in which particular communities are found is essen- tial before more geaerahzed models of communities ean be framed.

Communities show many temporal changes in their composition as a result Cf Speeiek evOlution, the establishment of' new niCheS within the cOmmunity, the eXtlaetlon Of sOQXe Speelek, RBd the iItunlgratlon Of etelnents frOB1 Othel' '8leas. changes of these kiack are found iII the communities of the soudleyan — Loag- viltian of' the Anglo-%'etkh area. For example, IIowellires Rnd solverby+lla are represented by tive stratigraphically arranged species (VAt tiams 1963), Reusdlella by two species Rnd 8 sub-species, Rad Balelarleikr by four species Mast of these aew species appear fo have arisen Iadlgeac<ukly wlthm 'the Anglo-%elsh area< Rnd Iadeecl most of the Soudleyan aad LongvHtian brachiopod. genera are either represented by earlier species within the area or appear for the 6rst time within this region, IB contrast, the appearance of some tritobites, notably Chaswops and EsIBIIiops, apparently results from immigration from the Baltic region (Dean I960a)

sonic of the Bud&aradoc CGIQIQUaltlek, RS defmed Rt the geaerIC level, Occur Blore widely within the Ordovician, The LIiIIOrllliv Rnd .kfaeroeoelia kub-COmmunitieS Rre well established by costonlaa tlrnes uI south ghropkhIfe and existed Until Rt least fbe top Soudteyan or lower LongmHian in %ales, Assemblages which are possibly related to the Dinoriltis community, with Horderlelvl/rI Rnd Rapwesqulna, together with coarsely ribbed orthids, such Rs Hesperorlhis Rad OrriIis, are fonnd as early as the upper Llanvirn of the I.taadeilo Distric (Williams 1953), but these early assemblages also include Da!mQIIella and Sov>erbyella as major etetaents, DalrIIllrIellu is again. RSSOCiated with l1ahnesqulna aud. 3&craeoelia in the upper Llandeilo Calcareous Ashes of the Herwya HiBs (MacGregor 1961), which suggests that the Dairnanellu Community might QOt have beCOme Clearly differentiated frOm the DirIBrtiIIS Carn- munity until the Caradoc. Faunas from the Spy %God Grit (Costonian) of' west ShropkhIre whIch contRIB DalrrIrIIIe/kt, BlcIIsplrIQ, KJPerIIIiI, and Poslrt'cellulQ pA'ittialas 1974) Rre mare renliniscent of' 8 mid-Caradoc Drllm<srIella community. Assoclatlons apparently SBBItar to the BQANcMzlkf coxnxnunlty Rre also present In the Marshbrookiaa of Shropshire (Dean 195g),

%e do Bot find records of low diversity assemblages doIxunated by Howellires and SOIVerbyella Outkide the SOudleyan-LOngVitliaa IB ContraSt, the ¹CIIkllXI COm- mu Bity appears to have a range extending from the Cost Gniaa at least mto the Akhgill where in zones 1-3 (Cautleyan) of the Cauttey area, Ingham (1966) records faunas

I'ALAEONTO'LOG«Y, VOLUME 22

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inctudillg.<vs.oiefhl > Dr<lerorlllfs, 6/vpto rills, PlntystAPpllfQ, SUP/pa> and Skenl'<h'Oi kr. Similar fauBal assocIattonk, with the addition of ChrlSAPIIlQ-,— ale kBown elsewhele ln the Ashgill, e g from Aank beds of the Boda Limestone of Sweden (persollat observa- tion), frOm. Belgium. (Sheehaa .1975), tile POrirane.Limestcae Of east Irelsnd (Wright 1964), aad from the Dr~uck Group Gf Girvan (Lamoat 1.935)., It is possible that this OffMore .OrdovIclan cOxnxnulutV subsequently developed IBto the Oleo<el(3'sIII— SAemdioides community of the Lower and Middle Ltaadcvery (Boucot 1975) witll the sddltIGQ. Gf aew Slhlrtan eleIaeatk.

Attempts hRVe been Iaade to generRhze LGWer PataeOzotc communities into 8 few major depth related types (Bxetsky 1969a; Anderson 1971), aad ia these. broad terms the North %ates.communities could atl be referred to as orthid-stmphomentd- tritobite communities; More recently, Boucot (1975) has advocated the use of benthic

mbtages w<hieh cOIaprise 8 gI.Oup Of cOmmunltlek that OCCUr repeatedlv ia different parts Gf 8 region ia the SRIBe position relative to 8 shorehne. Fottowiag thi's scheme the caradhc communities could etapirtmlty be allocated to the Benthic Assemblages of Boucot, Rs Illustrated ln cotulan A, below,

Benthic Assemblage 1 is ttyicalty represented elsewhere by 8 linguloid-bivalve dolmnated commumty which we haVe Bot observed ..<n the South Berwyns. The hve. communities which we have discusmd. have been assigned to benthIc assemblages according to their relative depth