Bryozoans from the Llanbedrog Mudstones (Caradoc), north Wales

16 Pages·1991·8.5 MB·English

by  C J Buttler

Bull. Br. Mus. not. Hist(Geol.)47(2): 153-168 Issued29November 1991
Bryozoans from the Llanbedrog Mudstones
(Caradoc), north Wales.
CAROLINE BUTTLER
J.
DepartmentofGeology, NationalMuseum ofWales, Cathays Park, Cardiff, CF1 3NP.
CONTENTS
Introduction 153
Material 153
SystematicPalaeontology 155
Stigmatellaoakleyisp.nov 155
Pedrogoporataylorigen.etsp.nov 157
DiplotrypapseudopetropolitanaAstrova 159
Diplotrypanontabulatasp.nov 159
Halloporacf. tolliBassler 160
Eridotrypacf.kilbartiensisPushkin 162
PrasoporagrayaeNicholson&Etheridge 163
PrasoporathoraliPrantl 163
Mesotrypasp 165
CeramoporelladistinctaUlrich 165
KukersellaborealisBassler 167
Acknowledgements 167
References 167
SYNOPSIS. A diverse bryozoan fauna is described from the Llanbedrog Mudstones (Soudleyan, Caradoc), near
Llanbedrog, Gwynedd, north Wales. The fauna is dominated by trepostomes (nine species), although one
cystoporate and one cyclostome species are present. Three new trepostome species, Stigmatella oakleyi, Pedrogopora
taylori, and Diplotrypanontabulataaredescribed; Pedrogoporaisanewgenus.
AlectotypeforPrasoporagrayaeNicholson& Etheridge 1877isselected.
Gwynedd, north Wales (National Grid Reference SH 322314).
INTRODUCTION Fossils are unevenly distributed throughout the rock, a
calcareous mudstone. Bands ofdecalcified bryozoans can be
identified on the surface ofthe outcrop. When these regions
Bryozoans are one of the major components of Ordovician wereexcavatedcalcifiedcolonieswere revealed. Brachiopods
faunas. Whilst they are well known from North America and (e.g. Leptaena sp.) and trilobites (e.g. Calymene sp. and
:he Soviet Union, their study has been largely neglected in Illaenus sp.) were also found at this locality.
Britain. This neglect may be partly attributed to poorpreser- Matley (1938) described the geology of the area around
vation andthetimetakentopreparespecimens, andpartlyto Pwllheli, LlanbedrogandMadryn. Thisincludesadescription
he lack of any strong tradition of bryozoan research in and faunal list of the Llanbedrog Mudstones. Recorded on
Britain. Bryozoan specimens from the Ordovician of Britain the list are 'cf. Monticulipora lens auct., Ptilodyctyal and
ire commonly present in museum collections (e.g. Natural indeterminate polyzoa'; no illustrations are included.
-listory Museum, Sedgwick Museum and British Geological The material examined herecomprisesspecimenscollected
>urvey), but they are usually decalcified moulds. This makes recentlyinthefield, andaNatural HistoryMuseumcollection
hem difficult to identify even to family level. Calcified fromthesame localitymadebythe late DrK. P. Oakley. The
pecimensfrequentlygounnoticedinthefieldexceptwhere a Oakley collection includes thin sections prepared from over
arge proportion ofthe rock consistsofbryozoans. thirty specimens.
The bryozoan fauna is diverse, consisting of 11 species,
three of which are new. Trepostomes dominate with nine
species, but one cystoporate and one cyclostome species are
MATERIAL also present. The cyclostome species is very small and quite
abundant but has only been recognized in randomly orien-
tatedpeels. Onlyabriefdescription ofthisspeciesisincluded
he Llanbedrog Mudstones (Soudleyan, Caradoc) crop out herein (p. 167) because it has recently been described in full
m the western side of the A449 road south of Llanbedrog, elsewhere (Buttler 1989).
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BRYOZOANSFROMLLANBEDROGMUDSTONES 155
A total of nine genera have been described from the Paratypes. BMNH PD 2639, 2655, 2658, 2664, 2664a, b,
CaradocLlanbedrogMudstones. Onlyoneofthese, Pedrogo- 2665-74, 2685-6, 2688, 2692-3, 2695, 2701, 2716, 2718, 8283-
pora gen. nov. (p. 157), is endemic to north Wales; all the 96; all from the same horizon and locality asthe holotype.
rest are cosmopolitan, known from North America and
Baltoscandia. Awidegenericdispersal mayhave beencaused Name. Forthe late Dr K. P. Oakley, formerlyofthe Natural
byalong-livedplanktotrophiclarval phasewhichencouraged History Museum, who first collected and sectioned
dispersal. Taylor & Cope (1987) consider that early steno- trepostome bryozoans from the LlanbedrogMudstones.
laematesmayhaveinheritedthisphasefromtheirctenostome Diagnosis. Colony large, ramose. Autozooecia have very
ancestors. thin, wavywalls in endozone, are parallel to branch axis, and
Four of the total 11 species have not been previously then curve out abruptly to zoarial surface. Zooecial walls
described outside north Wales. Three of these species are thicken irregularly in exozone. Autozooecia polygonal in
new and one has been left in open nomenclature. It is transverse section, rounded to petaloid in shallow tangential
impossible to tell if this apparent endemism reflects the true section. Polygonal-rounded mesozooecia common, originat-
condition, or is the result of selective preservation and/or ing in the outer endozone and inner exozone. Diaphragms
sampling. present in exozonal autozooecia, common in mesozooecia.
Three species, Hallopora cf. tolli, Ceramoporella distincta Acanthostyles abundant in exozone.
and Kukersella borealis have very broad biogeographical
ranges. They are known from both North America and Description. Zoariaerectwithcylindrical branches,on aver-
Baltoscandia. The majorityoftheBritish Isleswasseparated, age 8-26 mm in diameter. The surfaces of all specimens are
duringtheLowerOrdovician, fromNorth America(Laurentia) abraded. Autozooecia meander but generally are parallel to
by the Proto-Atlantic Iapetus Ocean. Pickering etal. (1988), the branch axis at the centre of the colony. They curve
usingavarietyofpalaeontological, stratigraphical, structural, outwardsabruptlyintheouterendozoneandmeetthezoarial
geophysical and igneous evidence considered that by the end surface at 90°. Autozooecia within the endozone have very
of the Ordovician the Iapetus Ocean was partially closed, thin walls.
with only narrow marine seaways persisting to the Middle The exozone is usually thick, with an average diameter of
Silurian. During the Caradoc the Iapetus did not form an 1-79 mm. It is recognized by simultaneous irregular thicken-
impenetrablebarriertobryozoandispersal. Prasoporagrayae ingofthe zooecial walls andachangeinzooecialorientation.
is also known from both sides of the Iapetus, because Transverse sections of autozooecia are polygonal in the
Scotlandwas located on the Laurentian margin. endozone, becoming rounded to petaloid in the exozone, as
seen in shallow tangential sections. Autozooecial diameters
average0-20mm x 0-29mm in theexozone. Orally-deflected
SYSTEMATIC PALAEONTOLOGY basal diaphragms are rare to absent in the endozone but
presentwithintheautozooeciaintheexozone,wheretheyare
spacedon average 0T5 mm apart.
The terminology in all descriptions isthat ofBoardman etal. Mesozoecia are common and originate in the outer endo-
(1983). Trepostomata genera are placed in families based on zone and exozone. They are polygonal-rounded in shallow
Astrova (1978) and Cystoporata genera on Utgaard (in tangential sections and have a maximum diameter which
uBnosaartidsmfaacntoertyal.in19P8a3l).aeFoazmoiiclytlreevpelosctloasmseisficaantidoniissgceunrerreanltllyy cavoemrmagoens 0inT2themmm.esoOzroaolelyc-idae,flsecptaecdedbaosnalavdeiraapghera0gmTs marme
being revised for the Treatise on Invertebrate Paleontology apart. In the outer endozone/innerexozone the mesozooecia
byR. S. Boardman. are thinner and constricted at the positions ofthe diaphragms,
Three taxa could not be conclusively identified to specific producing a beaded appearance.
l'sepv.e'l,.bIansetdheosnetchaesersectohemmsepnedcaietsioanrseorfefBeernrgesdttoona(s19'8c8f.)'.and diaAmceatnetrhosotfyl0e0s4armemshoarntdadnednsaibtuyndoafnt2,1 wpietrh manm2a.veTrhaegye
Biometric details for all trepostome species are tabulated originate throughout the exozone and frequently indent the
(sTpaebclieme1n).. ETahcehmmeeaasnureamnedntrwanagsemaadreeucpaltcoulsaetveedntfiomreseapcehr zAooehcyiaallinaepecrotrueresi,s psruordruocuinndgeda pbryonsotueenpcleyddpieptpailnogidcsohnaipcea.l
ppaubrlaimsehteedr.PhR.Da.wthdeastisa(cBuatntlbere1f98o8u)n.dAlilnstpheecimaeutnhsora'rse tuhni-n lamAiuntaoez.ooecial wall thickness averages 006 mm in the
sectionsoracetate peels. exozone. Wall microstructure is composed of inclined U-
shaped laminae. Zooecial wall boundaries consist of wide
PhyluSmTBERNYOOLZAOEAMAEhTrAenberg, 1831 driesgirounpsteodfbgyranlaurlgaer acaclacnittheo.stTyhlees.micSroomsteruzcotouerceiai,s fersepqeuceinatlllyy
SubOoCrlrdadseesrrTHRAELPLOOSPTOROOMIADTEAAUBlArosirtcgrh,o,v1a19,828612965 zbmoerasoroaizdaloUose-ucsrihfaaac,pee.adrIelnalmioinnfngialielt.euddiwniatlhselcatmiionnatrhiscailncfiitlelincglocsoensitsotsthoef
Family HETEROTRYPIDAEUlrich, 1890 It is common to find hollow 'cyst-like' structures within
GenusSTIGMATELLA Ulrich & Bassler, 1904 autozooecia in some zoaria (Figs 4b, c). These are spherical,
average 0-05 mm in diameter, and occur singly or in groups.
Stigmatellaoakleyi sp. nov. Figs 1-4, 8 They are attached to the sides ofthe zooecial walls or to the
centre ofdiaphragms, and their laminae are continuous with
-Iolotype. BMNH PD 2641; Llanbedrog Mudstones zooecial linings.
Soudleyan, Caradoc), near Llanbedrog, Gwynedd, Wales Maculae, consisting predominantly of mesozooecia, have
SH322314). been observed in shallow tangential section (Fig. 3). Large
156 C. J. BUTTLER
4c sf:'::~
.
BRYOZOANSFROMLLANBEDROGMUDSTONES
157
intraspecific overgrowths are common and are composed of The genus is similar to Leioclema Ulrich, 1882 but the
exozonal and outerendozonal elements. latter does not have such wide variation in wall thickness
Remarks. Stigmatella oakleyisp. nov. is primarily character- between the exozone and endozone. Thin-walled mesozooecia
ized by the erect colony form and very thin endozonal walls are also not recognized in Leioclema.
which thicken irregularly in the exozone. The acanthostyles The monotypic genus is currently known only from the
.are very abundant and frequently indent the autozooecial Upper Ordovician, at the type locality.
apertures producing a petaloid shape. Mesozooecia are
common and have abundant diaphragms. Pedrogopora taylorisp. nov Figs5-6, 8
BriSttaiignm,ataelltlhaouhgahs nitotispwreelvliokunsolywnbefernomdeNsocrritbhedAmferroimcaGraenadt Holotype. BMNH PD 2642; Llanbedrog Mudstones
the U.S.S.R. Stigmatellafoordi (Nicholson, 1889) from the (Soudleyan, Caradoc), near Llanbedrog, Gwynedd, Wales
Ordovician of Estonia has similar abundant acanthostyles (SH 322314).
which indent the autozooecial apertures. S. foordi can be Paratypes. BMNH PD 2646A, 2700, 8298-302; all from the
distinguished by the hemispherical colony form, smaller same locality and horizon as the holotype.
acanthostyles, straight mesozooecia and less extensive
thickeningofthe exozonal walls. Name. For Dr P. D. Taylor ofthe Natural History Museum,
S. spinosa Ulrich & Bassler (Ulrich & Bassler 1904: pi. ix, for hiswork on bryozoans.
figs 5-8; see also Bork & Perry 1968: pi. 48, figs 3-5), Diagnosis. As forgenus.
described from the Middle Ordovician of Iowa, has an
irregularly thickened exozone which is similar to that of S. Description.mmZoaria erect with cylindrical branches, on
oakleyi. It differs from the Welsh species by having a greater average 5-6 in diameter. Autozooecia are parallel to the
numberofdiaphragms in the autozooecia, fewer mesozooecia, branch axis within the endozone and then curve outwards
|and acanthostyles which do not inflect the autozooecial abruptly in the exozone to meet the zoarial surface at 90°.
apertures. Autozooecial walls within the endozone are exceptionally
thin and slightly wavy. The exozone is usually wide with an
average width of 1-67 mm. It is recognized by a thickeningof
GenusPEDROGOPORA nov. the zooecial walls and a simultaneous change in zooecial
orientation. Autozooecia all originate in the endozonewhere
Type species. Pedrogopora taylori sp. nov.; Llanbedrog theyhaveanirregular,compressed, rounded-polygonalshape
Vludstones(Soudleyan, Caradoc), nearLlanbedrog, Gwynedd in transverse section; some autozooecia have a vesicular
Wales (SH 322314). appearance. They become oval-circulartoslightly petaloid in
the exozone, as seen in tangential sections of branches.
Name. After Llanbedrog (= Church of St Pedrog), the Autozooecial diameters average 0-24 mm x 0-36 mm within
nearesttown tothe type locality. the exozone. Diaphragms are rare or absent in autozooecia
Diagnosis. Colony ramose. Autozooecia parallel to branch and,whenpresent, onlyoneortwoarefoundintheexozone.
lxis in endozone, then curve abruptly outwards to intersect They are basal diaphragms deflected orally at theirjunctions
;oarial surface. Walls very thin and wavy in the endozone, with the zooecial walls, and have laminae continuous with
greatly thickened in exozone. Autozooecia compressed autozooecial linings.
ounded-polygonal, vesicular appearence in transverse sec- Mesozooecia are common, originate in the outer parts of
ion, oval-circular to slightly petaloid in shallow tangential the endozone and inner parts of the exozone, and have an
ection. Oval mesozooecia common, some with extremely average diameterof0-14mm. Wall thicknessvariesconsider-
hin walls, originating in outer endozone and inner exozone. ably, and while some mesozooecia have a distinctly laminar
Diaphragms rare in autozooecia, present in exozonal meso- wall, othershave exceptionallynarrowwallsandappeartobe
:ooecia. Acanthostyles abundant in exozone. simple space-filling structures between the autozooecia (Fig.
I 6d). Mesozooecia are oval in shallow tangential section, and
Remarks. Pedrogopora is primarily characterized by its wall contain orally-deflected basal diaphragms in the outer endo-
tructure. The autozooecia within the endozone have very zone and exozone. The diaphragms are spaced on average
hin walls and often have an unusual vesicular appearance. 0-09 mm apart and generally increase in thickness distally
|n the exozone the walls increase greatly in thickness. alongthe mesozooecium.
i/Iesozooeciaoftenhaveexceptionallythinwallsandhavethe Acanthostyles are abundant with an average diameter of
ppearance of simple space-filling structures between the 0-06 mm and density of 7 per mm2 They are long, originate
utozooecia (Fig. 6d). Wall microstructure is composed of deep in the exozone and can occas.ionally indent the auto-
teeply inclined, V-shaped laminae and zooecial wall bound- zooecial apertures to produce a petaloid shape. They are
riesare dark and granular. usually found at the junctions of three or more zooecia but
ig2s6710-4(paSrtaitgympaet)e,lllaonogaiktluedyiinaslp.sencotvi.o;n,LlsahnobweidnrgovgeMryudtshtinoneensdo(zSoonuadllewyaalnl,s,Caxra2d0.ocF)i,g.ne2,arBLMlaNnHbedPrDog,26G41wy(nheoldodt,ypeW)a;le2sa., Ftirga.nsIv.erBseMNseHctiPonD,
BxxM215N1;0H.2bFP,iDg.ta2n36,g7e0nBt(MipaNalrHasteycPtpieDo)n;,246as6,h4olw(oipnganirgtautldyaiprnega)el,ascteaacnntgtiehonontsistayhlloewssiencitgnifoilnne,tcetrisnzhgooawtrihiaenlgaoumvtaeocrzugorlooaewcetiahc,,onsxxis24t85i;;ng42bcp,,rletodanongmgieitnnutdaiinantlallysesceotcfitoimnoe,nsosszhhoooowweiicnnigga,'mciycxsrt'o3ss5t.trruuFccittgu.urree4.s,
intheexozone, x 50;4c, longitudinalsectionshowingaclusterof'cyst'structuresintheexozone, x 145.
158 C. J. BUTTLER
^^H^^
:
BRYOZOANSFROMLLANBEDROGMUDSTONES 159
also occur in the walls between adjacent zooecia. A narrow maximum diameter which averages 0-16 mm. Mesozooecia
hyaline calcite core is surrounded by steeply dipping conical contain very abundant, orally-deflected, basal diaphragms
laminae. alongtheirentirelength;theyarespacedonaverage0-07 mm
mm
Autozooecial wall thickness averages 0-14 in the apart and increase very slightly in thickness at the distal end
exozone. Wall microstructure is composed of steeply inclined, of the mesozooecia. Cystiphragms are occasionally present
V-shaped laminae and zooecial wall boundaries are dark and instead of diaphragms throughout the mesozooecia; they
granular. Some zooecia, especially mesozooecia, are infilled occuron alternatingsides ofthe mesozooecia.
with laminar calcite close to the zoarial surface. In longi- Autozooecial walls are thin and the wall microstructure is
tudinalsectionthisinfillingconsistsofbroad U-shapedlaminae. composed of inclined U-shaped laminae. Zooecial boundaries
Maculae composed of mesozooecia have been recognized are wide granular regions, most clearly seen in shallow
in shallow tangential thin section in one specimen (PD 2642, tangential sections.
Fig. 6e). Intrazoarial overgrowths have been recognized in
several specimens (e.g. PD 8302) and they are composed of Remarks. Diplotrypa pseudopetropolitana Astrova, 1965 has
outerendozone andexozonal elements. been described previously from Vaigach Id and Pai-Khoi in
the Soviet Union by Astrova (1965) and Nekhorosheva
Remarks. This is the only species of Pedrogopora so far (1970). Astrova distinguished the new species from the similar
recognized. Within the species there is variation in the width D. petropolitana (Nicholson, 1879) by the presence of
of the endozone from 2-28 mm (in the type specimen PD rounded (not polygonal) autozooecial apertures and more
2642) to0-86mm (in PD 8299). The thin-walled mesozooecia abundant mesozooecia. D. petropolitana is a frequently
are notobserved in all specimens. described species, with several recorded varieties and sub-
species. It is very variable, especially in the number and
distribution ofdiaphragms within the autozooecia. Halloporal
FamilyHALLOPOROIDAEBassler, 1911 dybowskii Bassler, 1911 is extremely similar to D. petro-
GenusDIPLOTRYPA Nicholson, 1879 politana, asBasslerhimselfremarked(1911: 336), andshould
be synonymized.
D. nontabulata sp. nov. (below) is also described from
DiplotrypapseudopetropolitanaAstrova, 1965 Fig. 7 Llanbedrog. Itdiffersfrom D.pseudopetropolitanabyhaving
rare diaphragmswithin the autozooecia.
1948 Diplotrypapetropolitana (Nicholson); Astrova: 16; pi. Only one species of Diplotrypa has been previously de-
1965 Dip2,loftigrsyp4a-5p;speiu.do3,peftigr.op1.olitana Astrova: 189; pi. 30, scribed from Great Britain. This is D. hvergelmi McNamara,
1978 from the Applethwaite Beds (Ashgill) of Troutbeck
fig. 3; pi. 31, fig. 1. in the Lake District. This species has sinuous walls, with
1970 DiplotrypapseudopetropolitanaAstrova;Nekhorosheva diaphragmswidelyspacedthroughoutthe zooecia.
75; pi. 1, fig. 4.
Holotype. H No 1244/40, Yugorskiy Stage (Caradoc),
Diplotrypanontabulata sp. nov. Figs8-9
Pai-Khoi andVaigach Id, Zapadno-Arktichenskoy, U.S.S.R.
(Astrova 1948). 71968 Diplotrypasp. Bork & Perry: 340; pi. 45, figs 1-3.
Material examined. BMNH PD 2626, Llanbedrog Mud- Holotype. BMNH PD 8175; Llanbedrog Mudstones
stones (Soudleyan, Caradoc), near Llanbedrog, Gwynedd, (Soudleyan, Caradoc), near Llanbedrog, Gwynedd, Wales
Wales (SH 322314); D 28581, 'Bala Limestone', Gelli Grin, (SH 322314).
Gwynedd, Wales. Paratypes. BMNH PD 2697, 2719, 8174; all from same
Description. Zoaria are hemispherical in the fragmentary locality and horizon as the holotype.
specimens examined. Autozooecia originate at the centre of
the colony and curve outwards towards the zoarial surface. Other occurrence. ? Guttenberg Formation (Caradoc),
Autozooecial walls are straight and thin throughout most of Guttenberg, Iowa, U.S.A.
thecolony. Thereisnodifferentiationbetweenendozoneand Name. In consideration ofthe lack ofdiaphragms within the
;xozone. The autozooecia are rounded in transverse section autozooecia.
throughoutthezoaria,with an averagemaximum diameterof
3-44 mm in the exozone. Diaphragms are present, although Diagnosis. Colony hemispherical. Zooecia have straight,
not abundant, along the whole length of the autozooecia. thinwallsandare buddedfromthe basal lamina. Nodifferen-
Thesearebasal diaphragmsdeflected orallyattheirjunctions tiation between endozone and exozone is recognized. Auto-
vith the zooecialwalls. zooecia large, polygonal-circular in transverse section
Mesozooecia are numerous and originate throughout the throughout zoaria. Polygonal mesozooecia present. Diaph-
:olony. They are rounded in transverse section and have a ragms very rare in autozooecia, abundant in mesozooecia,
igs 5-6 Pedrogopora taylori gen. et sp. nov.; Llanbedrog Mudstones (Soudleyan, Caradoc), near Llanbedrog, Gwynedd, Wales. Fig. 5,
BMNH PD 8302 (paratype), longitudinal section, showingthin endozonal walls, x 28. Fig. 6, BMNH PD 2642 (holotype); 6a, longitudinal
section, x 35; 6b, transverse section, showing compressed rounded-polygonal autozooecia in the endozone, x 28; 6c, tangential section,
x 28;6d,tangentialsection,showingthin-walledspace-fillingmesozooecia, x 110;6e, tangentialsection, showingmaculae, x 28.
'ig. 7 Diplotrypa pseudopetropolitana Astrova, 1965; Llanbedrog Mudstones (Soudleyan, Caradoc), near Llanbedrog, Gwynedd, Wales.
BMNHPD2626;7a, longitudinal section, x 28;7b, tangentialsection, x 35.
160 C. J. BUTTLER
Fig.8 Longitudinal sketchesofthenewspeciesdescribedfromtheLlanbedrogMudstones. a,Stigmatellaoakleyi. b, Pedrogoporataylori.
c, Diplotrypanontabulata.
often constricting chamber width, producing beaded Bork & Perry (1968) described 'Diplotrypa sp.', from the
appearance. Guttenberg Formation (Caradoc), Guttenberg, Iowa, U.S.A.,
whichwascharacterizedbythe rare autozooecial diaphragms
Description. Zoaria massive and hemispherical, on average and a small number of mesozooecia with very abundant
9-5 mm in diameter. All specimens are fragmentary and no diaphragms. They only had one specimen and did not con-
bases are preserved. Autozooecia bud from the basal lamina sider that a species could be based on a single zoarium. It is
andcurve upwards to the zoarial surface. Autozooecial walls certainly very similar to D. nontabulata sp. nov., but the
are straight throughout the zoaria, and no differentiation Welsh species has more abundant mesozooecia. The Iowa
bzoeotewceieanareexolzarogneewaitnhdaenndavoezroangeecdainambeetedrisotfi0ng-u3i8smhemd.xAu0t-o4-4 ssmpaelcliemrennuismbteenrtatoifvemlyesaoszsoiogenecdiatomDa.ynsoinmtpalbuylarteaflehcetrei;nttrhae-
mm, andarepolygonal-circularintransverse sectionthrough-
specific variation.
out the colony. Diaphragms are absent or very rare in Bork & Perry(1968: 341) listedthree speciesofDiplotrypa
autozooecia; if present they occur in the outermost parts of which had few diaphragms: D. catenulata Coryell (1921:
the autozooecia. pi. 10, figs 6, 7) with crenulated zooecial walls; D. abnormis
Mesozooecia present, often in abundance, in all zoaria and (Modzalevskaya) (in Astrova, 1965: pi. 28, fig. lb) with
originate throughout the colony. Maximum diameter averages widely-spaced, curved diaphragmsthroughoutthe autozooecia;
0-23 mm; however, there is a large range (0-08-0-44 mm). and D. westoni Ulrich (1889: pi. 8, figs4-4b) with occasional
Mesozooecia are polygonal, often triangular or rectangular, diaphragmswithin the autozooecia.
frequently positioned at the autozooecial corners. Meso-
zooecia contain abundant diaphragms along their entire
length. These are basal diaphragms deflected orally at their
junctions with the zooecial walls, and spaced on average GenusHALLOPORA Bassler, 1911
0-16mm apart in the endozone and 015 mm in the exozone.
Mesozooecia are often constricted at the position of the Hallopora cf. tolli Bassler, 1911 Fig. 10
diaphragms, producing a beaded appearance in longitudinal
section. Material examined. BMNH PD 2666; Llanbedrog Mud-
Autozooecial walls are thin which, combined with poor stones (Soudleyan, Caradoc), near Llanbedrog, Gwynedd,
preservation, results in difficulties in distinguishing wall Wales (SH 322314).
microstructure. In one specimen (PD 8174) the wallsthicken Description. Zoarium erect with small cylindrical branches,
very slightly close to the zoarial surface, reaching a thickness on average5mmin diameter. Autozooeciaareparalleltothe
of0-032mm. Thereis an indistinctdark bandatthe centreof branch axiswithinthe endozoneandcurveoutwardsgentlyin
thewall, borderedby lighterbandson eitherside which have the outer endozone to meet the zoarial surface at 80°-90°.
a vague laminar appearance. Within the main body of the Theautozooeciawithintheendozone havethinstraightwalls.
zoarium the walls are dark and structureless. Theexozoneisnarrowwithanaveragewidthof0-86mm. Itis
Remarks. Diplotrypa nontabulata sp. nov. is primarily recognized by a slight thickening of the zooecial walls and a
characterized by the rare diaphragmswithin the autozooecia, simultaneous change in zooecial orientation. Autozooeciaall
small mesozooeciawith abundant diaphragms and thin straight originate in the endozone where they are polygonal-rounded
walls. in transverse section, becoming oval-rounded in the exozone
Fig. 9 Diplotrypanontabulatasp. nov.; LlanbedrogMudstones (Soudleyan, Caradoc), nearLlanbedrog, Gwynedd, Wales. BMNH PD8175
(holotype);9a, longitudinalsection,showingthelackofdiaphragmswithin theautozooecia, x 25;9b, tangentialsection, x 35.
Fig. 10 Halloporacf. tolliBassler, 1911;LlanbedrogMudstones(Soudleyan,Caradoc),nearLlanbedrog,Gwynedd,Wales. BMNHPD2666;
10a, longitudinal section, x 35; 10b,transversesection, x 35; 10c, tangentialsection, x 50.
Figs 11-12 Eridotrypacf. kilbartiensisPushkin {in Ropot&Pushkin, 1987);LlanbedrogMudstones(Soudleyan, Caradoc), nearLlanbedrog,
Gwynedd, Wales. Fig. 11, BMNH PD 2722; 11a, longitudinal section showing the exceptionally thickened exozonal walls, x 50; lib,
transversesection,showinglargeautozooeciainthecentreoftheendozone, x 50. Fig. 12,BMNHPD2691,tangentialsection, x 80.
BRYOZOANSFROMLLANBEDROGMUDSTONES
161
:'
162 C. J. BUTTLER
as seen in shallow tangential sections of branches. Auto- the endozonal zooecia are polygonal-rounded and the meso-
zooecial diameters average 0-23 mm x 0-31 mm within the zooecia are small, both characteristics of Parvohallopora.
exozone. Diaphragms in the endozone are not abundant and However, the occasional presence of diaphragms in the
are spaced on average 0-21 mm apart. They are spaced 0-11 autozooecia and the V-shaped laminae in the zooecial walls
mm apart in the exozone. Only one or two diaphragms are are attributes of Hallopora. This seems to show that the
found at the outerendsofautozooecia in the exozone. All of distinctionbetweenthetwogeneraislessclear-cutthan Singh
thesediaphragmsarebasal, deflectedorallyattheirjunctions suggests.
withthezooecialwalls, andtheirlaminaearecontinuouswith
autozooecial linings.
Mesozooecia are common, originating in the outer part of GenusERIDOTRYPA Ulrich, 1893
the endozone and having a maximum diameter which aver-
ages 0-13 mm in the exozone. They are circular to rounded- Eridotrypa cf. kilbartiensis Pushkin (in Ropot &
polygonal in shallow tangential sections. Mesozooecia con- Pushkin, 1987) Figs 11-12
tain abundant orally-deflected basal diaphragms, spaced on
average 0-06 mm apart, which occasionally increase in thick- Materialexamined. BMNHPD2691,2720,2722;Llanbedrog
nessdistally alongthe mesozooecia. Mudstones(Soudleyan, Caradoc), nearLlanbedrog, Gwynedd,
Autozooecial wall thickness averages 0-08 mm in the Wales (SH 322314).
exozone. Wall microstructure is composed of steeply in-
clined, V-shaped laminae. The zooecial wall boundaries are Description. Zoaria erect with very narrow cylindrical
wide, granular areas, most clearly seen in shallow tangential branches, on average 2-5 mm in diameter. The three speci-
section. Somezooeciaare infilledwithlaminarcalcitecloseto mens from Llanbedrog are poorly preserved. Autozooecia
the colony surface. In longitudinal section this infilling con- are parallel to the branch axis within the endozone and then
sistsofbroad U-shaped laminae. curve slightly in the exozone to meet the zoarial surface at
45°. The autozooecia within the endozone have thin, straight
Remarks. The single specimen described here is character- walls. The exozone is narrow with an average width of 0-76
ized bythe ramose colony form and thin-walled autozooecia. mm; it is recognized by an extensive thickening of the
Autozooecial apertures are polygonal-rounded in transverse zooecialwalls. Autozooecia all originate in theendozone and
zoarial section andoval-roundedin shallowtangential zoarial are polygonal in transverse section, becoming oval in the
section. Rounded-polygonal mesozooecia are common and exozone as seen in tangential sections of branches. In trans- |
originate in the endozone. Diaphragms are occasionally verse section the autozooecia are twice the diameter in the
present in the outer autozooecia and are abundant in the inner endozone than in the outer endozone. Autozooecial
mesozooecia. diameters average 0T0 mm x 0T2 mm within the exozone. '
Bassler (1911) described Hallopora tolli originally from Diaphragms are present in the outer endozone and exozone,
mm
the Jewe Limestone, Kuckers Shale and Kegal Limestone spaced on average 0-11 apart. These basal diaphragms
(Caradoc) of Estonia, and illustrated longitudinal and deep are all deflected orally at theirjunctionswith zooecial walls.
tangential sections. This species has long, straight meso- No mesozooecia have been conclusively identified in the
zooecia with abundant diaphragms, similar to the Welsh three specimens examined.
specimen. The autozooeciaare polygonal-rounded in tangen- Acanthostyles have been observed in the exozone of one
tial section (their shape in zoarial transverse section is not specimen (PD 2722). They are short and composed of a
known). However, Bassler's section is very deep and is hyaline calcite core surrounded bysteeplydipping laminae.
difficult tocompare directlywiththatofthe Welsh specimen. Autozooecial walls are thick in the exozone. Wall micro-
Fritz (1941) described H. tolli from the Whitehead Forma- structure is composed of steeply inclined, V-shaped laminae
tion, Gaspe, Quebec, Canada, but herdescription is not well (microstructure is slightly indistinct due to the age and
illustrated. Bork & Perry (1967) tentatively described one thickness ofthe thin sections). Zooecial boundaries have not
fragmentary specimen as H. cf. tolli from the Guttenberg been distinguished. Some zooecia are infilled with laminar
Formation (Caradoc) of Iowa, U.S.A. It differs from the calciteclosetothe zoarial surface. In longitudinal sectionthis
specimen illustrated by Bassler in having thinner walls. Until infillingconsists ofbroad U-shaped laminae.
the type material of H. tolli has been re-examined, the true
nature ofthe specieswill remain obscure. Because ofthisthe Remarks. The three poorly preserved specimens from
poorly preservedWelsh specimen is named H. cf. tolli. Llanbedrog assigned to Eridotrypa cf. kilbartiensis Pushkin
Singh (1979) proposed the genus Parvohallopora for some are characterized bytheirslenderbranches, and autozooecial
species previously assigned to Hallopora. He distinguished wallswhich are thin andstraight in the endozone andthicken
thetwogeneraonthebasisofthefollowingcharacteristics: in in the exozone. Autozooecial apertures are large and poly-
Parvohallopora, zooecia are of smaller diameter than in gonal in transverse section, small and oval in shallow tangen-
Hallopora; mural spinesare absentin Parvohalloporabutare tial sections. Diaphragms are present in the exozonal
present in the type species of Hallopora; mesozooecia are autozooecia and occasionally in the endozone.
generallysmallerin Parvohallopora; zooecia are polygonal in The specimens are very similar to E. kilbartiensis Pushkin
the endozone of Parvohallopora, whereas in Hallopora they (in Ropot & Pushkin, 1987), described from the Oanduskii
are rounded to subrounded; cystose diaphragms are rare to Stage (Caradoc), White Russia. E. kilbartiensis has thin,
absent in Parvohallopora but are more common in Hallopora; straight walls in the endozone which thicken extensively in
and the laminae of adjacent zooecia are broadly U/V-shaped the exozone (Ropot & Pushkin 1987: pi. 14, figs 3a, b). The
in Parvohallopora and distinctlyV-shaped in Hallopora. major difference is the greater number ofdiaphragms in the
The present specimen isassignedto Hallopora, despite the exozone of the Welsh specimens. There may be range of
possessionofcertaincharactersattributedto Parvohallopora: variabilitywithinthespecies, butuntilfurthermaterialcanbe