A review of the cyclostomiceratid nautiloids, including new taxa from the Lower Ordovician of Öland, Sweden

13 Pages·1998·5.9 MB·English

by  Andrew H King

A REVIEW OF THE CYCLOSTOMICERATID
NAUTILOIDS, INCLUDING NEW TAXA FROM THE
LOWER ORDOVICIAN OF OLAND, SWEDEN
by ANDREW H. KING
Abstract. CyclostomiceratidaeisadistinctfamilyofearlyOrdovician,small, gomphoceroidtobreviconic
ellesmeroceridnautiloidswhichpossessanadorallycontractingaperture.ThefamilyrangesinagefromArenig
toearlyLlanvirnandisrecordedfromtheUSA,northernArgentina,eastChinaandBaltoscandia.Thispaper
reviewsthefamily’sstatus,origin,stratigraphicaloccurrenceandsystematics.Threenewtaxaaredescribed
fromthelowerKundanStageofnorthernOland,Sweden: Pictetocerasoliviaesp.nov.,Parcyclostomiceras
paucitumidumsp.nov.andMicrostomicerasholmigen.etsp.nov.Thesecyclostomiceratidsareextremelywell
preservedandextendourknowledgeofthemorphology,biostratigraphyandpalaeogeographicaldistribution
ofthefamily.Microstomicerasgen.nov.isoneofthesmallestnautiloidsdescribedfromtheLowerPalaeozoic.
EllesmeroceridnautiloidsaresomeoftheoldestLowerPalaeozoiccephalopods.Theyevolved
fromtheancestralplectronoceridnautiloidsinChina(AnhuiandShaanxiprovinces)duringthelate
CambrianandrangeintotheupperOrdovicianwherethegroupisrepresentedbyasinglefamily
in the Ashgill (Rawtheyan Stage) of Cincinnati (King 1993). Following a brief ‘evolutionary
explosion’inthelateCambrianwhentheseearlycephalopodsspreadacrossnorthernandeastern
China, KazakhstanandcentralTexas, theyunderwent a dramatic and sharpdeclineinterms of
both taxonomic diversity and abundance. Three ofthe four nautiloid orders present in the late
Cambrian became extinct before the end ofthe period, and only a few ellesmerocerid genera
survived into the early Ordovician. From these forms, there developed a rich and diverse early
OrdovicianellesmeroceridfaunawhichhasbeendescribedextensivelybyUlrichetal.(1943, 1944)
andFlower(1964).
Whencomparedwithothernautiloidorders,theellesmeroceridsarerelativelylimitedintheform
ofmorphological structures that theydevelopedwithin theirconchs to aidbuoyancyregulation.
Typically, they exhibit small, narrowly camerate shells with short septal necks and thickened,
layeredconnectingrings.Theapicalportionsofsiphunclesofmanytaxacontainnumerousapically
directedsiphonaldiaphragms,althoughthisfeatureisnotexclusivetothegroup.Diversityamongst
thelowerOrdovicianellesmeroceridsis representedmainlybyvariationsin shell formwhichare
sufficienttoprovidethebasisforsystematicclassificationatfamilyandlowertaxonomiclevels.
The family Cyclostomiceratidae is a distinctive group ofArenig to lower Llanvirn nautiloids
whichexhibittypicalellesmerocerid features: theypossess small, narrowlycamerateconchswith
achoanitictoorthochoaniticseptalnecksandadorallycontractingapertures.Siphonaldiaphragms
have been recorded from at least one taxon (Mutvei and Stumbur 1971). Although the
Cyclostomiceratidaehashadavariedtaxonomichistory,thegeneracurrentlyassignedtothefamily
appeartorepresentanaturalgroupingand,followingFlower(1964),arepresentlyretainedwithin
theorderEllesmerocerida.Thisassignmentisprovisionalasthecyclostomiceratidmusclescarsare
fofrogmenetrhaeldvoernstormoymayrairainancfonodrimti(oMnutkvneiowanndtoStoucmcbuurrin19s71om;eTexott-hfeirg.e4lalehsemreer)ocwehriicdhsi(sDveewriytdzif1f8e8r0e)n.t
Future classifications may regard this difference as being ofhigh taxonomic significance and,
consequently, theorderEllesmeroceridamaybedividedfurther.
ThestudyofcollectionsofearlyLlanvirncephalopodsfromthelowerKundanStageofOland
[Palaeontology,Vol.41,Part2,1998,pp.335-347] ©ThePalaeontologicalAssociation
336 PALAEONTOLOGY,VOLUME41
text-fig. 1.a.GeographicallocationoftheislandofOlandoffsouth-easternSweden,b,NorthernOlandand
theOrdoviciaVnolbkehdorvoicakng(edoalrokgygraety)H,alKluundddaenns(hpaolreegarnedy)Eannedrupmos;tt-heKusntrdaatnaa(rneoloirmneasmtoennets)oafgeL.atorpianand
SWEDEN(Oland) SWEDEN(Oland) GREATBRITAIN GREATBRITAIN NORTHAMERICA
STAGES LIMESTONE SERIES GRAPTOLITE STAGES
FORMATIONS BIOZONE
Folkeslunda(part)
LASNAMAGIAN Seby
murchisoni
Skarlov LLANVIRN WHITEROCKIAN
ASERIAN Segerstad
KUNDAN Holen artus
hirundo VALHALLAN
VOLKHOVIAN Lanna
gibberulus
ARENIG nitidus CASSINIAN
LATORPIAN Latorp
deflexus
(approximate) JEFFERSONIAN
text-fig.2.LithostratigraphicalandchronostratigraphicalnomenclatureinSweden(Oland),comparedwith
BritishandNorthAmericanchronostratigraphyandbiostratigraphicalcorrelationinrelationtograptolite
biozones.
(heldintheSwedishMuseumofNaturalHistory,NaturhistoriskaRiksmuseet,Stockholmandthe
SwedishGeological Survey, SverigesGeologiskaUndersokning, Uppsala)hasrevealedthreenew
taxaofcyclostomiceratidnautiloids.Thespecimens,collectedbyGerhardandOliviaHolmbetween
1895 and 1909,comefrom thelowerHolenLimestone Formation(oftenreferred to by Gerhard
Holmasthe‘gravaginatumkalk’ orgreyvaginatumlimestone)ofHalluddenshoreandEnerum,
KING: ORDOVICIANNAUTILOIDS 337
northernOlandwheretheformationissuperblyexposedinthelow-lyingcliffsandforeshore(Text-
fig. 1).Acrossthisisland,stratadipslightlytotheeastand,withappropriatecollectingtechniques
usingdistinctive discontinuity surfaces, a high degreeofstratigraphicalcontrol can be obtained.
Furtherdetailsofthestratigraphy(summarizedinText-fig.2)aregivenbyJaanussonandMutvei
(1982).
The Swedish specimens are extremely well preserved and extend our knowledge of the
morphology, biostratigraphy and palaeogeographical distribution ofthe family Cyclostomicera-
tidae,whichhasnotbeenrecordedfromSwedenpreviously.ItisthererepresentedbyPictetoceras
oliviaesp. nov.,Paracyclostomiceraspaucitumidumsp.nov. andMicrostomicerasholmigen.etsp.
nov.TheonlyotherrecordsofCyclostomiceratidaefromBaltoscandiaareprovidedbydeVerneuil
(1845) and Mutvei and Stumbur(1971)who described Pictetoceraseichwaldi(deVerneuil, 1845)
fromtheupper Kundan Stage ofEstonia. The Swedishrepresentatives ofthe familyare slightly
older.
HISTORICAL REVIEW OF THE FAMILY CYCLOSTOMICERATIDAE
Foerste (1925) originally proposed Cyclostomiceratidae for the genera Cyclostomiceras Hyatt,
inZittel,1900andEremocerasHyatt,1884,believingthattheirsiphunclewallwascharacteristically
holochoanitic,althoughnomaterialwasapparentlysectionedtoconfirmthisfeature. Ulrichand
Foerste (1936) added their new genus Amphoroceras based upon Cyclostomiceras minimum
,
(Whitfield, 1886). Later, Ulrich et al. (1943) regarded Amphoroceras as a junior synonym of
CyclostomicerasandaddedtheirnewgeneraBuehleocerasandBridgeocerastothefamily.Theyalso
figured thin sections ofthe siphonal wall ofCyclostomiceras indicating that this genus actually
possessedveryshortseptalnecksandthick,layeredconnectingrings.Ulrichetal.(1944)lateradded
Dresseroceras to the Cyclostomiceratidae; the holotype ofits type species, and the only known
specimenofthismonospecificgenus,isrepresentedbytheinternalmouldofabody-chamberwhich
bearsprominentbutirregularlyspacedtransverseannulations.Thestructureofthesiphunclewall
remainsunknown.
Cecioni (1953) described the new cyclostomiceratid Paracyclostomiceras from the Llanvirn of
Serrania de Zapla, northern Argentina and illustrated the detail ofthe siphuncle wall. In their
description ofnautiloids from the lower Ordovician ofVirginia, Unklesbay and Young (1956)
includedWoosterocerasUlrich,Foerste,MillerandUnklesbay,1944withintheCyclostomiceratidae,
andlaterBalashov(1962,p. 74, pi. 5, fig. 13)includedPictetocerasFoerste, 1926,demonstrating
thattheconnectingringsofthislattergenuswereofthickenedellesmeroceridtype.
FurnishandGlenister(1964)placedCyclostomiceratidaeinsynonymywithEllesmeroceratidae
Kobayashi, 1934. This procedure united a large number of taxa with comparable siphuncular
features: short achoanitic to orthochoanitic septal necks, thick-layered connecting rings and
siphonaldiaphragms(whereknown). However, thesynonymyadoptedbyFurnishandGlenister
(1964) also incorporated taxa exhibiting a wide variety ofconch forms within the single family
Ellesmeroceratidaeandthisapproachisnowregardedasobscuringanumberofdistinctlineages.
The same authors regarded Dresseroceras as a synonym of the protocycloceratid genus
ProtocyclocerasHyatt,inZittel, 1900,althoughtheformergenusalsobearsastrongmorphological
resemblancetootherannulateprotocycloceratidssuchasCatoraphicerasUlrichandFoerste, 1936
and WalcottocerasUlrichandFoerste, 1936.
Flower(1964)recognizedavariedseriesofmorphotypeswithintheEllesmeroceratidaeranging
fromorthoconesandsimplecyrtoconestospecializedformswithcontractedor‘crested’apertures.
Heusedthesemorphotypesfordescriptivepurposesbuthismorphogroupswerenotintended to
representindependentlineageswithinthefamily. However,heacknowledgedthatthediversityof
taxa then assigned to the Ellesmeroceratidae was unrealistic and re-introduced a number of
previously used family names including Cyclostomiceratidae Foerste, 1925 for the genera
Cyclostomiceras, Paracyclostomiceras and Pictetoceras. Mainly on the basis ofconch form, he
retained Eremoceras, Buehleroceras and Woosteroceras within a still highly diverse family
Ellesmeroceratidae,andregardedBridgeocerasasanellesmeroceridofuncertainaffinities.Healso
338 PALAEONTOLOGY,VOLUME41
regarded Dresseroceras as a synonym of Bridgeoceras which itselfwas an ‘ellesmeroceroid of
uncertainposition’.
MutveiandStumbur(1971)providedacomprehensivestudyofPictetocerasfromtheLlanvirn
(Kundan; Aluojan Substage) ofEstonia, and noted the presence of siphonal diaphragms and
muscle-scarimpressionsinthetypespeciesP.eichwaldi(deVerneuil, 1845).ChenJun-yuan(inQi
et al. 1983) described the new genus Eocyclostomiceras from the lower Ordovician ofnorthern
JiangxiProvince,eastChina,butassignedthetaxontotheEllesmeroceratidae.Thislattergenusis
poorly known but available morphological and stratigraphical evidence tentatively supports
assignmenttotheCyclostomiceratidae.
Dzik(1984)recognizedthattheEllesmeroceratidaecontainedadiverserangeofconchformsand
lineages, and introduced the family Oneotoceratidae for relatively short, compressed, endo-
gastricallycurvedshellsincludingBuehleroceras.HealsodistinguishedtheCyclostomiceratidaeas
asmallgroupoflateArenigtoearlyLlanvirninflatedorthoconicnautiloidsbutsynonymizedboth
PictetocerasandParacyclostomiceraswithinCyclostomiceras.Forreasonsdescribedbelow,Iregard
thesethreegeneraasdistinct.
ORIGIN OF THE FAMILY CYCLOSTOMICERATIDAE
Flower (1964, pp. 123-124) regarded the Cyclostomiceratidae as being derived from the
ellesmerocerid family Baltoceratidae Kobayashi, 1935 during the late Canadian (Cassinian). He
noted, in particular, that the Cassinian genus Metabaltoceras Flower, 1964 was intermediate
between the two families in its overall form but departed from the generalized pattern in its
relativelylargeventralsiphuncleandsuturallobes.Thefusiformshellandachoaniticseptalnecks
inMetabaltocerasoffersomesupportforthispossiblerelationship.
Theconch cross section oftaxa assigned to the Cyclostomiceratidaeisusually sub-circularto
compressed, andismorereminiscentoftheBassleroceratidaethantheBaltoceratidaeinwhichit
tendstobeslightlydepressed. Bassleroceratidaehadawidespreaddistributioninthemidtolate
Canadianandseveralgenera(Avaoceras,DiaphorocerasandLawrenceocerasUlrich,Foerste,Miller
andUnklesbay, 1944)showfeaturesreminiscentoftheCyclostomiceratidae,namelycontractionof
thebody-chamberneartheapertureandveryshortseptalneckswiththickconnectingrings(Flower
1964,p. 152;FurnishandGlenister1964,p.K148).However,theBassleroceratidaearerepresented
mainly by longicones and consequently, despite superficial similarities, an origin for the
Cyclostomiceratidaefromabassleroceratidancestoristhoughttobeunlikely.
Cyclostomiceratids also bear a close external resemblance to some bathmoceratid genera
described from the upper Canadian of Argentina and Bolivia by Cecioni and Flower (1985).
Bathmoceratidae is a group of ellesmerocerid nautiloids characterized by inflated and greatly
thickened connecting rings which project adorally or laterally towards the siphuncle; these
connectingringsarecommonlyassociatedwithnumerousdiaphragms. However,thestructureof
thebathmoceratidsiphunclewalliswhollyunlikesiphonalfeaturesknownwithintheCyclostomi-
ceratidae. Inaddition, thebody-chambers belongingto thebathmoceratidsdescribedbyCecioni
and Flower (1985) do not appear to contract adorad which is a characteristic feature of
cyclostomiceratidtaxa. Consequently,thereappearstobenoclearevidencetolinktheoriginsof
the Cyclostomiceratidae and the Bathmoceratidae, and any similarity in external conch form is
regardedheretobeessentiallyhomeomorphy.
ApossibleoriginfortheCyclostomiceratidaefromwithintheOneotoceratidae(Dzik1984)does,
however, require seriousconsideration. Dzik’s (1984) interpretation ofthelatterfamilyincluded
stout breviconic or near-gomphoceroid endogastric forms in addition to distinctive, strongly
cyrtoconicbrevicones.Theoneotoceratidsappeartoexhibitveryshortseptalneckswiththickened,
layeredconnectingringswhicharehighlycomparabletothoseoftheCyclostomiceratidae,although
theformerfamilygenerallypossessesaslightlynarrowersiphuncle. Furtherevidenceforderiving
theCyclostomiceratidaefromtheOneotoceratidaeisprovidedherebyMicrostomicerasgen. nov.
whichindicatesthattheformerfamilymayalsobeendogastric.
KING: ORDOVICIANNAUTILOIDS 339
Current evidence would therefore indicate that the Cyclostomiceratidae developed in the late
Canadian (Cassinian) from either the Baltoceratidae (via Metabaltoceras) or more likely from
gomphoceroid-breviconicformsassignedtotheOneotoceratidae.
SYSTEMATIC PALAEONTOLOGY
Theterminologyusedhere followsthat ofFlower(1964) and Furnish and Glenister(1964). The
typematerialofnewlydescribedspeciesisheldincollectionsattheSwedishMuseumofNatural
History (Naturhistoriska Riksmuseet, RM), Stockholm and the Swedish Geological Survey
(SverigesGeologiskaUndersokning, SGU), Uppsala.
ClasscephalopodaCuvier, 1797
OrderellesmeroceridaFlower, inFlowerand Kummel, 1950
FamilycyclostomiceratidaeFoerste, 1925
Diagnosis. Gomphoceroid to vasiform brevicones, shell straight or with faint endogastric or
exogastric curvature. Conch section sub-circular to slightly compressed or depressed, rapidly
expanding over the phragmocone; aperture typicallycontracted but nevermarkedly constricted.
Sutures transverseorwithdorsal and ventral saddles andcorrespondinglaterallobes. Siphuncle
(sub)ventral, with achoanitic to suborthochoanitic or orthochoanitic septal necks; segments
concave and outlined by thick, layered connecting rings. Siphonal diaphragms known in some
genera.
Remarks. Conch form within the Cyclostomiceratidae varies: Cyclostomiceras and Paracyclo-
stomicerasare essentially straight brevicones (Ulrich et al. 1943, pis 31-34; Cecioni 1953, pi. 3)
whilstMicrostomicerasgen.nov.isclearlyendogastric.MutveiandStumbur’s(1971)reconstruction
ofPictetoceraseichwaldi(deVerneuil, 1945)proposesastraight,breviconicshellwithaverylarge
protoconch,althoughavailableevidence(basedonCyclostomicerascassinenseandC.depressumin
Ulrich et al. 1943 and Microstomiceras holmigen. et sp. nov.) indicates that the protoconch of
cyclostomiceratids was, in fact, relatively small. This is consistent with Dzik’s (1984, p. 22)
suggestionthattheapicalpartsofellesmeroceridsmayrepresentnautiloidswithasmallplanktonic
larvalstage.
Genera assigned. Cyclostomiceratidae is regarded here as a distinct family ofbreviconic ellesmerocerids
containing the following taxa: Cyclostomiceras Hyatt, in Zittel, 1900; Pictetoceras Foerste, 1926; Para-
cyclostomicerasCecioni, 1953;EocyclostomicerasChen,inQietal., 1983andMicrostomicerasgen.nov.
Occurrence. Arenig (upper Canadian, Cassinian) to lower Llanvirn ofthe USA, Baltoscandia, northern
ArgentinaandeastChina.
GenuscyclostomicerasHyatt, inZittel, 1900
(=AmphorocerasUlrichandFoerste, 1936)
Typespecies.GomphocerascassinenseWhitfield, 1886;byoriginaldesignation(Hyatt,inZittel1900,p.611).
Diagnosis.Gomphoceroid,breviconicorthoconeswithcirculartodepressedsection;phragmocone
expandingforwardfairlyrapidly,anteriorhalfofmaturebody-chamberslightlycontracted.Sutures
and growth lines straight and directly transverse. Siphuncle small, about 15 per cent, ofconch
diameter,ventralinpositionbutnotmarginal.Septalnecksorthochoanitic,connectingringsthick
andlayered.
Remarks.ApartfromthetypespeciesC.cassinense(Whitfield),thefollowingtaxaarealsoassigned
to the genus: C. minimum (Whitfield, 1886), C. depressum Ulrich, Foerste and Miller, 1943 and
PALAEONTOLOGY,VOLUME41
tape-axbrt,a-tfvyiepgnet.,r3aR.laMa-ndd,MolPelifc5tt8le4att4oec1rea;rlasvxioe2l-wi7sv.iaoeef-shhp,o.lPnoaotvry.apce;y,lcoRlwoeMsrtoHMmooilcleenr5a8Ls4i6pm0aeaus;ctioxtnueLm7Fi5o.drumcma-tds,ip.ovnen,notvHr.aa;llllauodnwddeernle,fHtonlloarettnehreaLlrinvmieOeslwtasondno.ef
Formation,northernOland.e-f,ventralandleftlateralviewsofholotype,SGUCeOOl,fromEnerum; x1-25.
G-H,ventralandleftlateralviewsofparatype,SGUCe002,fromHalludden; x1-75. I-L,Microstomiceras
hhoollomtiygpeen,.RetMsp.Mnoolv5.8;7l0o0w;erxH3o-l3.enJ-LLi,mevsenttornael,Forirgmhattilaotne,ralHaalnluddddeorns,alnovritehwesrnoOflpaanrda.tyi,per,ighRtMlateMroall5v8i6ew98o;f
x3-75.
KING: ORDOVICIANNAUTILOIDS 341
C. depressius Cecioni, 1953. Cyclostomicerasl vasiforme (Dwight, 1884) is ofuncertain status;
accordingtoFlower(1964,p. 124),theweatheredsectionofaBassleroceraswouldproduceaform
analogous to the type specimen. The expansion rate and conch form ofthe specimen tends to
confirmthatitsassignmenttotheCyclostomiceratidaeisincorrect.
Occurrence.Cecioni(1953)reportedC.depressiusfromtheArenigofQuebradadeCoquena,Purmamarca,
Chile;C.?vasiformeisrecordedfromtheMidCanadianRochdaleLimestoneofsouthernNewYork.Allthe
remainingtaxaoccurintheFortCassinLimestoneoftheChamplainValley,AddisonCounty,Vermontorthe
SmithvilleFormationofLawrenceCounty,Arkansas(bothoccurrencesareofArenigage).
GenuspictetocerasFoerste, 1926
Typespecies.GomphoceraseichwaldideVerneuil,1845,p.357;byoriginaldesignation(Foerste1926,p.327).
Diagnosis. Conch gomphoceroid, enlarging as far as anterior part of the phragmocone then
contractingadorally. Shellsectioncompressed;suturesinclinedforwardovertheventer. External
conch surface ornamented with weak, indistinct growth lines. Body-chamber relatively long,
aperturalmarginwithadistinct,unpaireddorsalsinusandweakerventralsinus cameraenarrow,
;
10-15percent,ofthedorso-ventraldiameter.Siphunclecircularinsection,incontactwithventral
wall,20-25percent,ofconchdiameter.Septalnecksorthochoanitic connectingringsthreetofour
timesthickerthanseptaandlayered.Apicalportionsofsiphuncletr;aversedbynumerous,adorally
archingcalcareousdiaphragms.
Remarks. Pictetoceraswas reviewed by Mutvei and Stumbur (1971) who regarded the genus as
monospecificanddescribedthetypespecies,P.eichwaldi fromtheupperKundanStage(Aluojan
Substage)oftheStPetersburgdistrictandEstonia. Asli,ghtlyolderspecies, P. oliviaesp. nov., is
describedherefromthelowerKundanofOland.ThisnewspeciesismuchsmallerthanP.eichwaldi
(basedontheassumptionthatcontractionoftheaperturewithseptalapproximationisindicative
ofmaturity in individual specimens). The type material ofP. oliviae sp. nov. does not exhibit
siphonal diaphragms, although it is likely that no material sufficiently apicad to contain such
structureshasbeensectioned.
Occurrence. Kundan Stage (uppermost Arenig to lower Llanvirn) ofOland, Sweden, Estonia and the St
Petersburgdistrict.
Pictetocerasoliviaesp. nov.
Text-figures3a-d,4,6a
Derivationofname.ForOliviaHolm,thedaughterofGerhardHolm,whocollectedtheholotypespecimenin
1896.
Material.Theholotype(RMMo158460a)isanunsectionedconchwithacompletebody-chamberandseveral
adoral camerae. Three paratypes (Mo158441, 158457, 158460b-d) consist of incomplete sectioned
phragmoconesand/orbody-chambers.AllthetypematerialwascollectedbyOliviaandGerhardHolmin
1890and 1896,fromthelowerHolenLimestoneFormation(lowerKundanStage)atHalludden,northern
Oland.
Diagnosis.Small,slenderPictetoceraswithsub-circularsectionandelongatebody-chamberwhich
graduallycontractsadorad.
Dceosmcprliepttieons.pecCiomnecnhexsammalilneda;ndlesnlgetnhde2r4,-2vmasmi,foardmo-rbarlev1i4c-o2nimcmwirtehpressuebn-tciinrgcubloardy-secchtaimonb.erH.olLaotttyeprereilsatmioveslty
long (estimated 40 percent, oftotal conch length) and contracting slowly towards aperture; contraction
342 PALAEONTOLOGY,VOLUME4
text-fig. 4. Pictetoceras oliviae sp. nov.; RM Mol58441, paratype; lower Holen Limestone Formation
Halludden,northernOland,a,ventralviewofbaseofbody-chambershowingannularretractormusclescar
(ms)withindistinctlobesconcentratedventrally; x4-6.b,shellcrosssection,venterandsiphuncleuppermost;
x5.
greateronventralside,dorsalsideonlyslightlyconcave.Externalshellsurfacesmoothwithveryfaintgrowth
lines which trace out feeble dorsal and ventral sinuses. Maximum conch width in holotype near top of
phragmoconewheredorsoventraldiameteris12-4mm;atapicalend,dorso-ventraldiameteris10-3mmwith
lateral width 9-4mm. Body-chamber bears faint muscle-scars consisting of annular retractor scar with
indistinct lobes concentrated ventrally (Text-fig. 4a). Sutures laterally transverse, becoming very weakly
inclinedoverventeranddorsum.Averagecameralheight ITmm,septalconcavityshallow.Atbaseofbody-
chamber,siphunclerepresents26percent,ofconchdiameter,reducingto 15percent,atapicalend. Septal
necksorthochoanitic,extendingapicad0-2camerae.Connectingringsthickandlayered;innerlayer(adjacent
tosiphuncle)darkandcomparableinthicknesstosepta,outerlayer(adjacenttocamerae)palerandthreeto
fourtimesseptalthickness(Text-fig.6a).
Remarks. P. oliviae sp. nov. differs from the type species, P. eichwaldi, in its smaller size and
relativelylongbody-chamber. Theventromyarianmusclescarsobservedatthebaseofthebody-
chamber in P. oliviae sp. nov. are similar in form to those described for the type species of
PictetocerasbyMutveiandStumbur(1971,p. 120).
Occurrence.ThespeciesisknownonlyfromthelowerHolenLimestoneFormation(lowerKundanStage)at
Halludden, northern Oland. The specimens collected in the last century were obtained from either the
HunderumianorValasteansubstages,orboth.SeveralotherspecimenscollectedrecentlyfromHalluddenare
alsolikelytobelongtothisspeciesbutrequirefurtherstudytoconfirmthisassignment.Thesespecimenscome
0f-r1o5mmthbeelloowwertmhoesmtapianrtdiosfctohnetiVnauliatsytesuarnfaScueb.stage(JaanussonandMutvei1982,p.16),between0-0-05mand
GenusparacyclostomicerasCecioni, 1953
Typesspecies.ParacyclostomicerasfloweriCecioni, 1953;byoriginaldesignation(Cecioni 1953,p.98).
Diagnosis.SimilartoCyclostomicerasbutsuturesundulatorywithwell-developeddorsalandventral
saddlesandcorrespondinglaterallobes.
Remarks. This genus is similarin overallconch form to Cyclostomicerasbut is readilyidentified
by the form of its sutures. Cecioni (1953) noted that the apertural contraction rate of
ParacyclostomiceraswassmallerthanthatknownforCyclostomiceras andthatthesiphunclewas
incontactwiththeventralwalloftheconch(accordingtoCecioni,thes,iphuncleisslightlyremoved
fromtheventerin Cyclostomiceras). Apartfromthetypespecies, healsodescribed P. depressum
KING: ORDOVICIANNAUTILOIDS 343
text-fig. 5. Polished dorso-ventral sections ofconchs, all from the lower HolenLimestone Formation of
northern Oland. A, Paracyclostomiceras paucitumidum sp. nov.; SGU CeOOl, holotype; Enerum. B-c.
Microstomicerasholmigen.etsp.nov.;RMMol58700,holotype;Halludden.b, x3-75.c,detailofsame; x7.
which(asitsnameimplies)hasamoredepressedconchsectionthanP.floweri.TheSwedishspecies,
P.paucitumidumsp. nov., hasalessinflatedconchformandisdistinctlycompressedin section.
Occurrence. The genus has been recorded only from the Llanvirn ofArgentina and Sweden. The South
American specimens were collected from Rio Las Capillas and Garrapatal, Serrania de Zapla, northern
Argentina from a horizon with Hoekaspis schlagintweiti Harrington and Leanza. Associated trilobite
(Hoekaspis-Famatinolithus)andgraptolite(Didymograptusbifidus-D.climacograptoidesgroup)faunassupport
anearlyLlanvirnage(Acenolaza 1976). P.paucitumidumsp.nov.isknownfromthelowerKundanStage
(uppermostArenigorlowerLlanvirn)ofnorthernOland.
Paracyclostomiceraspaucitumidum sp. nov.
Text-figures3e-h,5a,6b
Derivationofname.FromLatinpaucusmeaninglittleandtumidusmeaningswollen,referringtotherelatively
slenderconchformofthisspecies.
Material. The holotype (SGU CeOOl) and paratype (SGU Ce002) consist ofincomplete dorso-ventrally
sectionedphragmoconeswithportions ofthebody-chamberremaining. Both specimenswerecollected by
GerhardHolmin1895fromthelowerHolenLimestoneFormation(lowerKundanStage)ofnorthernOland;
theholotypeisfromEnerum,theparatypefromHalludden.
Diagnosis. RelativelyslenderParacyclostomiceraswithcompressedsection.
Description. Conch slender, vasiform-breviconicwith conspicuously compressed section. Body-chamber in
bothholotypeandparatypeincompletebutgraduallycontractingadorad.
Holotype41-9mmlong,adoral20-5mmrepresentingbody-chamber.Remainderofphragmoconeconsisting
Loaftneirnaelccaommeprraees,sieoanchocf.c2o-0ncmhmevhiidgehntexacteappticfaolreadnodrawlhecraemedroaresow-hveenrteraslepdtiaaampetperrox1i4m-a9tmedmaanndd0-l7atmermalawpiadrtth.
12-1mm.Suturesundulatory,formingbroadandconspicuousapicallydirectedlobes;septashallowlyconcave.
Siphuncle ventral, sub-circular, 25 per cent, ofconch diameter. Septal ridges distinct, comprising sharp,
adorallydirectedraisedlines2-2mmapartonventer.Septalnecksorthochoanitic,extendingapicad0-25to0-3
camerae.Connectingringsthickandlayered(Text-fig.6b),withinnerdarklayer(adjacenttosiphuncle)and
;
344 PALAEONTOLOGY,VOLUME41
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outer, paler thickerlayer (adjacent to camerae). Fragments ofexternal shell confirm conch surface to be
virtuallysmoothwithinconspicuousfeeblegrowthlines.
Paratype similar to holotype but smaller; 31-4mm long, adoral 12-7mm representing body-chamber;
remaining 18-7mmofphragmoconeconsistingoftencameraevaryinginheightfrom1-5mmto 1-9mm.
Remarks. Paracyclostomiceraspaucitumidum sp. nov. is readily distinguished from other species
assignedtothegenusbyitslaterallycompressedconchsection.
Occurrence. Lower Holen Limestone Formation (lower Kundan Stage; Hunderumian and/or Valastean
substages)ofEnerumandHalludden,northernOland.
GenuseocyclostomicerasChen, inQietal., 1983
Typespecies.EocyclostomicerasventrumChen,inQietal. 1983;byoriginaldesignation(Chen,inQietal.,
1983,p. 305).
Remarks. Eocyclostomicerasis a poorly known genus described from incomplete, sectioned (but
apparentlygomphoceroid)phragmoconeswiththeapicalportionofabody-chamberpreservedin
theholotypeofthetypespecies.Thesiphuncleisrelativelynarrow,accountingforc.10-15percent,
ofthe conch diameter; septal necks short, probably orthochoanitic, connecting rings thick and
layered. The sub-circular shell section and structure of the siphuncle wall tentatively support
assignmentofthegenustotheCyclostomiceratidae,althoughtheoverallconchformisunknown