Anatomical differentiation of isolated scales of amiiform
fishes (Amiiformes, Actinopterygii) from the Early Cretaceous
of Las Hoyas (Cuenca, Spain)
Différenciation anatomique d’écailles isolées de poissons ammiformes
(Amiiformes, Actinoptérygiens) du Crétacé inférieur de Las Hoyas
(Cuenca, Espagne)
Hugo Martín-Abad
a,b,c,∗aUnidad de Paleontología, Departamento de Biología, Universidad Autónoma de Madrid, Calle Darwin 2, 28049 Madrid, Spain bJURASSICA Museum, route de Fontenais 21, CH-2900 Porrentruy, Switzerland
cDepartment of Geosciences, University of Fribourg, chemin du Musée 6, CH-1700 Fribourg, Switzerland
Keywords: Scale anatomy Isolated scales Amiiformes Amia calva Early Cretaceous Las Hoyas El Montsec
a b s t r a c t
The scales of amiiform fishes are more different from each other than previously stated. The anatomy of the scales of the three amiiform taxa from the Early Cretaceous (Barremian) of Las Hoyas (Cuenca, Spain) is described in detail. The differences between them has allowed the segregation of isolated scales form the fossil record of this site into the three taxa, providing relatively large population samples that can be studied from a palaeobiological and palaeoecological point of view.
Mots clés :
Anatomie des écailles Écailles isolées Amiiformes Amia calva Crétacé inférieur Las Hoyas El Montsec
r é s u m é
Les écailles des poissons amiiformes sont plus différentes les unes des autres que ce qu’on avait précédemment affirmé. L’anatomie des écailles des trois taxons d’amiiformes du Crétacé inférieur (Barrémien) de Las Hoyas (Cuenca, Espagne) est décrite en détail. Les différences observées permettent la répartition des écailles isolées de cette localité entre ces trois taxons, ce qui permet de disposer d’un échantillonnage assez large, autorisant des études paléobiologiques et paléoécologiques.
∗ Unidad de Paleontología, Departamento de Biología, Universidad Autónoma de Madrid, Calle Darwin 2, 28049 Madrid, Spain. E-mail address:[email protected]
http://doc.rero.ch
Published in "Comptes Rendus Palevol 16(3): 257–265, 2017"
which should be cited to refer to this work.
1. Introduction
The scales of amiiform fishes have been thesubject of many studies, mostly in the classical literature. The anatomy, histologicaloriginanddevelopment,and ossi-fication process ofthescales oftheonly extantspecies ofthisorder,Amiacalva,havebeenstudiedindetailby numerousauthors,includingMackintosh(1878),Cockerell (1911),Degener(1924),Lagler(1947),Jollie(1984),and especially Kerr (1952). More recently, theirhistological microstructureandmineralizationhavebeendescribedby Meunier(1981).Grande andBemis(1998) describeand illustratetheontogeneticdevelopmentandindividualsize andshapevariationofA.calvascales.CooperandSchafer (1954),CartierandMagnin(1967)andSchiavone(1982) studiedtheageandgrowthofA.calvabymeansof identi-fyinggrowthcessationmarksontheirscales.
Schultze (1996) summarizes and discusses the phy-logenetic distribution of different scale anatomies in actinopterygian fishes, establishing different types of scales.The“amioid”typeofscalesisdescribedaselasmoid (flexible,round)withlongitudinallytoradiallyarranged ridgesorrodsontheoverlappedfield,insteadofforming circuliparalleltotheanteriormarginofthescale.“Amioid” scalesalsolackseveralstructurespresentinthescalesof otheractinopterygianfishes,suchasctenii,radii,enamel tissue,vascularcanals,thickbonybase,orpeg-and-socket articulation.Theterm“amioid”scaleshouldbeusedwith caution[hencetheusageofquotationmarkshere, follow-ingGrandeandBemis’(1998)recommendation],sincethis type ofscales is present in amiids but alsoin caturids, ionoscopids,andothernon-amioidactinopterygians(e.g., Arratia,2015;ArratiaandSchultze,2007;Schultze,1966, 1977).AccordingtoSchultze(1977),itisevenpresentin sarcopterygians.Onthecontrary,itisnotpresent,oratleast isnottheonlytypeofscalepresent,insinamiids,whichare partofAmioidea(GrandeandBemis,1998;Schultze,1996). Studiesonfossilamiiformscalesaremuchlesscommon. Schultze(1966)comparesthemorphologyandhistology ofseveralactinopterygians,includingtheamiiforms Catu-rusandUrocles(Solnhofenamia).MeunierandPoplin(1995) studiedthehistologicalmicrostructureandmineralization ofthescalesofAmiarobustafromthePalaeoceneofCernay (France),andshowedthatitispracticallythesamethan thatofthescalesofA.calva.Micklich(2012)studiedthe palaeobiologyand theageand growthcharacteristicsof Cycluruskehreri,arepresentativeofthesubfamilyAmiinae fromtheEoceneofMessel(Germany),incomparisonwith itsextantrelativeinpartbyexaminingmarkspreservedon itsscales.
On thebasis of a comparison betweenthe scalesof numerous actinopterygian groups, Schultze(1996) sug-geststhataconvergenceoccurredinthedevelopmentof thescalesofAmiidaeandCaturidae(moreparticularlythe genusCaturus),whicharemoresimilartoeachotherthan toanyotherofthegroupshestudied,including Sinami-idae.However,theactualanatomicaldifferencesbetween thescalesofthedifferenttaxabearing“amioid”-typescales haveneverbeenaddressed.Forinstance,intheir mono-graphicstudyonamiidfishes,GrandeandBemis(1998) describe thescalesofeveryamiidspeciesasbeingvery
similarorevenundistinguishablefromthoseofA.calva.Up todate,theonlysignificantanatomicaldifferencedetected isamoreovoidtosubrectangularcontourinamiidscales incomparisontoamoreroundedcontourincaturidscales (GrandeandBemis,1998;Schultze,1966,1996).
The objective of the present paper is to provide a detailed descriptionoftheanatomy ofthescalesofthe three differentamiiformtaxa fromLas Hoyasfossilsite (Spain),whichwillallowataxonomicalsegregationofthe isolated scalesfoundatthis locality. It isexpected that these descriptionswill serve asa starting point for the descriptionofthescalesofmanymoreamiiformtaxa.The finalaimofthisprojectistoofferawayoftaxonomically identifyingisolated scales,which canbefoundin fossil localities witha good qualityof preservation, including Konservat-Lagerstätten,whichcanthenprovideimportant palaebiologicalandpalaeoecologicalinformation.
2. Materialandmethods
For thepresent study, thescales of amiiform fishes fromtheEarlyCretaceous(Barremian)fossiliferous lami-natedlimestonesofLasHoyas(Cuenca,east-centralSpain) are utilized. Fossils areusually preserved completeand articulatedinthissite(e.g.,Martín-AbadandPoyato-Ariza, 2016);however,fishscales,duetotheirnature(i.e.,scales dropnaturallythroughoutfishlife,anddonotneedfrom taphonomiceventstodetachfromthebodies;fora dis-cussion onthis topic concerningboth extantand fossil fishscales,seeMicklich,2012),areoftenfoundisolated. AmiiformfishesarerelativelyrareatLasHoyas( Poyato-Ariza,2005).Onlyapproximately300outofover5000fish specimenscorrespondtoamiiforms;inaddition,alarge proportion of thesespecimens are isolated scales, thus accountingforanimportantpartoftheinformation avail-abletostudythepalaeobiologyofthesefishes.
Las Hoyas’ amiiforms belongto three different taxa, allof which arerepresented inthe siteby complete or almostcompletespecimens(Fig.1),whichalsopreserve theirscalesarticulated,thusallowingthecomparisonwith isolatedscales.Thesethree taxa havetraditionallybeen related to the three amiiform species described in the otherSpanishEarlyCretaceousKonservat-Lagerstätte,El Montsec:Caturustarraconensis,Amiopsiswoodwardi,and Vidalamiacatalunica(seeWenz,1968,1971,1988,1995; WenzandPoyato-Ariza,1994,1995).Althoughthereare clearlysomeresemblancesbetweenthespeciesofthetwo sites,adetailedanalysisofbothichthyofaunashasrevealed thatbothassociationsaremoredifferentfromeachother thanpreviouslythought(Poyato-Arizaand Martín-Abad, 2016).Afullreviewanddescriptionoftheamiiformtaxa fromLasHoyaswillbepublishedelsewhere;insummary, a detailed anatomical studyof thesefishes makesclear thatoneofthespeciesdoesnotfallwithinthe superfa-milyAmioidea(i.e.,itisprobablymorecloselyrelatedto CaturusandothertaxaofthesuperfamilyCaturoidea,or representsanewprimitive lineageofamiiforms), while theothertwoarecertainlymembersofthefamily Ami-idae(Martín-Abad,2015;Poyato-ArizaandMartín-Abad, 2016).Inholdofnewformalnamesforthesethreespecies, and for the sake of consistency, here the designations
Fig.1. AmiiformtaxafromLasHoyas:a:“Caturus”-liketaxon,specimenMCCMLH26022a;b:Newtaxon#1,specimenMCCMLH11286;c:Newtaxon#2, specimenMCCMLH28604.Scalebarsequal1cm.
Fig.1. TaxadepoissonsamiiformesdeLasHoyas:a:Caturus-liketaxon,spécimenMCCMLH26022a;b:Newtaxon#1,spécimenMCCMLH11286;c:
Newtaxon#2,spécimenMCCMLH28604.Barresd’échelle=1cm.
providedbyMartín-Abad(2015)willbeused: “Caturus”-like taxon(to refer tothespecies previously related to CaturusfromElMontsec,eventhoughitclearlydoesnot belongtothatgenus),Newtaxon#1(torefertothespecies previouslyrelatedtoAmiopsis),andNewtaxon#2(torefer tothespeciespreviouslyrelatedtoVidalamia).
2.1. Institutionalabbreviations
IEI:Institutd’EstudisIlerdencs,Lérida,Spain;MCCM:
MuseodelasCienciasdeCastilla-LaMancha,Cuenca,Spain (LH:LasHoyasCollection);MGB:MuseudeGeologiade Barcelona,Barcelona,Spain(MSE:ElMontsecCollection);
MNHN:Muséumnationald’histoirenaturelle(Institutde paléontologie),Paris,France;UAM:UniversidadAutónoma deMadrid(P-UAM-P:FishcollectionoftheUnidadde Pale-ontología),Madrid,Spain;UMMZ:UniversityofMichigan, MuseumofZoology,AnnArbor,USA.
2.2. Materialexamined
“Caturus”-like taxon (n=44): MCCM LH 520, 1176a, 2217a,6139, 6369a,7056a, 7060b,7244a, 9192,9622a, 15075b,15300,16215a,16241b,20136a,20485a,22382a, 23928b,26022b,26975b,28160a,30621,31052a/b,32032, 32056a,32057,32058,32068b,32077b,32195a,32242a/b,
32314,37166,37167,37168,37169,37170,37171,37173, 37174, 37175, 37176, 37177, 37178. Barremian of Las Hoyas(Spain).
Newtaxon#1(n=111):MCCMLH23,65b,76b,85R, 151Pa,162P,213b,234,374Rb,461,868,1082a,1275, 2147a, 2186,2318b, 2319,2401a, 5099a,6070a, 6371a, 7079a,7095b,7139b,7452,9113b,9159a,9226a,9474a, 9576b, 9621b, 9645, 11172a, 11286, 11359b, 13127a, 13360a/b, 13398b, 13657b, 15196a, 15305a, 15470a, 15583a,15783b,15827b,16040b,16257a,17139a,17274b, 18038,20008b,20192b,20226b,20241a,20263a,20312, 20620b,22115a,22385b,22600a,23062b,23322a,23354a, 23455a,23544b,23561a,23823a,26457b,27038,28235, 28258b, 28299, 28419, 28639, 29500a, 29606a, 29866, 30731a,30740a,30766a,30862,30878b,30933,30941b, 31108a/b, 31280a/b, 32022a, 32059, 32236, 32244a/b, 32313,32354b,33381b,33500b,33540b,35300a/b,36104, 37152,37153,37154,37155,37156,37157a/b,37158a/b, 37159a/b,37160a/b,37161a/b,37162,37163,37164a/b, 37165a/b.BarremianofLasHoyas(Spain).
New taxon#2 (n=29): MCCM LH 221b,2131, 2149, 4143,5389a,6866,7074a,9224a,9648a/b,13020,15835b, 16201a,17431,18017b,20602,22084b,22403a,26485b, 26530, 26772a,26924, 28604,29292b,31367b, 32023a, 32407a/b,32777,37150a/b,37151.BarremianofLasHoyas (Spain).
Caturustarraconensis(n=20):IEI800a,958,1269,1270, 2045, 3356, G 207, G616, G 624. MGB 513, 515, 551, V9633. MNHNMSE 219, 321, 434, 555, 576, 826, 872. Berriasian–ValanginianofElMontsec(Spain).
Amiopsis woodwardi (n=8): IEI 580, 5145, G 34, G 290, G 348. MGB V 9642. MNHN MSE 451, 844. Berriasian–ValanginianofElMontsec(Spain).
Vidalamia catalunica (n=8): IEI 1313, 1351, 2865. MGB 567, 568. MNHN MSE 320, 504, 848. Berriasian–ValanginianofElMontsec(Spain).
A.calva(n=3):P-UAM-P022.UMMZ190918-S, 205286-S.Extantspecies(NorthAmerica).
2.3. Terminologyusedforscaledescription
Terminologyforscalesdescriptionhasbeenextensively discussed in the classic literature (e.g.,Baudelot, 1873; Cockerell, 1911; Geinitz, 1868; Lagler, 1947; Schultze, 1966).TerminologyusedherefollowsDaniels(1996,figs. 7and8).
3. Descriptionofanatomicalfeaturesofthe amiiformscalesfromLasHoyas
3.1. Caturus-liketaxonscales
Theshapeofthescalesof“Caturus”-liketaxonisroughly thesameacrossthefishbody,beingroundedtoslightly quadrangular.Asfarascanbeseeninthefewarticulated specimensfoundtodate,thescalesareslightlysmallerat theendofthecaudalpeduncleaswellasinthe anterior-mostventralregionofthebody,betweenbothpectoralfins, butthisneedstobeconfirmedwithnew,morecomplete andbetterpreservedspecimens.
The scales of “Caturus”-like taxon (Fig. 2) have a well-defined focus, or centre of ossification, which is anoval-shaped structure fromwhere theridgesextend towardsthemarginsofthescale.Thefocusislocatedat one-thirdtoone-fourthofthelengthofthescalefromthe posteriormargin;inotherwords,theposteriorfield occu-piesone-thirdtoone-fourthofthelengthofthescale.The focusistransectedbythetransverseline,whichisalmost straight,andseparatestheposteriorfieldfromtheanterior andlateralfields.
Theridgesrunmostlyparalleltoeachotherfromthe focustowardsthemarginsofthescale;whenapproaching thefocus,someof them convergewitheach other.The ridgesarestraighterwhentheycontactthemiddleofthe focus,andmorecurvedwhentheycontactthelateraland postero-lateral edgesofit. Thereare approximatelythe sameproportionsofridgesendingattheanteriorfieldthan atanyofthetwolateralfields.Allridgespresentthesame thickness.
Crests(whichusuallycorrespondtogrowthcessation marks)arerelativelyconspicuousintheanteriorand lat-eralfields;inrelativelylargescales,thefirstandsecond crests(theclosesttothefocus)arelesseasilyidentifiable thantherest.Crestsaremoreseparatedfromeachotheron theanteriorfield,andbecomecloseronthelateralfields. Thelongitudinaldistancefromonecresttotheconsecutive onegetsmuchsmallerthefurtherawaytheyarefromthe focusintheanteriorfield.Inotherwords,crestsaremore crowdedneartheanteriormarginofthescale,especiallyin oldindividuals.Anastomoses,thatis,pointswheretwoor moreridgesconvergeandfusetoeachothertobecomeone onlyridgewiththesamethicknessthanthepreviousones, areeasilydistinguishable.Anastomosesoccurespeciallyat thecornerofthecrests,wheretheanteriorandlateralfields converge,andextendposteriorlyasmalldistancealongthe crestsinthelateralfields.Anastomosesarealso conspicu-ousinthetransverseline,wheretheyseemtoconcentrate inseveralpoints.Thefirstpointisveryclosetothefocus. Thefollowingpointscoincidewiththeposteriorendofthe crestswheretheyreachthetransverseline.Thelastcrest, whichroughlycoincideswiththeageatdeathofanygiven individual,usuallydoesnotpresentanastomoseswhereit reachesthetransverseline.Anastomosescanoccurspread
Fig.2.Scalesof“Caturus”-liketaxonfromtheBarremianofLasHoyas (Cuenca,Spain).a:DisarticulatedscalefromspecimenMCCMLH520a/b; theverticalstructureinthemiddleofthescaleisanartifactofan overlap-pingscale;b:specimenMCCMLH29589,laterallinescaleinmedialview showingthintube-likeossificationforpassageofthelaterallinesensory canal,innerornamentedsurface,andthickrimaroundtheposteriorfield;
c:schematicdrawingshowingthemostimportantcharacteristicsofthe scalesofthistaxon.Allthreescales,anteriorfacingleft.Scalebarsequal 2mm.
Fig.2. Écaillesd’unCaturus-liketaxonduBarrémiendeLasHoyas(Cuenca, Espagne).a:ÉcailledésarticuléeduspécimenMCCMLH520a/b;la struc-tureverticaledanslemilieudel’écailleestunartefactdûàuneautre écaillechevauchante;b:spécimenMCCMLH29589,écailledelaligne latéraleenvuemédialemontrantuneossificationminceenformedetube pourlepassageducanalsensorieldelalignelatérale,lasurfaceintérieure ornée,etlebordépaisautourduchamppostérieur;c:dessin schéma-tiquemontrantlescaractéristiqueslesplusimportantesdesécaillesdece taxon.Pourlestroisécailles,lapartieantérieuresetrouveverslagauche. Barresd’échelle=2mm.
acrossthesurfaceofthescale,notnecessarilyassociated withanyparticularstructure.
Theposteriorfield,whichcomprisesthesurfaceofthe scale posterior to thetransverse line, does not present ridges.Semicircularcrestsareveryconspicuousinthe pos-teriorfield; theyappear in higher number thanthat of crestsintheanteriorandlateralfields,andthusnotallof themcanbeinterpretedasgrowthcessationmarks.Apart fromthesemarks,theposteriorfieldischaracterizedby anornamentationpatternconsistinginnumerous small-sized,roundforamina,thelargestonesbeinglocatedcloser tothefocus,wheretheyseemtobemoreconcentrated. Theposteriorfieldissurroundedbyarimthatseemstobe slightlymorethicklyossified,givingtheposteriormargin ofthescaleadarkercolour;thisrimisespeciallyvisiblein theinnerviewofthescale.
In medial view,the scalespresent a thicker surface, whichcoverstheposteriorfieldandtheposteriorpartof theanteriorandlateralfieldsofthescale.Thissurfacehas anornamentationformedbysmall,tube-liketubercles.The scalesofthelaterallinebeara smallossifiedtube run-ninglongitudinallythroughoutthecentralaxisofthescale (Fig.2B).
3.2. Newtaxon#1scales
The shape of the scales is approximately constant throughout the body of New taxon #1, basicallyovoid incontourandlongerthandeep,exceptforthosescales coveringthecaudalfinrays,whichareshorterandmore circular.
ThescalesofNewtaxon#1(Fig.3)haveawell-defined focus,locatedatonethirdofthelengthofthescaleinthe posteriorpart(althoughitismoreposteriorlylocatedin veryyoungindividuals).Thetransverselinerunsfromthe focustotheposteroventralandposterodorsalcornersof thescale.Thetransverselineiswell-definedbynumerous anastomosesformingazigzag-likepattern.Theposterior fieldoccupies,thus,onethirdofthelengthofthescale.The ridgesinthisfieldrunfairlystraightfromthefocusand theanastomosesofthetransverselinetowardsthe poste-rioredgeofthescale.Theridgesinthelateralfieldsare curvedintheirorigininthefocusandanastomoses,but aremostlystraightandruntowardstheantero-dorsaland antero-ventralmarginofthescale,paralleltoeachother. Theridgesintheanteriorfield,allofthemoriginatingfrom thefocus,runmostlystraightfromthefocustotheanterior edgeofthescale.Theridgesofthelateralandanteriorfields appeartobeslightlynarrowerontheiroriginandbecome widerdistally.Thisismoreapparentintheridgescloserto thelongitudinalaxisofthescale.Crestsarehardly observ-ableintheanteriorfield,butaremoreconspicuousinthe lateralfields,andevenmoreintheposteriorfield. Anas-tomoses,usuallyassociatedtothecrestsinthelateraland anteriorfields,areveryhardtodistinguish.Thescalesofthe laterallinebearasmallossifiedtuberunninglongitudinally throughoutthecentralaxisofthescale(Fig.3B).Inmedial view,thescalespresentathickersurfacelocatedcloserto theposteriorthantotheanterioredgeofthescale.This surfaceextendsalmosthalfofthelengthofthescale,and hasanornamentationformedbysmall,tube-liketubercles.
Fig.3.ScalesofNewtaxon#1fromtheBarremianofLasHoyas(Cuenca, Spain).a:DisarticulatedscalefromspecimenMCCMLH27038;b: speci-menMCCMLH37152,laterallinescaleshowingthintube-likeossification forpassageofthelaterallinesensorycanal;c:schematicdrawing show-ingthemostimportantcharacteristicsofthescalesofthistaxon.Allthree scales,anteriorfacingleft.Scalebarsequal3mm.
Fig.3. ÉcaillesduNewtaxon#1duBarrémiendeLasHoyas(Cuenca, Espagne).a:ÉcailledésarticuléeduspécimenMCCMLH27038;b:
spécimenMCCMLH37152,écailledelalignelatérale,montrantune ossi-ficationminceenformedetubepourlepassageducanalsensorieldela lignelatérale;c:dessinschématiquemontrantlescaractéristiquesles plusimportantesdesécaillesdecetaxon.Pourlestroisécailles,lapartie antérieuresetrouveverslagauche.Barresd’échelle=3mm.
3.3. Newtaxon#2scales
The shape of thescales, elongated-ovoidin contour, remains approximately constant throughout the body of thefish,beingtheposteriormost onescomparatively shorter.
ThescalesofNewtaxon#2(Fig.4)haveawell-defined focus,locatedveryclosetotheposterioredgeofthescale. Consequently, the posterior field is very small,notably smallerthanthatinthescalesofNewtaxon#1.This pos-terior fieldis extremely thin,and it isusually not even preservedorisveryhardtoobserve.Thetransverseline iswell-definedandruns,thus,veryclosetotheposterior
Fig.4. ScalesofNewtaxon#2fromtheBarremianofLasHoyas(Cuenca, Spain).a:SpecimenMCCMLH26924,isolatedscale:b:schematicdrawing showingthemostimportantcharacteristicsofthescalesofthistaxon. Both,anteriorfacingleft.Scalebarsequal5mm.
Fig.4. ÉcaillesduNewtaxon#2duBarrémiendeLasHoyas(Cuenca, Espagne).a:SpécimenMCCMLH26924,écailleisolée;b:dessin schéma-tiquemontrantlescaractéristiqueslesplusimportantesdesécaillesdece taxon.Pourlesdeuxécailles,lapartieantérieuresesitueverslagauche. Barresd’échelle=5mm.
edgeofthescale,fromthefocustotheposteroventraland posterodorsalbordersofthescale.Theseborders,duetothe ellipsoidalratherthanquadrangularcontourofthescale, arelocatedanterior tothefocus;asa consequence,the transverselineiscurved.Allridgesstartfromthefocus, someofthem,themostlateralones,beingmuchcurvedat theirorigin.Theridgesinthelateralfieldstendtoendatthe antero-dorsalandantero-ventralcornersofthescale,and areslightlycurvedoutwards,insteadofinwardsasismore usualinthescalesofotheramiids.Intheanteriorfield,the ridgesrunstraighter,nearlyparalleltoeachother,although theyzigzagatthecrests,endingattheanteriormarginof thescale.Intheposteriorfield,theshortridgesrunfairly straightfromthefocustowardstheposterioredgeofthe scale.Crestsareclearlydistinguishableinallthefields,but especiallyintheanteriorfield.Anastomosesareveryhard toidentifyinthescalesofNewtaxon#2.Whentheyare observable,theyarelocatedatthecornersofthecrestsin thejunctionofthelateralandtheanteriorfieldsofthescale. Inmedialview,thescalespresentathickersurfacelocated closertotheposteriorthantotheanterioredgeofthescale, verysimilartothatpresentinthescalesofNewtaxon#1, butproportionallysmaller;italsopresentsan ornamen-tationformedbysmalltubulartubercles.Thescalesofthe laterallinebearasmallossifiedtuberunninglongitudinally throughoutthecentralaxisofthescale.
4. Resultsanddiscussion
Up to now, no detailed description has been pub-lished thatallowsfor aprecisetaxonomicalsegregation ofisolatedamiiformscales.Here,anumberofanatomical characteristicsofthescalesofthethreeamiiformtaxaof LasHoyas’fossilrecordarediscussedbasedonthestudyof thescalesfromarticulatedspecimens;themaindifferences aresummarizedbelow:
“Caturus”-liketaxonscalesareroundedtoquadrangular inshape.Theyarecharacterizedbythepresenceofathick rimaroundtheposteriorfield.Theposteriorfieldandthe focusareornamentedwithsmallroundforamina.Inthe posteriorfield,thecrestsareverynotoriousbutridgesare hardlyvisible.Anastomosesareclearlyvisibleoncrests.
Newtaxon#1scalesareovoidinshape.Theposterior fieldoccupiesonethirdofthelengthofthescale(lessin smallerspecimens).Thetransverselineiseasily observ-able, with numerous anastomoses. Many of the ridges runningthroughthelateralfieldsoriginateinthese anas-tomoses,insteadofinthefocus.Crestsarehardlyvisible ontheanteriorfield.
Newtaxon#2scalesaresubrectangularinshape,more roundedintheposterioredge.Thefocusislocatedvery closetotheposterioredgeofthescale,conformingtoa greatlyreducedposteriorfield.Thisposteriorfieldisvery thinlyossified.Allridgesapparentlyoriginateinthefocus. Crestsareclearlyvisible,especiallyintheanteriorfield.
Thedetailedstudyoftheanatomyofthesescaleshas allowedfortheidentificationofatotal of44individuals of“Caturus”-like taxon,111ofNew taxon#1and29 of Newtaxon#2,includingbotharticulatedindividualsand isolatedscales.
Beingabletotaxonomicallydifferentiateisolatedscales is also important from a palaeobiological and palaeoe-cological point of view.It allows us to know at which stratigraphiclevelsa given specieswaspresent, even if
Fig. 5.Isolated scales of the three amiiform taxa from the Berriasian–Valanginian of El Montsec (Lérida, Spain). a: Caturus tarraconensisMNHNMSE826a/b;b:AmiopsiswoodwardiMNHNMSE 844a/b;c:Vidalamia catalunica MNHNMSE 848a/b.Allthreescales, anteriorfacingleft.Scalebarsequal3mm.
Fig. 5. Écailles isolées des trois espèces d’amiiformes du Berriasien–Valanginien d’El Montsec (Lérida, Espagne). a : Caturus tarraconensisMNHNMSE826a/b;b:AmiopsiswoodwardiMNHNMSE 844a/b;c:VidalamiacatalunicaMNHNMSE848a/b.Pourlestroisécailles, lapartieantérieuresetrouveverslagauche.Barresd’échelle=3mm.
nocomplete or articulated material hasbeen found.In addition,itprovidesinformationaboutthetypeoffacies in which they are found; for instance, in the particu-larcaseofLas Hoyas,thestrongseasonalityinterpreted fortheWesternEuropeanBarremianlocalities(Haywood etal.,2004)resultedintwoeasilydistinguishabletypesof facies,wetanddry,whosefloralandfaunalassociations, and thus whose palaeoecology, werecompletely differ-ent(BuscalioniandFregenal-Martínez,2010).Likewise,it allowsustointerpretwhetherdistincttaxacohabitedin timeandspace,oriftheyinhabiteddifferentpartsofthe ecosystemoratdifferentregisteredepisodes.Inabroader sense, it allows us to interpret how the association of a locality evolvedin time. Moreover, it providesuseful palaeobiologicalinformationaswell,forexampleonthe fishpopulationstructureanddynamics(e.g.,Newbreyand Bozek,2003;Newbreyetal.,2008).
Interestingly enough, the scales of Las Hoyas’ ami-iformsareverysimilartothoseofthethreetaxafromEl Montsec, towhich theyhavebeen traditionallyrelated. “Caturus”-liketaxonscalesclearlyresemblethoseof Catu-rus tarraconensis (Fig. 5a) mainly in its ovoid, almost rounded contour and the presence of a thick posterior field.ThescalesofNewtaxon#1andAmiopsiswoodwardi (Fig.5b)aresimilarintheirovoid(notrounded)contour andarelativelyextendedposteriorfieldoccupyingabout onethirdofthelengthofthescale.Finally,thescalesof Newtaxon#2andVidalamiacatalunica(Fig.5c)aresimilar intheirsubrectangularcontour(especiallyintheanterior field)andtheveryposteriorpositionofthefocus,which definesaverythinandsmallposteriorfield.These similar-itiessuggestthat,eveniftheydonotrepresentthesame species,eachofthetaxafromLasHoyasisprobablyclose tooneofthetaxafromElMontsecfromasystematicpoint ofview.Inturn,thissuggestsapossibleusageofscalesfor phylogeneticalpurposes;however,inthecurrentstateof knowledge,littlecanbesaidaboutthis.
5. Conclusion
The“amioid”-typeofscalesispresentinseveralgroups ofosteichthyanfishes,includingallknownrepresentatives oftheorderAmiiformes,withthepossibleexceptionof theAsiaticsinamiids,whichhavebeenreportedtopresent both “amioid”andnon-“amioid”scales(e.g.,Grandeand Bemis, 1998). However, as presented here, the scales of amiiform fishes are much more different from each otherthanpreviouslyinterpreted.Aseriesofanatomical characteristicshavebeendetectedthathaveallowed dif-ferentiatingthescalesofthethreetaxaofamiiformfishes present intheSpanishCretaceouslocalityof LasHoyas, includingtheshapeofthecontour,positionanddefinition ofthefocusofossification,extensionoftheposteriorfield, presenceofathickerrimaroundtheposteriorfield, pres-enceofornamentationontheposteriorfield,presenceof conspicuousanastomosesinthecrestsand/orbetweenthe anteriorandlateralfieldsorthetransverseline, straight-nessofthetransverseline,conspicuousnessofcrestson theanterior,lateraland/orposteriorfields,presenceofan ornamentedsurfaceontheinnerview.
Onthebasisofthesecharacteristics,andby compar-ingisolatedscalesandscalesfromarticulatedspecimens, populationsamples of 44, 111and 29 individuals have beengatheredforthethreeamiiformtaxafromLasHoyas (“Caturus”-liketaxon,Newtaxon#1,andNewtaxon#2, respectively).Inthissense,taxonomicallyidentifiable iso-latedscaleshavegreatlyincreasedthenumberofknown individuals of thesethree species, since thenumber of articulated specimens is relatively low. In terms of the palaeobiology and palaeoecology of these fishes, larger populationsamplesopenthedoortoanalysesofpopulation dynamicsandoftheevolutionofboththefishassociation andeachparticularspeciesthroughouttheperiodoftime recordedinthelocality.
The final aimof this new lineof investigation is to provideakeyfortheidentificationofscalesofamiiform fishes based on a series of anatomical characteristics, includingtheonesalreadyhighlightedinthepresentpaper. For thispurpose, adescriptionoftheanatomy ofscales fromalargenumberofamiiformtaxawillneedtobe car-riedout,includinga highdiversityofboth amioidsand caturoids.Thissetofcharacteristicsmightproveespecially usefulinmicrovertebratesiteswherenocomplete spec-imensarefoundwitharticulatedscales,assuggestedby Arratia(2015),allowingfora taxonomicalidentification basedonisolatedscales.
Acknowledgements
FranciscoJoséPoyato-Ariza(UniversidadAutónomade Madrid,Spain)andMichaelG.Newbrey(ColumbusState University,Georgia,US,andformerlyRoyalTyrrellMuseum ofPaleontology,Alberta,Canada)areacknowledgedforthe thoroughdiscussionsonamiiformfishes.ÁngelaDelgado BuscalioniforherinputsonLasHoyas’palaeoecosystem andthescientificdirectionofLasHoyasResearchProject. JesúsMadero Jarabo(Museode lasCienciasde Castilla-LaMancha,Cuenca,Spain)fortheloaningofthematerial. Thoughtfulreviewsbytwoanonymousreviewersandthe editorialstaffsignificantlyimprovedthemanuscript.Fig.5 publishedwiththe permissionof theMuséum national d’histoirenaturelle(Paris,France).Fundingforthisproject comesfroma FPU grant providedtoH. M.-A. (AP2007-00894,MICINN,Spain).TwotravelgrantstovisittheRoyal TyrrellMuseumofPaleontologywereprovidedtoH.M.-A. byUniversidadAutónomadeMadrid.Thispaperisa con-tributiontoprojectsCGL-2009-11838(MICINN,Spain)and CGL-2013-42643P(MINECO,Spain).
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