a ceratopsian horncore from the olmos formation (early...

Porras-Múzquiz and Lehman262

ABSTRACT

An isolated supraorbital horncore collected from the Olmos Formation near Múzquiz is among the longest ever found, and records the presence in this area of a very large ceratopsid. The specimen probably pertains to a chasmosaurine, but differs significantly from the horncores in Coahuilaceratops known from the nearby Cerro del Pueblo Formation, and cannot be attributed with confidence to any other known ceratopsid.

Key words: ceratopsian, Cretaceous, Olmos Formation, Mexico.

RESUMEN

Un cuerno supraorbital aislado, colectado en la Formación Olmos cerca de Múzquiz, es uno de los más largos que se ha encontrado, y registra la presencia de un ceratópsido de gran talla en esta área. El espécimen probablemente pertenece a un chasmosaurino, pero difiere significativamente de los cuernos de Coahuilaceratops, reportado de la Formación Cerro del Pueblo, por lo que no puede ser atribuido con certeza a cualquier otro ceratópsido conocido.

Palabras clave: ceratópsido, Cretácico, Formación Olmos, México.

AceratopsianhorncorefromtheOlmosFormation(earlyMaastrichtian)nearMúzquiz,Mexico

Hector Gerardo Porras-Múzquiz1 and Thomas M. Lehman2,*

1 Museo de Múzquiz A. C., Zaragoza 209, Múzquiz, Coahuila, 26340, Mexico.2 Department of Geosciences, Texas Tech University, Lubbock, 79409-1053, Texas, USA.

*tom.lehman@ttu.edu

Revista Mexicana de Ciencias Geológicas, v. 28, núm. 2, 2011, p. 262-266

Porras-Múzquiz,H.G,Lehman,T.M,2011,AceratopsianhorncorefromtheOlmosFormation(earlyMaastrichtian)nearMúzquiz,Mexico:RevistaMexicanadeCienciasGeológicas,v.28,núm.2,p.262-266.

INTRODUCTION

In1984theHistoricMuseumofMúzquizwasestab-lished,andresidentsoftheregionatthattimewereaskedtodonateobjectsofhistoricalinteresttothemuseum.Inadditiontoculturalandarcheologicalmaterials,manyfossilsweredonated;however,mostofthesehadbeenrecoveredyearsagoandwithoutproperdocumentation.Beginningin2005,agroupoflocalamateurpaleontologists(MuseodeMúzquiz,A.C.)begancollectingadditionalspecimens,documentingandpreservingtheseinthemuseum.In2008,themuseumcollectionswereenrolledinthePublicRegistryofArcheologicalZonesandMonuments(INAH).

Amongtheobjectsdonatedtothemuseumin1984wasthesuprorbitalhorncoreofaceratopsiandinosaur(MUZ309;Figure1).Thelabelassociatedwiththespeci-menindicatesonly“OlmosFormation,SuperiorCretaceous,MúzquizCountylocality.”Cataloguedwiththespecimenwereanumberofadditionalfragmentarydinosaurbones,includingthedistalendofaceratopsianhumerus(MUZ310)andotherindeterminatefragments.Althoughtherearetwelvedinosaur-bearinglocalitiesknownintheMúzquizarea,theexactcollectionsiteforthehorncorehasnotbeenpreciselyestablished.

DinosaurboneshadbeenpreviouslyreportedfromtheOlmosFormation.In1968,afragmentaryskeletonof

A ceratopsian horncore from the Olmos Formation, Múzquiz, Mexico 263

aceratopsiandinosaurwasreportedfrom“PalaúinthecoalzoneoftheOlmosFormation”(Ojeda-Riveraet al.,1968).Thespecimenconsistedofpartsofbothhind-andforelimbs, and was tentatively identified as ChasmosaurusbyWannLangston,Jr.(Ojeda-Riveraet al., 1968; their fig. 8).However,thepresentwhereaboutsofthespecimenanditsoriginalcollectionsiteareunknown.Meyeret al.(2005)describedfootprintsoftheropoddinosaursandmentionedtheoccurrenceofhadrosaurandankylosaurbonesintheOlmosFormation.Kirklandet al.(2006)describedapar-tialskeletonofthehadrosaurKritosaurussp.,andnotedtheoccurrenceherealsooftyrannosauriddinosaurteeth.TheMúzquizhorncoreisofinterestthereforeinproperlydocumentingthepresencehereofceratopsiandinosaurs.Althoughthisisanisolatedfragmentaryspecimen,itisuse-fultodescribeandillustrateitforpurposesofcomparisonwithceratopsiansfromnearbyandcorrelativedeposits.

The fossil flora of the Olmos Formation is much better knownthanitsfossilvertebratefauna,andincludesawidevarietyofferns,conifers,andangiosperms(Weber,1978;Cervallos-FerrizandRicalde-Moreno,1995;Estrada-Ruizet al.,2007,2008,2010).SomeofthefossilwoodtypesfoundintheOlmosFormationhavealsobeenfoundintheAgujaandJavelinaformationsinBigBendNationalPark,about250kmnorthwestofMúzquizinTexas(WheelerandLehman,2009).MarineinvertebratesindicatethattheOlmosFormationisLateCampaniantoEarlyMaastrichtianinage(reviewedbyKirklandet al., 2006). The flora suggests correlationoftheterrestrialfaciesoftheOlmosFormation

withthelowerpartoftheJavelinaFormationoruppermostpartoftheAgujaFormationinTexas,andaccordsbestwithanearlyMaastrichtianage.

DESCRIPTION

TheMúzquizhorncoreisbrokenintofoursegments,butthesecanbejoinedtoshowitsentireform(Figure1).Themedialsideiswellpreservedandexhibitsthelongitu-dinalvascularimpressionstypicalofceratopsidhorncores.Thelateralsidewasnodoubtexposedwhenthespecimenwasdiscovered,andmuchofthecorticalbonehasbeenweatheredoffofthatside.Thebaseofthehorncorehasaspongiosa-filledcentralaxis,andislackingthedeepcornualsinusfoundinmanyotherlargeceratopsids(e.g.,Farke,2006).Itspreservedlength,asrestored,is95.2cm(straight-linedistance).

Althoughtherimoftheorbitisnotpreserved,sev-eral features allow for identification of the medial, lateral, anterior,andposteriorsurfaces.Inmostorallceratopsids,theposteriorbaseofthehorncoresmoothlycontinuesthecurvature of the rear flange of the postorbital where it ap-proachesthesquamosalsuture.Incontrast,theanteriorbaseofthehorncoretypicallyendsinasharpanglewitharugosesurfacewhereitissuturedtothepalpebral,producinganantorbital‘buttress’(e.g.,Lehman,1989).Thispermitsidentification of the anterior and posterior surfaces of the Múzquizspecimen(Figure1).

Figure1.TheMúzquizhorncore(MUZ309)alargeleftsupraorbitalhorncorefromachasmosaurineceratopsidin(a)lateral,and(b)medialview.

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onlyfragmentsofthesupraorbitalhorncoreswererecoveredwiththetypespecimen(CPC276),andtheauthorsrestoredthehorncorestoalengthofonlyabout60cm(Loewenet al., 2010; their fig. 7.4), substantially smaller than in the Múzquizspecimen.Moreover,oneofthehorncoresinthetypespecimenhasacornualsinus,andabasalwidthratio(0.77)muchgreaterthanintheMúzquizspecimen.TheauthorsindescribingCoahuilaceratopswerealsouncer-tainastowhetherthehorncoreswerecurvedanteriorlyorposteriorly.Theauthorsrestoredthehorncoreswithanteriorcurvature(Loewenet al., 2010; their fig. 7.4).

Thelateralsideofthehorncoreinmostceratopsidsis flattened above the dorsal rim of the orbit, whereas the medial side flares away from the horncore to form the roof ofthefrontalsinus(or‘postfrontalfontanelle’ofauthors,e.g., Lehman, 1989). This permits identification of the inner andoutersidesofthehorncoreinMUZ309(Figure1).Iftheinnerandouter,forwardandrearsidesoftheMúzquizspecimen are correctly identified as described above, then thisisaleftsupraorbitalhorncoreandit isposteriorlycurved,ratherthananteriorlycurvedasismoretypicalforlargeceratopsids.

DISCUSSION

TheMúzquizhorncoreisverylong,aconditiontypicalforsupraorbitalhorncoresofchasmosaurineceratopsidsbutalsoknowninbasalcentrosaurinessuchasAlbertaceratops(Ryan,2007).Thishorncoreis,however,longerthaninanyceratopsianotherthanthelargestknownspecimenofPentaceratops(OklahomaMuseumofNaturalHistory,OMNH10165;Lehman,1998).Thehornappearstolackacornualsinus,aconditionamongchasmosaurinesonlyfoundinChasmosaurus(otherrecentlynamedtaxasuchas‘Agujaceratops’, ‘Mojoceratops’,‘Utahceratops’,and‘Vagaceratops’aresimilarinthisrespect,andperhapscongenericwithChasmosaurus;e.g.,Paul,2010).Aru-dimentarycornualsinusisfoundinAnchiceratopsandinPentaceratops(Farke,2006).BecausethebaseoftheMúzquizhorncoreisbroken,itwouldhavebeensomewhatlongerthanpreserved,andsoitispossiblethatasinuscouldhavebeenpresentintheunpreservedpart.However,thedegreeofexpansionshowninthepreservedbasesuggeststhatverylittleismissing,andweconsideritunlikelythatacornualsinuswaspresent.Iftheorientationsuggestedaboveiscorrect,thehornisposteriorlycurved,aconditionknownonlyinChasmosaurus(andcloselyrelatedtaxalistedabove).

TheMúzquizhorncoreisalsoquiteslenderforitslength(Figure2;basalanteroposteriorwidth=20.8cm,transversewidth=12.2cm).Theratioofthebasalanteropos-teriorwidthtothelengthofthehorncore(0.22)islowerthaninanychasmosaurineforwhichmeasurementsareavailable,otherthanthelargestknownspecimenofChasmosaurusmariscalensis(TMM43098-1).Largechasmosaurines,suchasTriceratopsandTorosaurustypicallyhavehorncoresthatarerelativelymuchwideratthebase.Thebaseofthehorncoreisextendedposteriorlyinlargechasmosaurines(Triceratops,Torosaurus,andPentaceratops)suchthatthebasaltransversetoanteroposteriorwidthratioislow(<0.5).TheMúzquizhorncoreretainsarelativelyhighbasalwidthratio(0.59;Figure3).

ThesizeandformoftheMúzquizhorncore,aswellastheproximityofitscollectionsite,suggestthatitmightpertaintoCoahuilaceratopsfoundintheCerrodelPuebloFormationnearSaltillo(Loewenet al.,2010).However,

Figure2.Bivariateplotshowingtherelationshipbetweenbasalantero-posteriorwidthandlengthofsupraorbitalhorncoresinchasmosaurineceratopsidsforwhichmeasurementsareavailable.DataforChasmosaurusarefromLehman(1989;C.mariscalensis, filled circles) and Godfrey andHolmes(1995;C.russelli,graysquares);AnchiceratopsfromBrown(1914,opendiamonds);ArrhinoceratopsfromParks(1925,blacksquare);PentaceratopsfromLehman(1998;graydiamonds);TriceratopsfromHatcheret al.(1907;darktriangles);TorosaurusfromColbertandBump(1947;stars);comparedwiththeMúzquizhorncore(MUZ309;opencircle).ThehorncorelengthforCoahuilaceratopsisestimatedbasedontherestorationgivenbyLoewenet al.(2010;questionmark).Drawingsshow variation among chasmosaurine supraorbital horncores (modified fromLehman,1990),comparisonwiththeMúzquizhorncoreandalter-nateinterpretationsshowinganteriorly-orposteriorly-curvedorientation(toscale).

A ceratopsian horncore from the Olmos Formation, Múzquiz, Mexico 265

Al though t he sup rao rb i t a l ho rnco re s i nCoahuilaceratopsaresmaller,haveagreaterbasalwidthratio,andpossessacornualsinus,thesedifferencescouldbewithintherealmofindividualontogeneticvariationob-servedinotherchasmosaurines(e.g.,Lehman,1989,1998).Iftheanteriorlycurvedorientationshownintherestora-tionofCPC276ismistaken,oriftheposteriorlycurvedorientationsuggestedhereforMUZ309isincorrect,thentheMúzquizspecimencouldpertaintoCoahuilaceratops;albeitprobablyfromamuchlargerindividual.However,thetypeskullofCoahuilaceratopsisalsofromaverylargematureanimal,andmagnetostratigraphyoftheCerrodelPuebloFormationsuggeststhatthesestrataareprobablyolderthantheOlmosFormation(e.g.,Eberthet al.,2004).Therefore,itseemsdoubtfulthattheMúzquizspecimenbelongstoCoahuilaceratops,anditmayinsteadrepresentadistincttaxon.

CONCLUSIONS

TheMúzquizhorncoreisverylongandslender,andprobablypertainstoachasmosaurineceratopsid.Itsgreatlength,lowbasalwidthratio,apparentposteriorcurvature,andlackofacornualsinus,suggestthatitcannotbeat-tributedtoCoahuilaceratopsmagnacuernaknownfromthenearbyCerrodelPuebloFormationinSaltillo.ItisalsoclearthattheMúzquizspecimenpertainstoaspeciesunlikeeitherChasmosaurusmariscalensis(Lehman,1989)

orTorosauruscf.utahensis(HuntandLehman,2008)theonlytwoceratopsiansthusfarknownfromthenearbyAgujaandJavelinaformationsinTexas.Inalllikelihoodthehorncore belongs to an unknown taxon. This could reflect eithermarkedprovincialityorenvironmentalsegregationofhorneddinosaursatthattime(e.g.,Lehman,1997)orpossiblythattheOlmosFormationwasdepositedduringatimenotrepresentedbytheCerrodelPuebloFormationortheAguja-JavelinasuccessioninTexas.

ACKNOWLEDGMENTS

WethankDr.CiprianoPortales,presidentofMúzquizmunicipality,forhissupportofthisresearch,Drs.MarkLoewenandAndrewFarkeforhelpfulcommentsthatim-provedthecontentofthispaper,andDr.WannLangston,Jr.forprovidinginformationaboutthehistoryofcollectingintheOlmosFormation.

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Figure3.Bivariateplotshowingtherelationshipbetweenbasalwidthratio(transverse/anteroposteriorwidth)andlengthofsupraorbitalhorncoresinchasmosaurineceratopsidsforwhichmeasurementsareavailable.DataforChasmosaurusarefromLehman(1989;C.mariscalensis, filled circles) and GodfreyandHolmes(1995;C.russelli,graysquares);PentaceratopsfromLehman(1998;graydiamonds);TriceratopsfromHatcheret al.(1907;darktriangles);TorosaurusfromColbertandBump(1947;stars);com-paredwiththeMúzquizhorncore(MUZ309;opencircle).ThehorncorelengthforCoahuilaceratopsisestimatedbasedontherestorationgivenbyLoewenet al.(2010;questionmark).

Porras-Múzquiz and Lehman266

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Manuscriptreceived:February21,2011Corretedmanuscriptrecived:April11,2011Manuscriptreceived:April26,2011