A new archaic homodont toothed cetacean (Mammalia, Cetacea, Odontoceti) from the early Miocene of Peru

摘要

Apartfromafewexceptions,extantodontocetes(toothedcetaceans)exhibitaroughlyhomodontdentition.Thetransitionfrombasilosaurid-likedouble-rootedcheekteethwithaccessorydenticlestosingle-rootedconicalteethoccurredduringthelateOligocene-earlyMiocene.Atthattime,severalcladesofnowextinct,homodontandpredominantlylong-snoutedodontocetesappearedinthefossilrecord.Amongthem,membersofthegeneraArgyrocetusLydekker,1893andMacrodelphinusWilson,1935,fromtheearlyMioceneoftheNortheastPacificandArgentina,weretentativelyattributedtothefamilyEurhinodelphinidae.However,duetothefragmentarystateofthespecimens,unambiguousapomorphiesofthefamilycouldnotbedetected.Basedontwowell-preservedskullswithassociatedmandibularelements,discoveredinearlyMiocenelayersoftheChilcatayFormation(PiscoBasin,Peru),wereportonanewgenusandspeciesoflong-snoutedhomodontodontocete,Chilcacetuscavirhinusn.gen.,n.sp.Characterizedby,amongothers,thepresenceofalveoliontheanteriorpremaxillaryportionoftherostrum,thelackofalateralgrooveontherostrum,anterodorsallyelevatednasals,apossiblyautapomorphiccavitybetweennasalsandmesethmoidintheposteriorwallofthebonynares,ahightemporalfossa,andtheabsenceofankylosisalongthemandibularsymphysis,C.cavirhinusn.gen.,n.sp.doesnotfitinanyoftheknownodontocetefamilies,butsharesseveralmorphologicalfeatureswithArgyrocetusspp.andMacrodelphinus.Ourphylogeneticanalysis,basedon77charactersfor35odontocetetaxa,suggeststheexistenceofanearlyMioceneEasternPacificlong-snoutedhomodontodontoceteclade(withanhypotheticalSouthAtlanticmember,thepoorlyknownArgyrocetuspatagonicusLydekker,1893),distinctfromtheonlysuperficiallysimilareoplatanistidsandeurhinodelphinids.Furthermore,ourconsensustreeindicatesanearlybranchingofthisnewcladecomparedtootherhomodontodontocetelineages.Unfortunately,theresultsofthecladisticanalysispresentedherearenotwellsupported;areappraisalofArgyrocetusandMacrodelphinusisneededtomoreclearlydefinethenewcladeandbolsteritsphylogenticposition.