Originalarticle
A delayed response of the trace fossil community at the Cretaceous-Paleogene boundary in the Bottaccione section, Gubbio, Central Italy
§Paolo Monacoa,*,Francisco J.Rodrı´guez-Tovarb,Alfred Uchmanc
aDipartimentodiFisicaeGeologia,Universita` degliStudidiPerugia,viaG.Pascolisnc,06123Perugia,Italy
bDepartamentodeEstratigrafı´ayPaleontologı´a,FacultaddeCiencias,UniversidaddeGranada,18002Granada,Spain
cJagiellonianUniversity,InstituteofGeologicalSciences,OleandryStreet2a,30-063Krako´w,Poland
1. Introduction
TheCretaceous-Paleogene(K-Pg)boundaryevent(65.5m.y.) resultedinoneofthemajorPhanerozoicmassextinctions(Schulte etal.,2010).Alargebolideimpactrepresentedbythe Chicxulub craterinYucatan,Mexico,iswidelyaccepted.Environmentaleffects of this event include acid rains,wildfires,elevated atmospheric temperatures,dustandaerosolsintheatmosphere,destructionof ozonelayer,andtheincreaseofgreenhousegases(Wolbachetal., 1985;Gale et al., 2001;Beerlinget al., 2002;Kring, 2007). The catastrophicvs.gradualdisappearanceorchangeofbiotaorrecovery of the involved communities is still a matter of debate and controversy(Kring,2007;MacLeodetal.,1997;MacLeod,1998).
Someauthors(Kring,2007;Kelleretal.,2009;Schulteetal., 2010)pointouttheabsenceofasimilarreactionofdifferentbiota attheboundary,suggestinginfluencesofseveralfactorsleadingto variable effects on marine vs. continental communities, verte- bratesvs.invertebrates,planktonicvs.benthic,K-vs.r-strategists,
ordepositvs.suspensionfeeders(PowellandMacGregor,2011;
Labandeiraetal.,inpress).Marinedeposit-feederslivingindeep- sea substrates and feeding upon detritus or dead organisms (ArthurandZachos,1987;Twichett,2006)werelessaffectedby theextinctionepisodethansuspensionandfilterfeederslivingin theshallowwaterconditions,havingmoredirecttrophiclinksto photosynthesis(SheehanandHansen,1986). However,Levinton (1996)suggestedthatextinctionlevelsofdepositandsuspension feeders are consistent with a near simultaneous extinction, possiblywithinamaximumof3to6months.
The response of marine infaunal invertebrates to the K-Pg boundaryeventcanbedecipheredfromichnologicaldata.Trace fossilanalysisofthedeep-marineK-Pgboundarysectionsfromthe Czech Republic, southern (Agost and Caravaca) and northern (Sopelana)Spain,andinsouthwesternFrance(Bidart),revealslittle ichnotaxonomicvariationbetweenpre-andpost-eventichnoas- semblages(Rodrı´guez-TovarandUchman,2004a,b;Uchmanetal., 2005;Rodrı´guez-Tovaretal.,2011;Table1).Thisindicatesaminor influenceoftheK-Pgboundaryeventondeep-seaichnofauna.In theAgostsection,isotopicanalysisoftracefossilfillingsallowed characterization of different phases of colonization during the earliest Danian (Rodrı´guez-Tovar et al., 2002,2004, 2006). The presence of iron-oxide spherules in fillings of Thalassinoides ARTICLE INFO
Articlehistory:
Received11June2014 Accepted3February2015 Availableonline23February2015
Keywords:
Ichnology
Cretaceous-Paleogeneboundary Bottaccione
Apennines Italy
ABSTRACT
Abed-by-bedichnologicalanalysisattheclassicBottaccionesection(Gubbioarea,Italy),revealsan unusualresponseofichnofaunatotheenvironmentalchangesassociatedwiththeCretaceous-Paleogene (K-Pg) boundary event. The trace fossil assemblage, consisting of Chondrites isp., Planolites isp., Thalassinoidesisp.,Trichichnusisp.,andZoophycosisp.,issimilartothatregisteredduringCenomanian andTuroniantimes,showingpersistenceofthesamecommunityforalongtimeintervalindeep-sea carbonate deposits. Absence of significant changes through theK-Pg boundaryconfirms that the extinctiondidnottouchtheichnofauna.Thedeclineoftracefossilsoccursat6cmabovetheK-Pg boundary, with the total absence of biogenic structures in a 3cm-thick layer showing parallel laminations.AgradualrecordofZoophycosandPlanolitesresumesintheoverlyinglayers.Thissuggestsa progression of inhospitable conditions for the macrobenthos, followed by a delayed response of ichnofaunatodrasticdiminutionofavailablefoodstartingabout15k.y.aftertheK-Pgboundaryevent, andthenagradualrecoveryleadingtopre-eventenvironmentalconditions.
ß2015ElsevierMassonSAS.Allrightsreserved.
§Correspondingeditor:EmmanuelFara.
* Correspondingauthor.
E-mailaddress:[email protected](P.Monaco).
Availableonlineat
ScienceDirect
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http://dx.doi.org/10.1016/j.geobios.2015.02.001
0016-6995/ß2015ElsevierMassonSAS.Allrightsreserved.
confirms nearly contemporaneous macrobenthic colonization with deposition of the red iron-rich layer (Rodrı´guez-Tovar, 2005). IntheCaravacasection,therediron-richboundarylayer atthebaseofthedarkboundarylayeris crosscutvertically by Zoophycosand Chondrites,and penetratedhorizontallybyChon- drites,confirmingrapidre-colonizationofthesubstrateaftertheK- Pgboundaryeventbyopportunistictracemakers(Rodrı´guez-Tovar andUchman,2006,2008;Ke˛dzierskietal.,2011).
Theaimofthispaperistopresentaparticularresponseofdeep- sea ichnofaunatothe K-Pgevent in theBottaccionesection at Gubbio, Italy. Our objective is not to address the catastrophic vs.gradual debate,but rathertopresentnewdatarevealingan alternativetothetraditionalhypothesesontheevolutionaryand ecological dynamics associated with the K-Pg boundary event, includingtherecoveryofmarinebiota.
2. Geologicalsetting
Fromthe numerous K-Pgboundarysections availableworld- wide,theBottaccionesectionatGubbiointheUmbrianApennines, CentralItaly,representstheclassicandprobablymostimportant andintenselystudiedone(Bonarelli,1890;Renz,1936;Luterbacher, 1964;Luterbacherand PremoliSilva,1964;PremoliSilva,2012;
Fig.1).ThissectionislocatedalongthestateroadSR298,crossingthe BottaccioneGorgeontheslopewestoftheGPSpointN43821.9230- E012834.958011m(seeCoccionietal.,2012,forageologyofthe Gubbioarea).ThissectionisareferencesectionoftheK-Pgboundary, beingthefirstwheretheiridiumanomaly,togetherwithothertrace elements, werediscovered and interpreted asevidences ofa large extraterrestrialimpact(Alvarezetal.,1980;MontanariandKoeberl, 2000;Alvarez,2009).
The Maastrichtian-Danian sediments belong to the Scaglia Rossa Formation (early Turonian-middle Eocene; Fig. 2(A)). A 117cm-thick interval of uppermost Maastrichtian, including
62cmofredtowhitecalcilutiteswithabundantmicrobioclasts, andthelowermostDanianinterval,dominatedby55cm-thick redcalcilutitesandredmarlstonesontop,werestudied.TheK-Pg boundary is marked by a 5mm-thick black to red clay layer (Fig.2(B,C)).TheK-Pgboundarylayeriswellexposedatm382.6, intheupperpartofChron29r(Coccionietal.,2012).Atthetop of the red limestones, there is a 60cm-thick bed of white pelagiclimestone(Maastrichtian)containingabundantZoophycos (MontanariandKoeberl,2000;Fig.3(G)).Thisisthefirstreportof ichnologicaldatafromthesection.
Recentbio-magnetostratigraphicstudyoftheupperCampanian- MaastrichtianfromtheBottaccioneandContessasectionsofthe Gubbiosectionsrevealeda425k.y.-longhiatuswithinChronC31n atBottaccione(wellbelowthestudiedinterval).Thesedimentation rateforChronC29r(topmostMaastrichtian)isestimatedat1.23cm/
k.y.(Gardinetal.,2012).Previousstudiesrevealedlowsedimenta- tion rates in the Gubbio area, i.e., 6–10mm/k.y. for the top of the Cretaceous and 2–5mm/k.y. for the lowermost Paleocene (MontanariandKoeberl,2000).Otherauthorsindicate4–6mm/k.y.
forthelatestCretaceousand2–3mm/k.y.fortheearlyandmiddle Paleocene (Mukhopadhyayet al., 2001; Cronholm and Schmitz, 2007).UpperCretaceousandPaleocenesedimentsfromtheGubbio, UmbriaMarcheBasinweredepositedinadeepbathyalpositionwith awaterdepthof1500–2500m,accordingtoagglutinatedforami- niferaassemblages(Kuhnt,1990;CronholmandSchmitz, 2007).
Coccioni et al. (2012) indicated that Cretaceous and Paleogene Fig.1.LocationmapoftheGubbioCretaceous-Tertiary(GBKT)boundarysection
andCretaceous-Paleogene(K-Pg)boundary.
ModifiedfromCoccionietal.(2012).
Table1
Pre-andpost-K-Pgboundarytracefossilsinselectedmarinesections.
Section Pre-K/Pg(UpperMaastrichtian)boundary
traces
Post-K/Pg(LowerDanian)boundarytraces References
StevnsKlint(Denmark) Chondrites,Thalassinoides,Zoophycos Phycosiphon(former‘‘smallChondrites-like forms),Planolites,Thalassinoides
EkdaleandBromley(1984)
MillersFerry(Alabama) Chondrites,Planolites,Thalassinoides,Zoophycos Ophiomorpha,Planolites,Thalassinoides Savrda(1993) LosRamones,ElMimbral,etc.
(NEMexico)
Chondrites,Ophiomorpha,Planolites,Zoophycos EkdaleandStinnesbeck(1998)
Agost(SESpain) Chondrites,Planolites,Zoophycos,unidentified structures
Alcyonidiopsis,Chondrites,Diplocraterion, Planolites,Thalassinoides,Zoophycos, unidentifiedstructures
Rodrı´guez-TovarandUchman (2004a,b)
Caravaca(SESpain) Chondrites,Planolites,Thalassinoides, Trichichnus,Zoophycos
Alcyonidiopsis,Chondrites,Planolites, Thalassinoides,Zoophycos
Rodrı´guez-TovarandUchman (2006)
Bidart(SWFrance)&Sopelana (NSpain)
Planolites,Thalassinoides Chondrites,Planolites,Thalassinoides, Trichichnus,Zoophycos
Rodrı´guez-Tovaretal.(2010, 2011)
Gubbio(Italy) Chondrites,Planolites,Thalassinoides, Zoophycos
Chondrites,Planolites,Thalassinoides, Trichichnus,Zoophycos
Thispaper K-Pg:Cretaceous-Paleogene;NE:northeast;SE:southeast;SW:southwest;N:north.
P.Monacoetal./Geobios48(2015)137–145 138
pelagicsuccessionsoftheContessaValleyweredepositedatmiddle tolowerbathyaldepths(1000–1500m).
3. Tracefossilassemblages
Theichnological analysesfocused on characterizationof the Maastrichtian and Danian trace fossil assemblages, based on outcrop and laboratory studies. An ichnofabric approach was difficultduetoproblemsin recognizingthebioturbationindex, crosscutting relationships or tiering patterns. Samples were collectedbed-by-bed,cutindifferentorientedsections,polished, and photographed. They are housed in the Earth Science DepartmentofPerugiaUniversity(labeledGBKT).Fiveichnotaxa wererecognized:Chondritesisp.,Planolitesisp.,Thalassinoidesisp.,
Trichichnusisp.,andZoophycosisp.(Fig.4).Poorpreservationand common stylolitizationpreventsa determinationat theichnos- pecieslevel.
Chondrites isp. appears as small groups of patches that are usuallydarkerthanthehostrock(Fig.5(B)).Thiscontrastswiththe abundantlightChondritescharacterizingtheBonarelliEventinthe same Bottaccione section (Monaco et al., 2012). Patches have circular to elliptical cross sections; short, vertical or oblique segmentswithoccasionalbranches0.5–0.8mm-wide(Fig.3(A,B, E))canbepresent.Chondritesisadeep-tiertracefossilprobably produced by an organism most likely living in dysaerobic conditions,thatis,achemosymbiont(Seilacher,1990;Fu,1991).
Chondrites is usually related to an opportunistic behavior (r-strategy)ofatracemakertolerantofverylowoxygenconditions Fig.2.TheCretaceous-Paleogene(K-Pg)boundarysectionatBottaccione.A.Generalview;theK-Pgboundaryismarkedbythedashline.Themeasuringstickis1mlong.
B.Detailoftheboundary,whichislocatedjustatthetopofthewhitecalcilutite.C.Crosssectionoftheboundaryinapolishedslab.Thalassinoides(Th)filledwithdarker sedimentandtransformedbystylolithization.Scalebars:1cm(B,C).
(BromleyandEkdale,1984;SavrdaandBottjer,1986,1989,1991;
Tyszka, 1994; Wetzel, 1994) and/or organic-rich sediments (VosslerandPemberton,1988).
Planolitesisp.isassignedtohorizontal/oblique,straight,simple flattenedcylinders.Theyhavea darkcolorthatdiffersfromthe hostsediment.Theypresentasharpboundary(Fig.3(F)),andthey areupto150mmlongwhen occurringin planesurfaces.Long specimensusuallyoccurwithanendichnial/epichnialstratinomic positioninlimestonebeds.Whenobservedincrosssections,they appear as 2–3mm-wide elliptical spots (Figs. 3(A–D, F), 5(B)).
Planolitesis a facies-crossingform,interpretedasa pascichnion (i.e.,a grazing trace), produced by a number of different soft- bodied invertebrates(Pemberton and Frey,1982; Keighley and Pickerill,1995).
Thalassinoides isp. occurs as straight, horizontal to oblique, locallyverticalbranchedcylinders, upto 20mm-wide in plane view(immediatelybelowtheK-Pgboundary;Fig.5(A)).Theyoccur usually as epichnialspecimens, although some hypichnial and endichnialpositionscanbeobservedinmanymudstones.Theycan beobservedmostlyincrosssectionsas4.5–11mm-wideelliptical spots(Figs.2(C),3(A–C)).Thalassinoidesoccursinagreatvarietyof marineenvironments, beingmostly interpretedasa domichnial (i.e.,dwelling)andfodinichnial(i.e.,feeding)structureproduced byarthropods,mostcommonlydecapodcrustaceans(Freyetal., 1984;Ekdale,1992;Schlirf,2000).
Trichichnusisp.appearsasavertical,oftenbranchedstructure.
TherarityofobliquetoflatformsintheScagliaRossaFormation, comparedtootherformationsintheNorthernApennines(Monaco and Trecci, 2014), canbe due to a slow sedimentation rate of pelagic carbonates in contrast with siliciclastic turbidites (MacBride and Picard, 1991; Monaco and Trecci, 2014). This ichnotaxonappearsasthread-likecylindersfilledwithaferrugi- nous substance, with a whitish, diagenetic halo around the cylinder(usuallylessthan1mmindiameter;Fig.3(D)).Trichich- nusis a eurybathicmarinetracefossil,commonin fine-grained deposits (Fillion and Pickerill, 1990). Trichichnus is regarded as domichnialburrowsofmarinemeiofaunaldeposit-feeders(Frey, 1970).TheproducerofTrichichnushasalsobeensuggestedasa chemosymbiont(Uchman,1995).
Zoophycos isp. is observed in cross section as horizontal or subhorizontalspreitenstripes,2–3mm-thick(Fig.3(E,F,H,J)).In the outcrop, fragments ofthe burrow systems are observed as planarspreitelobessurroundedbyamarginaltunnelandarranged inhelicalwhorls(atleasttwo)withadistinctapex(Fig.3(G)).The outerdiameterofthewhorlsatthetopofMaastrichtiancanreach 35–40cm,whilethePaleogenespecimensaresmaller,from5to 20cm (exceptionally up to 25cm). A progressive increase in dimensions and complexity of Zoophycos in the Gubbio area resumesfromtheLatePaleocene/Eocene(Fig.5(C)),andespecially inOligocenetoearlyMiocenetimes(upperScagliaRossa,Scaglia Fig.3.TracefossilsfromtheCretaceous-Paleogene(K-Pg)boundarysectionatBottaccione.Samplesindicatedinthelowerpartofthephotographs.Tracefossils:Ch:
Chondrites;Pl:Planolites;Th:Thalassinoides;Tr:Trichichnus;Zo:Zoophycos.A–D.Horizontalcrosssectionsofpolishedslabs.E,F.Verticalcrosssectionsofpolishedslabs.
G.LargeZoophycosbelowtheK-Pgboundarywithadistinctapexandatleasttwowhorls(Zo1andZo2).H.Zoophycostruncatedbystylolith;verticalcrosssectionsofpolished slab.I.SampleGBKT12withpossibleprimarylaminations(lam);verticalcrosssectionsofpolishedslab.J.Zoophycostransformedbystylolithization;verticalcrosssectionsof polishedslab.Scalebars:1cm.
P.Monacoetal./Geobios48(2015)137–145 140
Variegata, ScagliaCinerea, Bisciaro, and Marnoso-arenacea For- mations), with helicoidal to lobate forms up to 120cm wide (L. Bracchini, personal communication). Zoophycos has been interpretedasa feedingstructure (fodinichnia;Seilacher,1967;
WetzelandWerner,1981;EkdaleandLewis, 1991;Oliveroand Gaillard, 1996). Other ethological interpretations, however, consider that Zoophycos may have been produced by surface ingestors of organic detritus that accumulate fecal pellets in a Fig.4.Thestudiedintervalwithbed-by-bedindicationofsamples,rangesoftracefossilsandnumberofichnotaxa.Intervalsreferredtoichnofaunacrisesareshadowedin grey.
subsurfacestructure(Kotake,1989,1991;Lo¨wemarketal.,2007), orasacombinationstructureofdepositfeedingandchemosym- biotics(BromleyandHanken,2003).
Thecompositionofthetracefossilassemblageinthestudied intervalshowsnosignificantdifferencesbelowandabovetheK-Pg boundary(Figs.3,4).OnlythequestionableTrichichnusabovethe boundaryisabsentbelow.However,thediversityandabundance of trace fossils vary throughout the investigated interval. A concentrationoflarge,superimposedZoophycosisp.ispresentin thewhitelimestonesat thetopoftheCretaceous(inthesame stratigraphicpositionasintheContessasection,whichis2.5km away;MontanariandKoeberl,2000).Thedeclineoftracefossils (onlysporadicZoophycosand Planolites)inan 18cm-thicklevel (sample GbKT12-GbKT16A) starting at 6cm above the K-Pg boundaryis moststriking.At thebaseofthis interval, primary parallellaminationispresentinathin3cm-thicklayer(Figs.3(I), 4), in and around which trace fossils are absent. Above, the diversityincreasesanddropsagain.BelowtheK-Pgboundary,the diversitydrops inthreelevels,whichdo notshowanyprimary lamination(Fig.4).AttheK-Pgboundaryandjustaboveandbelow it,thediversityoftracefossilsishighandtheirrecordiscontinuous (Fig.4).
4. Discussion
Thestudiedtracefossilassemblage(Chondritesisp.,Planolites isp., Thalassinoides isp., Trichichnus isp., and Zoophycos isp.) is typical of the Zoophycos ichnofacies that characterizes pelagic sedimentsbelowtherangeofstormcurrentsandbeyondflysch sedimentation(WetzelandUchman,2012).Itisverysimilartothe ichnoassemblageintheBonarelliLevel(uppermostCenomanian) inthesamearea(Monacoetal.,2012),suggestingapersistenceof the same ichnofauna over a long time interval. Environmental change, such as fluctuation in oxygenation and food supply, apparentlyhadaminoreffectontheichnofauna,causing onlya variationinabundanceanddiversity,butwithoutextinction.The analysisofthetracefossilassemblagerevealsseveralsignificant points.
4.1. Rhythmicpattern
Ingeneral,thefluctuationsofdiversityandabundanceoftrace fossilsthroughthestudiedintervalshowamoreorlessdeveloped rhythmicpattern(Fig.4),whichmayberelatedtofluctuationsof environmentalconditions.Cyclostratigraphicanalysisconducted on the Contessa and Bottaccione sections revealed an orbital controlontheMaastrichtiansedimentation,withtheidentification ofprecessionandeccentricitycycles(Hussonetal.,2014).Thus,it is possible that theichnological fluctuations arerelated to the Milankovitchcyclicity,thoughthedataare,atpresent,insufficient toprovethis hypothesis.However, takingintoaccountthatthe fluctuations in trace fossil diversity and abundance are in 20–
30cm-thickintervals(Fig.4),andtherateofsedimentationduring thelatestMaastrichtianwas1.23cm/k.y.(Gardinetal.,2012),the periodicityofchangesrangesfrom16to24k.y.,fittingwellwithin theprecessioncycle.Amuchsmallersedimentationratepostulat- edforthelowerPaleocene(2–3mm/k.y.;Mukhopadhyayet al., 2001;CronholmandSchmitz,2007)stillgivesperiodicitywithin therangeofdurationoftheorbitalcycles.
4.2. Continuousdistribution
DistributionoftracefossiltaxaacrossthestudiedCretaceous- Palaeogeneintervalshowsanevidentcontinuity,withonlyabrief absenceofdiscretetracefossils(seebelow).Thisichnotaxonomic continuitysuggeststhatthecatastrophicphenomenainterpreted Fig.5.Ichnologicalfeaturesofthestudiedsection.A.Bioturbatedlimestoneatthe
top of the Maastrichtiancarbonates, with Thalassinoides (Th). B. Bioturbated limestonebedsatthetopoftheMaastrichtiancarbonates(sampleGBKT10on Fig.2),withPlanolites(Pl)andChondrites(Ch)indifferentlevels;scalebar:2cm.C.A gradualrecordoflargeZoophycos(Zo)resumesintheoverlyinglatePaleocene-early EocenelayersoftheBottaccionesection.
P.Monacoetal./Geobios48(2015)137–145 142
