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Geosciences2020,10,39;doi:10.3390/geosciences10020039www.mdpi.com/journal/geosciences
Article
Ichnofossils,CracksorCrystals?ATest
forBiogenicityofStick‐LikeStructuresfromVera
RubinRidge,Mars
AndreaBaucon
1,
*,CarlosNetoDeCarvalho
2,3
,FabrizioFelletti
4
andRobertoCabella
5
1
DISTAV,UniversityofGenova,CorsoEuropa52,16132Genova,Italy
2
GeologyOffice,NaturtejoUNESCOGlobalGeopark,AvenidaZonaNovadeExpansão,
6060‐101Idanha‐a‐Nova,Portugal;carlos.praedichnia@gmail.com
3
InstitutoD.Luiz,UniversityofLisbon.FaculdadedeCiênciasdaUniversidadedeLisboa,
CampoGrandeEdifícioC1,Piso1,1749‐016Lisbon,Portugal
4
DipartimentodiScienzedellaTerra‘A.Desio’,UniversitàdegliStudidiMilano,ViaMangiagalli34,
20133Milano,Italy;fabrizio.felletti@unimi.it
5
DISTAV,UniversityofGenova,CorsoEuropa52,16132Genova,Italy;cabella@dipteris.unige.it
*Correspondence:andrea@tracemaker.com
Received:8October2019;Accepted:27December2019;Published:21January2020
Abstract:NewimagesfromMarsroverCuriositydisplaymillimetric,elongatestick‐likestructures
inthefluvio‐lacustrinedepositsofVeraRubinRidge,thedepositionalenvironmentofwhichhas
beenpreviouslyacknowledgedashabitable.Morphology,sizeandtopologyofthestructuresare
yetincompletelyknownandtheirbiogenicityremainsuntested.Hereweprovidethefirst
quantitativedescriptionoftheVeraRubinRidgestructures,showingthatichnofossils,i.e.,the
productoflife‐substrateinteractions,areamongtheirclosestmorphologicalanalogues.Crystal
growthandsedimentarycrackingareplausiblenon‐biologicalgeneticprocessesforthestructures,
althoughcrystals,desiccationandsyneresiscracksdonottypicallypresentallthemorphological
andtopologicalfeaturesoftheVeraRubinRidgestructures.Morphologicalanalogydoesnot
necessarilyimplybiogenicitybut,giventhatnoneoftheavailableobservationsfalsifiesthe
ichnofossilhypothesis,VeraRubinRidgeanditssedimentaryfeaturesarehererecognizedasa
privilegedtargetforastrobiologicalresearch.
Keywords:palaeontology;ichnology;ichnofossils;pseudofossils;astrobiology;biosignatures;Mars;
VeraRubinRidge;GaleCrater;Haroldswick
1.Introduction
NewobservationsatVeraRubinRidgebytheMarsSpaceLaboratoryRoverCuriosityshow
millimetric,elongatestructurespreservedinsedimentaryrocksdepositedinfluvio‐lacustrine
environmentswithinGaleCrater.OnEarth,suchsettingsareinhabitedbyanenormousdiversityof
macro‐andmicroorganismsproducingtracesoflife‐substrateinteractions[1,2]thataresimilartothe
structuresofVeraRubinRidge.
Eversincetheywereannounced,thestructuresfromVeraRubinRidgehavebeencontroversial.
Inablogpost,NASAscientistslabelledthestructuresasenigmaticstick‐likefeatures,proposingan
abioticoriginaserosion‐resistantmineralveins[3].Bycontrast,DiGregorio[4]suggestedapotential
biogenicnatureasthefossilizedproductsoflife‐substrateinteractions(ichnofossils).Thisbiogenic
hypothesiswasinformallydisputedinasetofblogpostsincludinginterviewsofEarthandplanetary
scientists[5–7].Despiteofthisdebate,morphology,sizeandtopologyofthestick‐likestructuresare
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yetincompletelyknown,andtheirpotentialbiogenicityremainsuntested.Todate,thereisnopeer‐
reviewedpaperdiscussingtheabiotichypothesisforthestick‐likestructures.Inthispaperweaimto
fillthesegapsbytestingtheirbiogenicity.Accordingly,thenullhypothesisisthattheobserved
featuresareabioticinorigin;thealternativehypothesisisthattheiroriginisbiological.
AllthestructuresthatwedescribeherecomefromtherocktargetHaroldswick,whichislocated
onthenorthernfoothillofAeolisMons(Mt.Sharp)atVeraRubinRidge,withinGaleCrater.Vera
RubinRidgeisanelevatedcrest,250mwide,onAeolisMons[8,9].Itischaracterizedbyahematite
signatureinspectra,forwhichreasonitisalsoknownasHematiteRidge[9,10].TheVeraRubinRidge
consistsofMurrayformationdepositsthatexperiencedenhancedcementationresultinginits
preservationasaridge[11].Understandingtheoriginoftheridgehasbeenakeyobjectiveforthe
MarsScienceLaboratorymissionsincebeforelanding[12].VeraRubinRidgehasbeenrecognizedas
aniron‐bearingenvironmentappropriateforbiosignaturepreservation[13].
Thestick‐likestructureshavebeenimagedbytheMarsScienceLaboratoryroverCuriosity
duringMartiandays(Sols)1905,1921and1923.Theirhostrockpertainstothefluvio‐lacustrine
MurrayFormation,whichisevidenceofthepresenceofancientlakesintheGalecraterandindicates
theexistenceofhabitableenvironmentsbetween~3.8billionand3.1billionyearsago[14–18](fora
geologicaloverview,seealso[16,17,19,20]).
2.MaterialsandMethods
SourceimageshavebeenacquiredbyroverCuriosityusing(1)MastcamduringMartiandays
(Sols)1905and1921,(2)ChemCamonSol1921,(3)MarsHandLensImager(MAHLI)onSols1922
and1923.Imageswithstick‐likestructureshavebeenanalysedwiththeimageanalysissoftwareFiji
1.51w(http://fiji.sc),animageprocessingpackagebasedonImageJ.Resultsofwidth,lengthand
angleanalysisarelistedintheAppendixA,FiguresA1–A3andTablesA1–A3.Sizeofthestructures
havebeenderivedfromsourceimagesusingtheMAHLIequations[21].
3.Results
11stick‐likestructuresarepreservedaserosion‐enhancedreliefswithinasiltstoneoutcrop
(Figure1:specimenss1tos11).Theyconsistofhorizontal,straightsegmentsthatarefrequently
connectedbycurvedturns(Figure2a).Structuresareunlined.Contactwiththehostrockissharp,
buttherearenomanifestgrain‐sizedifferencesbetweenthestick‐likestructuresandthehostrock.
OurinvestigationsusingimageanalysisandMAHLIequations[21]showanaveragewidthof0.7
mm.Theendsoffewstructures(s1,s11)appeartaperedbutitisunclearwhethertaperingresults
fromprimary(i.e.,structure‐producing)orsecondary(e.g.,weathering,fragmentation)processes;as
such,endsofthesestructureshavebeenexcludedfromimageanalysis.Accordingly,eachstructure
retainsaconstantwidthalongitslength(averageper‐specimenvariationinwidth:0.1mm).Widths
tendtobeconstantalsobetweenstructures(widthrange:0.5–0.9mm).Resultsofimageanalysisshow
anaveragelengthof4mm,butthisvalueunderestimatestheoriginallengthbecausestick‐like
structuresshowclearevidenceoffragmentation(Figure2a).Someofthestructuresintersect
(specimenss4–s5;s8–s9;s10–s11;Figure2a–c)orcoalescewithoutintersecting(structuress1–s5;
Figure2a).Averageintersectionangleis63.1°.SeeappendixAforadditionalinformationabout
morphometricanalysis(FiguresA1–A3,TablesA1–A3).
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Figure1.Thestick‐likestructuresatVeraRubinRidge,Mars.Labelsindicateindividualspecimensof
stick‐likestructures(s1‐s11)andbulbousstructures(b).OutcropimagedbyroverCuriosityusing
MAHLI(image1922MH0001520010703174C00_DXXX).
Figure2.Stick‐likestructuresfromMars.(a)Stick‐likestructures4intersectingtheS‐shapedforms5,
whichchangesitscurvatureatshortdistancefromspecimens1.(b)T‐junctionformedbyspecimens
s8ands9.(c)Intersectionbetweenspecimenss10ands11.SeeFigure1forspecimennumbers.
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4.Discussion
Inordertotestthenullhypothesis(abioticorigin),thestick‐likestructuresarecomparedwith
similarnon‐biologicalstructuresfoundonEarthandknowntooccuronMars,accordingtotheir(a)
morphology,(b)size,(c)geologicalcontextand(d)topologicalinterrelationships.
Themorphologicalfeaturesofthestick‐likestructuresaresharedwithcrackfills.Desiccation
cracksareacommoncomponentofcontinentaldepositsonEarth(Figure3a),whereastheyhavebeen
reportedonMarsfromtheMurrayFormation[14]andMeridianiPlanum[22].Martiandesiccation
cracksmeetorthogonally,formingT‐junctions[14];thiscomparesfavourablywiththeT‐junction
observedinapairofstick‐likestructures(s8,s9;Figure2b).Subaqueoussedimentarycracks,also
knownassynaeresiscracksorintrastratalshrinkagecracks[23,24],havebeendocumentedonMars
intheSheepbedmudstone[25].SimilarlytotheVeraRubinRidgestructures,subaqueous
sedimentarycrackscanformincompletepolygonsconsistingofcurved(curlicue‐like;Figure3b),at
timesbranchedelements;unlikethestick‐likestructures,theyareusuallyspindle‐shapedandcan
presenttaperedtips[23](Figure3b–d).OnEarthtaperingisanimportantfeatureofichnofossil‐like
structuresbecauseitarguesagainstanichnologicalorigin[26].MostoftheVeraRubinRidge
structuresarenottapered;theonlyexceptionisrepresentedbyspecimenss1ands11,butitisunclear
whethertheirnarrowtipsresultedfromprimary(i.e.,structure‐producing)orsecondary(e.g.,
weathering,fragmentation)processes.
Thecommonlysteepflanksofthestick‐likestructures(Figure2a–c)areconsistentwithcrack
fills(Figure3b–d).Becauseofmorphologicalsimilaritiesandgeologicalcontext,sedimentarycracks
areaplausibleanalogueforthestick‐likestructures.OnEarth,formationofcracksiscommonly
mediatedbymicrobialmats,whereasonMarscracksareassociatedtostructuresresembling
microbialites[27].Itshouldbehowevernotedthatabioticprocessescanexplaintheproductionof
cracks[23,24].
Non‐sedimentarycrackingisanadditionalcandidateprocesstoexplainthestick‐likestructures
becauseoftheabundantfracturesobservedonMars,wheresulfate‐mineralizedfracturesattributed
tohydraulicfracturingarefoundintheMurray[14]andYellowknifeBayformations[16].The
immediatesurroundingsofthestick‐likestructuresshowabundantnon‐sedimentarycracks,but
theirsize,lackoffillandorientationaremarkedlydifferentwithrespecttothestick‐likestructures.
Filledfracturescanalsoberejectedbecauseoftheirtypicaltabularshape,whereasstick‐like
structuresareroughlyprismatic.Tensiongashesarealsoexcludedbecausetheyhavetaperedends
andbecausetheytendtoformconjugatesets.
Figure3.Sedimentarycracksandtheirsalientfeatures.(a)Mudcracksshowingtapering(Ta)andT‐
junctions(Tj).GorgesdeDaluis,France(Permian).(b)Shrinkagecrackswithcurvedshape(Cu),
taperededges(Ta)andsteepflanks(Sf).Kalshanehsection,Tabas,CentralIran(LalunFm.,Lower
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Cambrian).(c)Shrinkagecrackswithsteepflanks(Sf).ValedaMuasection,Portugal(Armorican
QuartziteFormation,LowerOrdovician).(d)Shrinkagecrackswithsteepflanks(Sf)andT‐junctions
(Tj).PenhaGarciaIchnologicalPark,Portugal(Lower‐to‐MiddleOrdovician).
Thestick‐likestructuresmightbeexplainedassinglemineralcrystalsbecauseoftheirelongate
shape,thecommonlypolygonalcross‐sectionandtheflatfacesmeetingat~90°.Thesefeaturesare
shared,forinstance,withgypsumcrystalsgrowninsabkhaenvironmentsonEarth(Figure4a,b;see
alsoFigures9aand11ain[28]).Cross‐cuttingbetweens4ands5remindsofpenetrationtwinning,
i.e.,theprocessbywhichmineralsintergrowandresultininterpenetratingindividuals[29].Curved
morphologyofthestick‐likestructuresdoesnotdisprovethenull(abiotic)hypothesissincecurved,
needle‐likegypsumcrystalsofcomparablesizehavebeenreportedfromterrestrialevaporitic
settings[30,31].Sulfateminerals,includinggypsum,havebeendocumentedontheMartiansurface
withdatafromsatellites,landers,androvers[32].Amongotherprocesses,curvatureofmillimetre‐
sizedgypsumcrystalscanresultfromimpurityincorporationfollowedbycrackformationand
mechanicaltwinning[33].Nevertheless,euhedralmonocrystalsandtheirpseudomorphsarenot
perfectanaloguesofthestick‐likestructures.Infact,thetipsofmoststick‐likestructuresdonotshow
thesmoothflatfacesandthesharpcrystal‐likeoutlinesthattypifyeuhedralmonocrystals[29]and
theirpseudomorphs(Figure4c,d).Itshouldbenotedthattipsofspecimenss4–s6aresmoothand
flat,buttheycouldreflectsecondary(i.e.,fragmentation)processes.Inaddition,cleavageisnot
detectable.Forinstance,thereisnoevidenceoftheeasy[010]perfectcleavagethatistypicalof
gypsumandanhydrite[29](Figure4a).
Tectographs,i.e.,pseudofossilsproducedbythemovementofrocklayersrelativetoeachother
[34],canberejectedasanexplanationfortheVeraRubinRidgestructuresbecausethestick‐like
featuresarenotparallelbetweeneachother.
Figure4.Crystalsandcrystalpseudomorphs.(a)Exhumedlargegypsumcrystalsfromsabkha
deposits.Crystalsshowsanelongateshapeandthetendencytosplitalongdefiniteplanesof
weakness(cleavage:Cl).Gypsumcrystalsformedearlierthanthosedevelopingtoday.SabkhaofBarr
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AlHikman,SultanateofOman.Largercrystalsare12cmlong.Imagefrom[35].(b)Nestoffibrous
gypsumcrystals10–20cmbelowthesabkhasurface.Gypsumcrystalsshowanelongateshape(El).
SabkhaofBarrAlHikman,SultanateofOman.Imagefrom[35].(c)Computedtomographyscan(CT‐
scan)ofcrystalpseudomorphs.Crystalsshowapolygonalcross‐section(Po).Largearrowsshow
pointedspursclosetosomevertices,whichareatypicalfeatureofaragonitecontacttwins.3.48Ga‐
oldDresserFormation(PilbaraCraton,Australia).Imagefrom[36].(d)Glendonitemouldsfromthe
shallowmarineRurikfjelletFormation[37]showingsharpcontactwiththehostrock(Sh).Early
CretaceousofSvalbard.
Althoughthenullhypothesis(abioticorigin)cannotbereadilydisproved,itshouldbenoted
thatthemorphologyofthestick‐likestructuresresemblesthatofhorizontalburrowssuchas
Helminthoidichnites(Figure5a)andPlanolites,whicharecommoncomponentsoffluvio‐lacustrine
settingsonEarth[38–40].ConstantwidthisatypicalfeatureofburrowsonEarth(Figure5a–d)since
tracemakerstendtominimizeenergyexpenditure,producingburrowsaswideastheirwidth[41].
Stick‐likestructuresarewithinthesizeandshaperangeofichnofossilsonEarth(seeFigure5).Most
specimensfromVeraRubinRidgepresentpolygonalcross‐sections,assuggestedbyfragmented
specimensandtheprevailinglyflatuppersurfaces(Figure2a–c).Polygonalcross‐sectionis
apparentlynon‐adaptiveforburrowingbehaviour,buttherearefewexamplesofichnofossilson
Earthwithsquarishsections,e.g.,Arthrophycus[42],(Figure5e,f)Bulbichnnus[43]andOblongichnus.
Subcylindricalspecimenswithroughlyellipticalcross‐sectionarealsofound(e.g.,s2),althoughthey
arerarerthanpolygonalones.
Figure5.Ichnofossilsandtheirsalientfeatures.(a)Fossilburrows(Helminthoidichnites)showing
constantwidth(Co),curvedmorphology(Cu)andcolorcontrastwiththehostrock(Cc).Upper
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CarboniferousofthePramolloBasin,Italy‐Austria[39,40].(b)Fossilburrows(Taenidiumisp.)
displayingconstantwidth(Co)andintersections(In).CortePereirosection,TocadaMouraComplex,
Ossa‐MorenaZoneinsouthernPortugal(Carboniferous:Mississipian).(c)Helminthoidichnitesand
Cruzianaproblematicadisplayingconstantwidth(Co)andsteepflanks(Sf).IchnologicalParkofPenha
Garcia,Portugal(Ordovician).(d)Nereites(=Helminthoida)irregularisshowingconstantwidthand
thigmotaxis,i.e.,abehaviouraccordingtowhichanorganismstaysclosetoanexistingentity.Piani
diCreto,Italy(Cretaceous).Specimen24175/TF16oftheUniversityofGenova.(e)Arthrophycus
alleghaniensisdisplayingpolygonal,squarirshcross‐section(Po).Mação,Portugal(LowerOrdovician).
Specimen13,184oftheGeologicalMuseumofLisbon[44].(f)Detailofthesquarecross‐section(dashed
lines)ofArthrophycusalleghaniensis.ThemagnifiedareaisindicatedbyPoinFigure5e.
Moststick‐likestructuresdonotgeometricallyinteractwitheachother,butsomepairsare
coalescingandintersecting.AprominentexampleconsistsofaS‐shapedstructure(s5)thatabruptly
changesitscurvatureatshortdistancefromanotherstick‐likefeature(s1),stayinginclosecontact
withitbutwithoutintersecting(Figure2a).Thisindicatesthatthedevelopmentofstick‐like
structurescanbeinfluencedbyexisting(syngenetic)ones.OnEarth,thisisatypicaloutcomeoftaxes,
i.e.,theorientationandturningofanorganisminresponsetoaperceivedexternalstimulusfield[45].
SimilarlytotheichnotaxaHelminthoidichnites(Figure5a),HelminthorhapheandNereites
(=Helminthoida)irregularis(Figure5d),theS‐shapedstructurefromVeraRubinRidgecanbemodelled
byusingathigmotaxisrule,compellinganagenttostayclosetoanexistingentity[46,47].
Nevertheless,evaporiticmineralscangrowveryrapidly,henceweexpectthatacrystalcanactasa
physicalconstraintforthegrowthofsuccessive,almostsyngenetical,ones.Thisphenomenoncould
replicatethigmotaxisbehaviour,posingsignificantproblemsindisprovingthenull(abiotic)
hypothesis.
Soft‐sedimentdeformation,fluidescapingandsedimentarycrackingmayaccountforthe
genesisofichnofossil‐likesedimentarystructures[48]butsuchabioticexplanationsoftheobserved
thigmotaxis‐liketopologicalrelationshiparedifficulttoproveforthoseprocesses.Verticalfluid
escapecanproducetinyverticaltubes(pillars)slicingsteeplythroughmassiveorlaminatedsand
[49].Alternatively,sand‐filledfractures(sanddykes),commonlyrootedinastructurelesssandbed,
canbeformedbytheintroductionofmaterialfromabove,eitherunderpressureorbysimplefilling
ofpre‐existingcracksorfissures.Sanddykesarestraighttosinuousinverticalview;theirsinuosity
canbeincreasedbypost‐depositionalcompaction,withthedevelopmentofptygmaticfolds[50].
However,theseprocessesarenotknowntoproducethetopologicalrelationshipsobservedin
Martianspecimens.Similarly,toourknowledgetherearenoexamplesofsedimentarycracksthat
changetheircurvaturewhenencounteringothers.
TheS‐shapedstructureiscross‐cutbyanotherstick‐likefeature(Figure1:s4),showingthatthe
herestudiedstructurescanalsointersectduringtheirdevelopment.Thisfeatureissharedwith
ichnofossils(Figure5b).Activedisplacementofsediment,includingcross‐cutting,isconsidereda
strongindicatorofanichnologicalnatureforterrestrialstructures[26].Itshouldbehowevernoted
thatcross‐cuttingisalsosharedwithcracksandsyngeneticalcrystalintergrowth.Ourimageanalysis
revealsanaverageintersectionangleof63.1°(intersectionanglerange:39.8°–85.6°).SomeoftheVera
RubinRidgestructurespresentangularterminations(e.g.,s5),whichisexpectedfrombreakageof
anorganismwithamodulargrowth[26].Similarsizeofdifferentstructuresandsinuousorientation
arealsocompatiblewithbodyfossilsofflexibleorganisms,whichhowevercannotbepreservedas
cross‐cuttingstructures.Cross‐cuttingprecludesthehypothesisofstick‐likestructuresasbody
fossils.Inaddition,carbonizedremains,whichareacriterionforrecognizingthebodyfossilorigin
ofsedimentarystructures[26],arenotdocumentedfromtheVeraRubinRidgestructures.
Thestick‐shapedfeaturesareunlined;contactwiththehostrockissharpandwell‐defined,
althoughthereisapparentlynodifferenceingrain‐sizebetweenthestick‐likestructuresandthehost
rock.Sharpcontacts,andtheelevationofthestick‐likestructuresrelativetothehostrock,suggest
thatthehostrockweatheredatafasterratethanthestick‐likestructures(differentialweathering).
Hence,thestick‐likestructuresarelikelytobemoreresistantthanthehostrock,possiblybecauseof
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moreintensecementation.Resistancetoerosionisconfirmedbythepresenceofanisolatedfragment
(Figure1:s2)thatexperiencedshort‐distancetransport.
Stick‐likestructuresfromVeraRubinRidgearedarker‐tonedthanthealteredhostrockbutshow
similarhueswithrespecttotheunalteredsubstrate.Unlikethesubstrate,brokensegmentsoften
displaymetallichues(s5inFigures1and2a).Thisdoesnotprecludeanichnologicaloriginofthe
stick‐likestructuresbecauseichnofossilswithmetallichuescanresultfromearlydiagenetic
processes,e.g.,onEarthpyriteandbarite‐filledburrowshavebeendocumentedfromavarietyof
depositionalsettings,includingpostglaciallacustrineclays[51,52].Thestick‐likestructuresarefound
overanareaofapproximately15cm2.Suchclustereddistributionimpliesthattheprocesses
responsibleforthedevelopmentand/orpreservationofthestructuresshouldhavebeenactive
locally.Thisiscompatiblewithbioticprocesses,e.g.,onEarthstressedenvironmentsare
characterizedbylowdiversityassemblages[53]withunevendistributionofichnofossils[54,55].
However,thisdoesnotprecludeanabioticoriginforthestick‐likestructures.Forinstance,in
sedimentaryrocks,crystalsmaygrowinaggregatesdependingontheunevenlyhighconcentration
ofdissolvedions.
Thestick‐likestructuresoccurinageologicalcontextthatfitswithevaporiticcrystalsand
sedimentarycracks.Thegeologicalcontextalsofitswiththeacceptedcriteriafortheidentificationof
ichnofossilsonEarth[56],i.e.,theyaresyngeneticwiththeprimaryfabricofthehostrockwhichis
sedimentaryandwellunderstoodintheregionalgeologicalcontextofGaleCrater;stick‐like
structuresarecrosscutbylaterstagefracturesandarefilledwithmaterialcapabletosurvivetothe
diagenetichistoryoftherocks.Inaddition,stick‐likestructuresarerelatedtoanancienthabitable
environment[14–18].Bulbousstructures(e.g.,structurebinFigure1)areembeddedinthehostrock
ofthestick‐likestructures.Theyarehereinterpretedasconcretions,intendedasconfinedbodiesof
clasticsedimentlithifiedbyauthigenicminerals[57].OnEarth,formationofconcretionsiscommonly
mediatedbybiogenicprocessessuchasmicrobialactivityandthepresenceoforganicmatter—
includingmucusonburrowwalls[58]—butfullyabioticprocessescanalsoexplaintheoriginofsome
concretions[59].Nevertheless,toourknowledgetherearenoexamplesofabioticconcretionssharing
themainmorphologicalfeaturesofthestick‐likestructures,whichareroughlypolygonalshape,with
constantwidthandcurvedorientation.OnEarth,curvedburrowsrepresentthemain
physicochemicalsettingforthedevelopmentofcurvedconcretions[58].Therearenoconclusive
evidencesshowingacommonoriginoftheconcretionsfromHaroldswickandthestick‐like
structures.Infact,concretionsshareasimilartexture,sizeandcolourwiththestick‐likestructures,
buttheylackthemetallichuesthataresometimesobservedinstick‐likestructures.Inaddition,they
differfromthestick‐likestructuresinhavingapredominantlyellipticalcrosssectionwithalarger
size(majoraxisistypically~1.5mm).Otherpost‐depositionalstructureswithintheMurray
Formationincludeenhancedrelieffeatures,darkraised‐ridgesandveins[60].Amongthese,
enhancedrelieffeaturesshareseveralfeatureswiththestick‐likestructuresofHaroldswick,i.e.,they
areembeddedinthehostrock,showtopographicreliefabovethesurface,theyarenotclearlyrelated
tofracturepatterns,theyaredarkerthanthealteredhostrock[60].Unlikethestick‐likestructures,
enhancedrelieffeaturestendtobedendritic,theirwidthisnotconstantand,mostimportantofall,
thecontactswiththehostrockshownoevidenceofdistinctboundaries.
Thehypothesis‐testingapproachweappliedfindsastrongparallelintheprotocolproposedby
BrasierandWacey[61,62]fordemonstratingbiogenicityofcandidatebodyfossilsandstromatolites.
Followingthisapproach,candidatefossilstructuresshouldnotbeacceptedasofbiologicalorigin
untilallplausibleexplanationsoftheirnon‐biologicaloriginhavebeentestedandfalsified,thatis,
untilthenullhypothesisofanabiogenicoriginisrejected.Ourresultsshowthatnoneoftheplausible
non‐biologicalexplanationsofthestick‐likestructuresisfullysatisfactory.Itmaybetherefore
temptingtoclaimthebiogenicityofthestick‐likestructures,alsobecausenoneoftheavailable
observationsfalsifiesaneventualichnofossiloriginofthem.
Nevertheless,ourresultsdonotallowtoclaimthebiogenicityofthestick‐likestructures.Infact,
availableevidencecannotdisprovethenullhypothesis,thatis,theabioticnatureofthestick‐like
structures.CarlSagan’stenet(“Extraordinaryclaimsrequireextraordinaryevidence”;[63])doesnot
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onlysuggestcautionwheninterpretingextraterrestrialichnofossil‐likestructures,butitalso
encouragesthesearchforextraordinaryevidence(e.g.,regularlymeanderingichnofossils)onMars.
Insum,ourobservationsdonotnecessarilyimplythatthestick‐likestructuresarebiogenicbut,
giventhatnoneoftheavailableobservationsatVeraRubinRidgefalsifiestheirichnofossilorigin,the
stick‐likestructuresarehereidentifiedasaprivilegedtargetforastrobiologicalresearch.Assuch,the
stick‐likestructuresfromVeraRubinRidgefollowtherecommendationofNASA’sMars2020Science
DefinitionTeam[64]inthattheymighthaveabiologicaloriginandthuscompelresearcherstogather
moredatabeforereachingaconclusionastothepresenceorabsenceofpastlife(seealso[65]).
Ourresultsalsohighlighttheurgentneedofestablishingarobustsetofquantitativecriteriafor
demonstratingthebiogenicityofichnofossil‐likestructures,microscopicandmacroscopic.Inother
words,thereistheneedofdefiningthemorphospaceofichnofossilsandabiogenicichnofossil‐like
structures.Thiswillallowtorejectthenullhypothesisofanabiogenicoriginforcandidate
ichnofossilsviamorphospaceanalysis.Thisclearlyparallelsthecellmorphospaceapproach[61].
Theestablishmentofbiogenicitycriteriawouldbenefitnotonlythesearchforextraterrestrial
life,butalsothesearchforPrecambrianlifeonEarth.Infact,thePrecambriangeologicalrecord
comprisesseveralproblematicstructuresthathavebeenrejectedastracefossilsandreinterpretedas
eitherabiogenicstructuresorbodyfossilsinsomecases[26,66].Ichnofossil‐likepseudofossilshave
alsobeenrecognizedinmorerecentdeposits[67].Ametazoan‐likebodyplanisnotessentialfor
producingmacroscopictraces,asevidencedbymicrobial‐relatedstructuresinOrdovicianlow‐
oxygenenvironments[68].Microorganismscanevenproducemacroscopictrailsorburrows,as
evidencedby2.1Gaichnofossilsattributedtomulticellularorsyncytialorganismabletomigrate
laterallyandvertically[69].Thesenon‐metazoanmacroscopicichnofossilsencourageresearchof
extraterrestrialanalogues,i.e.,ichnofossilsproducedbytheaggregationofamoeboidcellsintoa
motileslug‐likephase,similartothatofterrestrialslimemolds.
Inaddition,fivepropertiesofichnofossilshighlighttheastrobiologicalpotentialofthese
biogenicstructuresandthereforeencouragetheestablishmentofbiogenicitycriteriaforichnofossil‐
likestructures[70].First,theypreservetheactivityofsoft‐bodied(e.g.,[71])orpoorlymineralized
(e.g.,[72])organisms,togetherwithskeletonizedforms.Second,theyareresilienttoprocessesthat
obliterateotherbiosignatures,asevidencedbytracefossilspreservedinmetamorphosedandhighly
tectonizedrocks[73–75].Becauseoftheirpreservationpotentialandabundance,ichnofossilsare
amongthemostabundantevidenceofmicroscopicandmacroscopicpastlife,asexemplifiedby3.7
billion‐years‐oldstromatolites[76],2.1billion‐years‐oldburrows[69],andca.551million‐years‐old
trackways[77].Third,theyareveryvisiblebiosignatures,eitherbecausetheytendtobeenhancedby
diagenesis[78]orbytheeffectsoftracemakingitself[79].Fourth,themorphologyofichnofossils
oftenreflectenvironmentalconditionsatthetimeofdeposition[80].Fifth,ichnofossilsarenotonly
directevidenceofbiologicalbehaviour[81,82],buttheirmorphologyiscommonlyindependentfrom
themorphology,sizeandbiochemistryofthetracemaker.Sincethatextraterrestriallifemaydiffer
significantlyfromtheoneonEarthinmorphology,sizeandbiochemistry,ichnofossilsare
biosignaturesthatareideallysuitedfordetectinganytypeoflife[70].Thesepropertiesshowthe
astrobiologicalpotentialofichnofossils,henceestablishingbiogenicitycriteriawouldbeof
fundamentalhelpforthesearchofextraterrestriallifeandofearlylifeonEarth.
5.Conclusions
Thenewobservationalandmorphometricdatareportedherehavefar‐reachingimplicationsfor
thesearchofextraterrestriallife.Ourmorphometricandtopologicdataareuniquetothestick‐like
structuresamongMartiangeologicalfeaturesandshowthatichnofossilsareamongtheclosest
morphologicalanaloguesoftheseuniquefeatures.
Infact,thestick‐likestructuresshowmorphological(elongateshape;presenceofcurves;
constantwidth;clustereddistribution)andtopological(cross‐cutting,coalescingrelationships)
featuresthatarecommonlyobservedinichnofossils(Table1).Geologicalcontextandsizearealso
compatiblewithichnofossils.Bycontrast,steepflanksandthesomewhatpolygonalcross‐sectionare
Geosciences2020,10,3910of18
rarelyobservedinichnofossilsonEarth,withfewexceptions(Arthrophycus,Oblongichnus,
Bulbichnus).
Table1.Characteristicsofthestick‐likestructuresandtheiroccurrenceintheirclosestmorphological
analogues.Fiveoccurrenceclassesareused(fromthemostfrequenttothelessfrequent:always,
usually,often,rarely,never).Colorscalereflectsfrequencyofoccurrence.
Stick‐LikeStructuresSynaeresis
Cracks
EuhedralSingle
Crystals
Bioturbational
Ichnofossils
Geological
context
syngeneticwiththehostrockalwaysalwaysalways
fluvial‐lacustrineoftenoftenoften
Morphology
elongateshapeinbeddingplane
viewusuallyusuallyusually
curvedusuallyrarelyusually
constantwidth(notapering)rarelyusuallyusually
T‐junctions,incompletepolygonsalwaysoftenoften
polygonalcross‐sectionusuallyalwaysrarely
notipswithsmoothflatfacesusuallyrarelyusually
notendencytosplitalongdefinite
planesofweaknessalwaysrarely(cleavage)often
sharpcontactwiththehostrockalwaysalwaysusually
steepflanksalwaysalwaysrarely
Topology
cross‐cuttingrarelyusually
(twinning)
usually(false
branching)
coalescing(changetheircurvature
whenencounteringothers)rarelyrarelyoften
straightsegmentsarenotparallel
betweeneachotheralwaysusuallyusually
Other
metallichuesneveroftenrarely
millimetricwidthrarelyoftenoften
clustereddistributionoftenoftenoften
Followingtheinterpretationofotherextraterrestrialbiogenic‐likestructures[65],theabsenceof
unequivocalevidenceforlifeonMarsistheonlyevidencethatfavoursanabioticoriginofthestick‐
likestructures.Nevertheless,availabledatacannotfullydisprovetwomajorabiotichypotheses,that
aresedimentarycrackingandevaporiticcrystalgrowthasgeneticprocessesforthestructures.The
polygonalsectionandthesteepflankssupportthecrystalhypothesis,althoughthestick‐like
structureslackthesmoothflatfacesandthesharpcrystal‐likeoutlinesthattypifythetipsofeuhedral
monocrystals.TheT‐junctionsandincompletepolygonsformedbythestick‐likestructuresremind
offilledsedimentarycracks,althoughshrinkagecracks,unlikethestick‐likestructures,areusually
spindle‐shapedandcanpresenttaperedtips(Table1).Accordingly,theavailableobservationsat
VeraRubinRidgedonotallowtoclaimthebiogenicityofthestick‐likestructures,buttheyencourage
thecollectionoffurtherdata,especiallygeochemical,aboutthiskindofsedimentarystructureson
Mars.
Consequently,thiscasestudyraisesthequestionofhowtodisprovetheabioticoriginofan
ichnofossil‐likestructureinancientsedimentarydepositsonEarthandbeyond.Theanswertothis
questionnecessarilyliesinthestudyofmorphologyandgeologicalcontext,sincethechemical
compositionofpassively‐filledichnofossilsisnotnecessarilyrelatedtobiologicprocesses.This
question,anditsconverse(howtoprovebiogenicityofichnofossil‐likestructures)areoffundamental
importancenotonlybecauseichnofossilsarethemostpersistentmacroscopicevidenceforlifeon
Earth,butalsobecausetheyareindependentfrommorphology,sizeandbiochemistryofthe
producer.Assuch,ichnofossilswouldallowtorecognizelifethatdiffersfromknown–terrestrial–
life.BydescribingtheuniqueVeraRubinRidgestructuresfromMars,thisstudyemphasizesthe
potentialofichnologyasanewfrontierinastrobiology.
Geosciences2020,10,3911of18
AuthorContributions:Conceptualization,A.B.;Datacuration,A.B.,C.N.D.C.,F.F.andR.C.;Formalanalysis,
A.B.,C.N.D.C.,F.F.andR.C.;Fundingacquisition,A.B.;Investigation,A.B.,C.N.D.C.,F.F.andR.C.;
Methodology,A.B.;Projectadministration,A.B.;Supervision,A.B.;Validation,A.B.;Visualization,A.B.;
Writing—originaldraft,A.B.;Writing—review&editing,A.B.,C.N.D.C.,F.F.andR.C.
Funding:TheworkofA.B.hasbeensupportedbytheUNIVERSITYOFGENOVA,projectsALAN‐X,
PALEOGIANTS,CURIOSITY.
Acknowledgments:Twoanonymousreviewersarethankedfortheirconstructivecommentswhichgreatly
improvedthepaper.DerekBurgess(MartinMarietta,Raleigh,NC, USA)andDavidLoope(Universityof
Nebraska‐Lincoln,Lincoln,NE,USA)isthankedfordiscussiononconcretions.WethankBarrydiGregorio
(UniversityofBuckingam,Buckingham,UK)fordiscussiononthebiogenicityofthestructuresandwe
acknowledgehimasthefirsttomaketheparallelbetweenichnofossilsandthestick‐likestructures.Lorenzo
Bonini,AngeloDeMin,RomanaMelis,StefanoFurlani,MaurizioPonton,FrancescoPrincivalle,NevioPugliese
(UniversityofTrieste,Trieste,Italy)arethankedfordiscussiononthemorphologyofthestructures.Six
anonymousreviewersarethankedforrevisingapreliminaryversionofthemanuscript.AramBayet‐Goll(IASBS
ofZanjan,Zanjan,Iran)coordinatedthefieldworkinIran;GilMachado(Inst.D.Luiz,Lisbon,Portugal)and
NoelMoreira(Univ.Évora,Évora,Portugal),aswellasSebastiãoFraústo,arewarmlythankedforthelocation
ofCortePereiroandValedaMuasections,respectively.Theauthorsgreatlyappreciatetheaccesstothe
collectionsoftheGeologicalMuseumofLisbon,especiallytoitsexecutivedirectorProf.Dr.MiguelRamalho.
MoniqueMettraux(Geosolutions,Pau,France)andEliasSamanakassou(IAS,UniversityofGeneva,Geneva,
Switzerland)arethankedforprovidingpicturesofevaporitecrystals(Figure4a,b).JuanManuelGarcía‐Ruiz
(CSIC,UniversidaddeGranada,Granada,Spain)isthankedforprovidingapseudomorphpicture(Figure4c).
MadelineVickers(UniversityofCopenaghen,Copenaghen,Denmark)isacknowledgedforprovidinga
glendonitepicture(Figure4d).AlexanderE.S.VanDriessche(ISTerre‐CNRS&Univ.GrenobleAlpes)and
MaciejBąbel(UniversityofWarsaw,Warsaw,Poland)arethankedforcrystalimages.
ConflictsofInterest:Theauthorsdeclarenoconflictofinterest.
AppendixA
FigureA1.Widthanalysismap.Numbers1–34refertothemeasurementIDinTableA1.
Geosciences2020,10,3912of18
FigureA2.Lengthanalysismap.
FigureA3.Angleanalysismap.
TableA1.(followingpage).Widthmeasuresofthestick‐likestructuresfromVeraRubinRidge,Mars.
SeeFigureA1forlocationofmeasurements.
MeasurementIDMeasuredSpecimenWidth(cm)
1s10.08
2s10.08
3s10.08
4s10.07
5s10.06
6s30.09
7s40.07
8s40.06
Geosciences2020,10,3913of18
9s40.06
10s50.09
11s50.09
12s50.08
13s50.08
14s50.09
15s60.06
16s60.06
17s60.07
18s70.07
19s80.05
20s80.07
21s80.07
22s90.08
23s90.08
26s90.08
24s90.08
27s100.07
25s90.08
28s100.08
29s100.07
30s100.07
31s100.07
32s110.04
33s110.05
34s110.04
Mean0.07
Min0.04
Max0.09
TableA2.Lengthmeasuresofthestick‐likestructuresfromVeraRubinRidge,Mars.SeeFigureA2
forlocationofmeasurements.
SpecimenLength(cm)
s10.40
s20.30
s30.26
s40.22
s51.05
s60.25
s70.24
s80.26
s90.43
s100.81
s110.23
Mean0.40
Min0.22
Max1.05
Geosciences2020,10,3914of18
TableA3.Anglemeasuresofthestick‐likestructuresfromVeraRubinRidge,Mars.SeeFigureA3for
locationofmeasurements.
AngleIDRaysoftheAngleAngle(°)
α s4–s563.1
β s8–s985.6
γ
s10–s1139.8
Mean63.1
Min39.8
Max85.6
References
1. Asikainen,C.A.;Francus,P.;Brigham‐Grette,J.Sedimentology,claymineralogyandgrain‐sizeas
indicatorsof65kaofclimatechangefromEl’gygytgynCraterLake,NortheasternSiberia.J.Paleolimnol.
2007,37,105–122.
2. Scott,J.J.;Renaut,R.W.;Buatois,L.A.;Owen,R.B.Biogenicstructuresinexhumedsurfacesaroundsaline
lakes:AnexamplefromLakeBogoria,KenyaRiftValley.Palaeogeogr.Palaeoclimatol.Palaeoecol.2009,272,
176–198.
3. Greicius,T.Stick‐Shape,Rice‐SizeFeaturesonMartianRock“Haroldswick”.Availableonline:
https://www.nasa.gov/image‐feature/jpl/pia22213/stick‐shape‐rice‐size‐features‐on‐martian‐rock‐
haroldswick(accessedon8October2019).
4. DiGregorio,B.IchnologicalevidenceforbioturbationinanancientlakeatVeraRubinRidge,GaleCrater,
Mars.InProceedingsofthe3rdInternationalConventiononGeosciencesandRemoteSensing,19‐20
October2018,Ottawa,ON,Canada,2018;pp.1–7.
5. Howell,E.No,ThoseAren’tAnimalTracksonMars.Availableonline:https://www.space.com/39894‐
mars‐rock‐features‐not‐animal‐tracks.html(accessedon8October2019).
6. Howell,E.DidCrystalsfromAncientLakesonMarsFormTheseTiny,WeirdThings?Availableonline:
https://www.space.com/39687‐ancient‐mars‐lakes‐made‐weird‐crystal‐features.html(accessedon8
October2019).
7. David,L.CuriosityRoverSpotsWeirdTube‐LikeStructuresonMars.Availableonline:
https://www.space.com/39294‐mars‐rover‐curiosity‐weird‐tube‐structures.html(accessedon8October
2019).
8. Frydenvang,J.;Mangold,N.;Wiens,R.C.;Fraeman,A.A.;Edgar,L.A.;Fedo,C.;L’Haridon,J.;Gupta,S.;
Grotzinger,J.P.;Bedford,C.;etal.Theroleoflarge‐scalediagenesisintheformationofVeraRubinRidge
inGalecrater,Mars,asimpliedbyChemCamobservations.InProceedingsofthe50thLunarandPlanetary
ScienceConference,TheWoodlands,TX,USA,18–22March2019;Volume2019,LPIContributionNo.2132.
9. Fraeman,A.A.;Arvidson,R.E.;Catalano,J.G.;Grotzinger,J.P.;Morris,R.V.;Murchie,S.L.;Stack,K.M.;
Humm,D.C.;McGovern,J.A.A.;Seelos,F.P.P.;etal.Ahematite‐bearinglayerinGalecrater,Mars:
Mappingandimplicationsforpastaqueousconditions.Geology2013,41,1103–1106.
10. L’Haridon,J.;Mangold,N.;Rapin,W.;Cousin,A.;Johnson,J.R.;Fraeman,A.A.;Meslin,P.‐Y.;Gasnault,
O.;Maurice,S.;Wiens,R.DiageneticironenrichmentsobservedbyChemCamonVeraRubinRidge,Gale
Crater,Mars.InProceedingsofthe49thLunarandPlanetaryScienceConference2018,TheWoodlands,
TX,USA,1923March2018;LPIContributionNo.2083.
11. Heydari,E.;Parker,T.J.;Calef,F.J.;Schroeder,J.F.;VanBeek,J.;Rowland,S.K.;Fairen,A.G.Characteristics
andtheoriginoftheVeraRubinRidge,GaleCrater,Mars.InProceedingsofthe49thLunarandPlanetary
ScienceConference2018,TheWoodlands,TX,USA,19–23March2018;LPIContribNo.2083.
12. Frydenvang,J.;Mangold,N.;Wiens,R.C.;Fraeman,A.A.;Edgar,L.A.;Fedo,C.M.;L’Haridon,J.;Bedford,
C.C.;Gupta,S.;Grotzinger,J.P.;etal.ThechemostratigraphyofthelacustrineMurrayformationinGale
crater,Mars,andevidenceforlarge‐scalediagenesisinVeraRubinridgebedrockasimpliedbyChemCam
observations.InProceedingsoftheNinthInternationalConferenceonMars2019,Pasadena,CA,USA,22–
25July2019;p.LPIContrib.No.2089.
13. Williams,A.J.;Sumner,D.Y.;Alpers,C.N.;Karunatillake,S.;Hofmann,B.A.PreservedFilamentous
MicrobialBiosignaturesintheBrickFlatGossan,IronMountain,California.Astrobiology2015,15,637–668.
Geosciences2020,10,3915of18
14. Stein,N.;Grotzinger,J.P.;Schieber,J.;Mangold,N.;Hallet,B.;Newsom,H.;Stack,K.M.;Berger,J.A.;
Thompson,L.;Siebach,K.L.;etal.DesiccationcracksprovideevidenceoflakedryingonMars,Sutton
Islandmember,Murrayformation,GaleCrater.Geology2018,46,515–518.
15. Rampe,E.B.;Ming,D.W.;Blake,D.F.;Bristow,T.F.;Chipera,S.J.;Grotzinger,J.P.;Morris,R.V.;Morrison,
S.M.;Vaniman,D.T.;Yen,A.S.;etal.Mineralogyofanancientlacustrinemudstonesuccessionfromthe
Murrayformation,Galecrater,Mars.EarthPlanet.Sci.Lett.2017,471,172–185.
16. Grotzinger,J.P.;Sumner,D.Y.;Kah,L.C.;Stack,K.;Gupta,S.;Edgar,L.;Rubin,D.;Lewis,K.;Schieber,J.;
Mangold,N.;etal.Ahabitablefluvio‐lacustrineenvironmentatYellowknifeBay,GaleCrater,Mars.Science
2014,343,1242777.
17. Grotzinger,J.P.;Gupta,S.;Malin,M.C.;Rubin,D.M.;Schieber,J.;Siebach,K.;Sumner,D.Y.;Stack,K.M.;
Vasavada,A.R.;Arvidson,R.E.;etal.Deposition,exhumation,andpaleoclimateofanancientlakedeposit,
Galecrater,Mars.Science2015,350,aac7575.
18. Hurowitz,J.A.;Grotzinger,J.P.;Fischer,W.W.;McLennan,S.M.;Milliken,R.E.;Stein,N.;Vasavada,A.R.;
Blake,D.F.;Dehouck,E.;Eigenbrode,J.L.;etal.RedoxstratificationofanancientlakeinGalecrater,Mars.
Science2017,356,eaah6849.
19. Dohm,J.M.;Ferris,J.C.;Baker,V.R.;Anderson,R.C.;Hare,T.M.;Strom,R.G.;Barlow,N.G.;Tanaka,K.L.;
Klemaszewski,J.E.;Scott,D.H.AncientdrainagebasinoftheTharsisregion,Mars:Potentialsourcefor
outflowchannelsystemsandputativeoceansorpaleolakes.J.Geophys.Res.Planets2001,106,32943–32958.
20. Edgar,L.A.;Fraeman,A.A.;Gupta,S.;Fedo,C.M.;Grotzinger,J.P.;Stack,K.M.;Bennett,K.A.;Sun,V.Z.;
Banham,S.G.;Stein,N.T.;etal.SedimentologyandstratigraphyobservedatVeraRubinRidgebytheMars
sciencelaboratoryCuriosityRover.InProceedingsofthe49thLunarandPlanetaryScienceConference
2018,TheWoodlands,TX,USA,19–23March2018;Volume2018,pp.5–6.
21. Yingst,R.A.;Edgett,K.S.;Kennedy,M.R.;Krezoski,G.M.;McBride,M.J.;Minitti,M.E.;Ravine,M.A.;
Williams,R.M.E.MAHLIonMars:Lessonslearnedoperatingageosciencecameraonalandedpayload
roboticarm.Geosci.Instrum.MethodsDataSyst.2016,5,205–217.
22. Grotzinger,J.;Bell,I.;Herkenhoff,K.;Johnson,J.;Knoll,A.;McCartney,E.;McLennan,S.;Metz,J.;Moore,
J.;Squyres,S.;etal.Sedimentarytexturesformedbyaqueousprocesses,ErebuscraterMeridianiPlanum,
Mars.Geology2006,34,1085–1088.
23. McMahon,S.;vanS.Hood,A.;McIlroy,D.Theoriginandoccurrenceofsubaqueoussedimentarycracks.
InEarthSystemEvolutionandEarlyLife:ACelebrationoftheWorkofMartinBrasier.SpecialPublications448;
Brasier,A.T.,McIlroy,D.,McLoughlin,N.,Eds.;GeologicalSociety:London,UK,2017;pp.285–309.
24. Harazim,D.;Callow,R.H.T.;Mcilroy,D.Microbialmatsimplicatedinthegenerationofintrastratal
shrinkage(‘synaeresis’)cracks.Sedimentology2013,60,1621–1638.
25. Siebach,K.L.;Grotzinger,J.P.;Kah,L.C.;Stack,K.M.;Malin,M.;Léveillé,R.;Sumner,D.Y.Subaqueous
shrinkagecracksintheSheepbedmudstone:Implicationsforearlyfluiddiagenesis,Galecrater,Mars.J.
Geophys.Res.Planets2014,119,1597–1613.
26. Jensen,S.;Droser,M.L.;Gehling,J.G.Tracefossilpreservationandtheearlyevolutionofanimals.
Palaeogeogr.Palaeoclimatol.Palaeoecol.2005,220,19–29.
27. Noffke,N.AncientSedimentaryStructuresinthe<3.7GaGillespieLakeMember,Mars,ThatResemble
MacroscopicMorphology,SpatialAssociations,andTemporalSuccessioninTerrestrialMicrobialites.
Astrobiology2015,15,169–192.
28. Al‐Youssef,M.GypsumCrystalsFormationandHabits,UmmSaidSabkha,Qatar.InSabkhaEcosystems
VolumeIV:CashCropHalophyteandBiodiversityConservation.PartoftheTasksforVegetationScienceBookSeries
(TAVS,Volume47);Khan,M.A.,Böer,B.,Öztürk,M.,Zahran,T.,Abdessalaam,A.,Clüsener‐Godt,M.,Gul,
B.,Eds.;Springer:Berlin,Germany,2014;pp.23–54.
29. Klein,C.;Philpotts,A.R.EarthMaterials:IntroductiontoMineralogyandPetrology;CambridgeUniversity
Press:Cambridge,UK,2013.
30. Becker,A.FaciesdevelopmentoftheBadenian(MiddleMiocene)gypsumdepositsintheRacławicearea
(MiechówUpland,southernPoland).Ann.Soc.Geol.Pol.2016,75,111–120.
31. Babel,M.;Bogucki,A.TheBadenianevaporitebasinofthenorthernCarpathianForedeepasamodelofa
meromicticselenitebasin.InEvaporitesThroughSpaceandTime.GeologicalSocietyofLondonSpecial
Publications;Schreiber,B.,Lugli,S.,Babel,M.,Eds.;TheGeologicalSocietyofLondon:London,UK,2007;
Volume285,pp.219–246.
Geosciences2020,10,3916of18
32. Benison,K.C.;Karmanocky,F.J.III.CouldmicroorganismsbepreservedinMarsgypsum?Insightsfrom
terrestrialexamples.Geology2014,42,615–618.
33. Rinaudo,C.;Franchini‐angela,M.;Boistelle,R.Curvatureofgypsumcrystalsinducedbygrowthinthe
presenceofimpurities.Mineral.Mag.1989,53,479–482.
34. Seilacher,A.;Meschede,M.;Bolton,E.W.;Luginsland,H.Precambrian“fossil”Vermiformaisatectograph.
Geology2000,28,235–238.
35. Mettraux,M.;Homewood,P.W.;Kwarteng,A.Y.;Mattner,J.CoastalandcontinentalsabkhasofBarrAl
Hikman,SultanateofOman.InQuaternaryCarbonateandEvaporiteSedimentaryFaciesandTheirAncient
Analogues:ATributetoDouglasJamesShearman;TheInternationalAssociationofSedimentologistsSpecial
Publications:Wiley‐Blackwell,Hoboken,2011;Volume43,pp.183–204,ISBN9781444339109.
36. Otálora,F.;Mazurier,A.;Garcia‐Ruiz,J.M.;Kranendonk,M.J.;VanKotopoulou,E.;ElAlbani,A.;Garridoa,
C.J.Acrystallographicstudyofcrystallinecastsandpseudomorphsfromthe3.5GaDresserFormation,
PilbaraCraton(Australia).J.Appl.Crystallogr.2018,51,1–9.
37. Vickers,M.;Watkinson,M.;Price,G.D.;Jerrett,R.Animprovedmodelfortheikaite‐glendonite
transformation:EvidencefromtheLowerCretaceousofSpitsbergen,Svalbard.Nor.J.Geol.2018,98,1–15.
38. Minter,N.J.;Krainer,K.;Lucas,S.G.;Braddy,S.J.;Hunt,A.P.PalaeoecologyofanEarlyPermianplayalake
tracefossilassemblagefromCastlePeak,Texas,USA.Palaeogeogr.Palaeoclimatol.Palaeoecol.2007,246,390–
423.
39. Baucon,A.;Venturini,C.;NetodeCarvalho,C.;Felletti,F.;Muttoni,G.Behavioursmappedbynew
geographies:IchnonetworkanalysisoftheValDolceFormation(lowerPermian;Italy‐Austria).Geosphere
2015,11,744–776.
40. Baucon,A.;Carvalho,C.Fromtherivertothesea:Pramollo,anewichnolagerstättefromtheCarnicAlps.
Stud.Trent.Sci.Nat.ActaGeol.2008,83,87–114.
41. Kowalewski,M.;Demko,T.M.Tracefossilsandpopulationpaleoecology:Comparativeanalysisofsize‐
frequencydistributionsderivedfromburrows.Lethaia1996,29,113–124.
42. Mángano,M.G.;Carmona,N.;Buatois,L.;MuñizGuinea,F.ANewIchnospeciesofArthrophycusfrom
theUpperCambrian‐LowerTremadocianofNorthwestArgentina:ImplicationsfortheArthrophycid
LineageandPotentialinIchnostratigraphy.Ichnos2005,12,179–190.
43. Monaco,P.Bulbichnusgiorniin.ichnogen.andn.ichnosp.:Adeep‐waterdomichnion‐praedichnionmade
byaneunicidpolychaete(Marnoso‐ArenaceaFormation,Miocene,NorthernApennines,centralItaly).Boll.
DellaSoc.Paleontol.Ital.2016,55,172.
44. NetodeCarvalho,C.;Fernandes,A.C.S.;Borghi,L.Diferenciaçãodasicnoespéciesevariantesde
ArthrophycusesuautilizaçãoproblemáticaemIcnoestratigrafia:Oresultadodehomoplasias
comportamentaisentreanelídeoseartrópodes?[DistinctionbetweenArthrophycusichnospeciesand
variantsandtheird.Rev.EspañolaPaleontol.2003,18,221–228.
45. Plotnick,R.E.Behavioralbiologyoftracefossils.Paleobiology2012,38,459–473.
46. Sims,D.W.;Reynolds,A.M.;Humphries,N.E.;Southall,E.J.;Wearmouth,V.J.;Metcalfe,B.;Twitchett,R.J.
Hierarchicalrandomwalksintracefossilsandtheoriginofoptimalsearchbehavior.Proc.Natl.Acad.Sci.
USA2014,111,11073–11078.
47. Raup,D.M.;Seilacher,A.Fossilforagingbehavior:ComputerSimulation.Science1969,166,994–995.
48. Berra,F.;Felletti,F.Syndepositionaltectonicsrecordedbysoft‐sedimentdeformationandliquefaction
structures(continentalLowerPermiansediments,SouthernAlps,NorthernItaly):Stratigraphic
significance.Sediment.Geol.2011,235,249–263.
49. Lowe,D.R.;LoPiccolo,R.D.Thecharacteristicsandoriginsofdishandpillarstructures.J.Sediment.Res.
1974,44,484–501.
50. Laubach,S.E.;Schultz‐Ela,D.D.;Tyler,R.Differentialcompactionofinterbeddedsandstoneandcoal.Geol.
Soc.Lond.Spec.Publ.1999,169,51–60.
51. Virtasalo,J.J.;Löwemark,L.;Papunen,H.;Kotilainen,A.T.;Whitehouse,M.J.Pyriticandbariticburrows
andmicrobialfilamentsinpostglaciallacustrineclaysinthenorthernBalticSea.J.Geol.Soc.Lond.2010,167,
1185–1198.
52. Virtasalo,J.J.;Whitehouse,M.J.;Kotilainen,A.T.IronisotopeheterogeneityinpyritefillingsofHolocene
wormburrows.Geology2013,41,39–42.
Geosciences2020,10,3917of18
53. Baucon,A.;Ronchi,A.;Felletti,F.;NetodeCarvalho,C.EvolutionofCrustaceansattheedgeoftheend‐
Permiancrisis:IchnonetworkanalysisofthefluvialsuccessionofNurra(Permian‐Triassic,Sardinia,Italy).
Palaeogeogr.Palaeoclimatol.Palaeoecol.2014,410,74–103.
54. Gingras,M.K.;MacEachern,J.A.;Dashtgard,S.E.Processichnologyandtheelucidationofphysico‐
chemicalstress.Sediment.Geol.2011,237,115–134.
55. Baucon,A.;NetodeCarvalho,C.Starsoftheaftermath:AsteriacitesbedsfromtheLowerTriassicofthe
CarnicAlps(WerfenFormation,SaurisdiSopra),Italy.Palaios2016,31,161–176.
56. Wacey,D.EarlyLifeonEarth:APracticalGuide;SpringerScience+BusinessMediaB.V.:Berlin,Germany,
2009.
57. Baumann,L.M.F.;Birgel,D.;Wagreich,M.;Peckmann,J.Microbially‐drivenformationofCenozoicsiderite
andcalciteconcretionsfromeasternAustria.AustrianJ.EarthSci.2016,109,doi:10.17738/ajes.2016.0016.
58. Brown,B.J.;Farrow,G.E.RecentdolomiticconcretionsofcrustaceanburroworiginfromLochSunart,west
coastofScotland.J.Sediment.Res.1978,48,825–833.
59. Cox,T.L.;Oze,C.;Horton,T.W.IronconcretionswithinahighlyalteredunitoftheBerlinsPorphyry,New
Zealand:Anabioticorbioticstory?Mineral.Petrol.2017,111,173–181.
60. Nachon,M.;Mangold,N.;Forni,O.;Kah,L.C.;Cousin,A.;Wiens,R.C.;Anderson,R.;Blaney,D.;Blank,
J.G.;Calef,F.;etal.ChemistryofdiageneticfeaturesanalyzedbyChemCamatPahrumpHills,Galecrater,
Mars.Icarus2017,281,121–136.
61. Brasier,M.D.;Wacey,D.Fossilsandastrobiology:Newprotocolsforcellevolutionindeeptime.Int.J.
Astrobiol.2012,11,217–228.
62. Brasier,M.D.TowardsaNullHypothesisforStromatolites.InEarliestLifeonEarth:Habitats,Environments
andMethodsofDetection;Golding,S.D.,Glikson,M.,Eds.;SpringerScience+BusinessMediaB.V.:Dordecht,
TheNetherlands,2011;pp.115–125,ISBN9789048187942.
63. Zahnle,K.;Freedman,R.S.;Catling,D.C.IstheremethaneonMars?Icarus2011,212,493–503.
64. Mustard,J.F.;Adler,M.;Allwood,A.;Bass,D.S.;Beaty,D.W.;Bell,J.F.,III;Brinckerhoff,W.B.;Carr,M.;
Marais,D.J.D.;Drake,B.;etal.ReportoftheMars2020ScienceDefinitionTeam;Availableonline:
http://mepag.jpl.nasa.gov/reports/MEP/Mars_2020_SDT_Report_Final.pdf;2013.
65. Ruff,S.W.;Farmer,J.D.SilicadepositsonMarswithfeaturesresemblinghotspringbiosignaturesatEl
TatioinChile.Nat.Commun.2016,7,13554.
66. Jensen,S.;Droser,M.L.;Gehling,J.G.ACriticalLookattheEdiacaranTraceFossilRecord.InNeoproterozoic
GeobiologyandPaleobiology;Xiao,S.,Kaufman,A.J.,Eds.;Springer:Dordecht,TheNetherlands,2006;pp.
115–157.
67. Knaust,D.;Hauschke,N.TracefossilsversuspseudofossilsinLowerTriassicplayadeposits,Germany.
Palaeogeogr.Palaeoclimatol.Palaeoecol.2004,215,87–97.
68. NetodeCarvalho,C.;Couto,H.;Figueiredo,M.V.;Baucon,A.Microbial‐relatedbiogenicstructuresfrom
theMiddleOrdovicianslatesofCanelas(northernPortugal).Comun.Geológicas2016,103,23–37.
69. ElAlbani,A.;Mangano,M.G.;Buatois,L.A.;Bengtson,S.;Riboulleau,A.;Bekker,A.;Konhauser,K.;Lyons,
T.;Rollion‐Bard,C.;Bankole,O.;etal.Organismmotilityinanoxygenatedshallow‐marine.Proc.Natl.Acad.
Sci.USA2018,116,3431–3436.
70. Baucon,A.;NetodeCarvalho,C.;Barbieri,R.;Bernardini,F.;Cardini,A.;Cavalazzi,B.;Celani,A.;Felletti,
F.;Ferretti,A.;Schoenlaub,H.P.;etal.Organism‐substrateinteractionsandastrobiology:Potential,models,
methods.EarthSci.Rev.2016,171,141–180.
71. Seilacher,A.;Buatois,L.A.;Mángano,G.TracefossilsintheEdiacaran‐Cambriantransition:Behavioral
diversification,ecologicalturnoverandenvironmentalshift.Palaeogeogr.Palaeoclimatol.Palaeoecol.2005,227,
323–356.
72. NetodeCarvalho,C.;Rodrigues,N.P.C.;Viegas,P.A.;Baucon,A.;Santos,V.F.Patternsofoccurrenceand
distributionofcrustaceanichnofossilsintheLowerJurassic‐UpperCretaceousofAtlanticoccidental
marginbasins,Portugal.ActaGeol.Pol.2010,60,19–28.
73. NetoDeCarvalho,C.;Baucon,A.Gianttrilobiteburrowsandtheirpaleobiologicalsignificance(Lower‐to‐
MiddleOrdovicianfromPenhaGarcia,Portugal).Comun.Geol.2016,103,71–82.
74. NetoDeCarvalho,C.;Baucon,A.;Gonçalves,D.Daedalusmega‐ichnositefromtheMuradalMountain
(NaturtejoGlobalGeopark,CentralPortugal):BetweentheAgronomicRevolutionandtheOrdovician
Radiation.Comun.Geol.2016,103,59–70.
Geosciences2020,10,3918of18
75. Hollocher,K.APictorialGuidetoMetamorphicRocksintheField;CRCPress/Balkema:Leiden,The
Netherlands,2014.
76. Nutman,A.P.;Bennett,V.C.;Friend,C.R.L.;VanKranendonk,M.J.;Chivas,A.R.Rapidemergenceoflife
shownbydiscoveryof3,700‐million‐year‐oldmicrobialstructures.Nature2016,1,1–12.
77. Chen,Z.;Chen,X.;Zhou,C.;Yuan,X.;Xiao,S.LateEdiacarantrackwaysproducedbybilateriananimals
withpairedappendages.Sci.Adv.2018,4,1–8.
78. Savrda,C.E.Taphonomyoftracefossils.InTraceFossils.Concepts,Problems,Prospects.;Miller,W.,Ed.;
Elsevier,Amsterdam,TheNetherlands,2007;pp.92–109.
79. Seike,K.;Yanagishima,S.I.;Nara,M.;Sasaki,T.LargeMacaronichnusinmodernshorefacesediments:
Identificationoftheproducer,themodeofformation,andpaleoenvironmentalimplications.Palaeogeogr.
Palaeoclimatol.Palaeoecol.2011,311,224–229.
80. Crippa,G.;Baucon,A.;Felletti,F.;Raineri,G.;Scarponi,D.Amultidisciplinarystudyofecosystem
evolutionthroughearlyPleistoceneclimatechangefromthemarineArdaRiversection,Italy.Quat.Res.
2018,89,533–562.
81. DeCarvalho,C.N.;Pereira,B.;Klompmaker,A.;Baucon,A.;Moita,J.A.;Pereira,P.Runningcrabs,walking
crinoids,grazinggastropods:BehavioraldiversityandevolutionaryimplicationsoftheCabeçodaLadeira
Lagerstätte(MiddleJurassic,Portugal).Comun.Geol.2016,103,39–54.
82. Vallon,L.H.;Rindsberg,A.K.;Bromley,R.G.Anupdatedclassificationofanimalbehaviourpreservedin
substrates.Geodin.Acta2016,28,5–20.
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