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Ichnofossils, Cracks or Crystals? A Test for Biogenicity of Stick-Like Structures from Vera Rubin Ridge, Mars

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New images from Mars rover Curiosity display millimetric, elongate stick- like structures in the fluvio-lacustrine deposits of Vera Rubin Ridge, the depositional environment of which has been previously acknowledged as habitable. Morphology, size and topology of the structures are yet incompletely known and their biogenicity remains untested. Here we provide the first quantitative description of the Vera Rubin Ridge structures, showing that ichnofossils, i.e., the product of life-substrate interactions, are among their closest morphological analogues. Crystal growth and sedimentary cracking are plausible non-biological genetic processes for the structures, although crystals, desiccation and syneresis cracks do not typically present all the morphological and topological features of the Vera Rubin Ridge structures. Morphological analogy does not necessarily imply biogenicity but, given that none of the available observations falsifies the ichnofossil hypothesis, Vera Rubin Ridge and its sedimentary features are here recognized as a privileged target for astrobiological research.
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Geosciences2020,10,39;doi:10.3390/geosciences10020039www.mdpi.com/journal/geosciences
Article
Ichnofossils,CracksorCrystals?ATest
forBiogenicityofStickLikeStructuresfromVera
RubinRidge,Mars
AndreaBaucon
1,
*,CarlosNetoDeCarvalho
2,3
,FabrizioFelletti
4
andRobertoCabella
5
1
DISTAV,UniversityofGenova,CorsoEuropa52,16132Genova,Italy
2
GeologyOffice,NaturtejoUNESCOGlobalGeopark,AvenidaZonaNovadeExpansão,
6060101IdanhaaNova,Portugal;carlos.praedichnia@gmail.com
3
InstitutoD.Luiz,UniversityofLisbon.FaculdadedeCiênciasdaUniversidadedeLisboa,
CampoGrandeEdifícioC1,Piso1,1749016Lisbon,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
inthefluviolacustrinedepositsofVeraRubinRidge,thedepositionalenvironmentofwhichhas
beenpreviouslyacknowledgedashabitable.Morphology,sizeandtopologyofthestructuresare
yetincompletelyknownandtheirbiogenicityremainsuntested.Hereweprovidethefirst
quantitativedescriptionoftheVeraRubinRidgestructures,showingthatichnofossils,i.e.,the
productoflifesubstrateinteractions,areamongtheirclosestmorphologicalanalogues.Crystal
growthandsedimentarycrackingareplausiblenonbiologicalgeneticprocessesforthestructures,
althoughcrystals,desiccationandsyneresiscracksdonottypicallypresentallthemorphological
andtopologicalfeaturesoftheVeraRubinRidgestructures.Morphologicalanalogydoesnot
necessarilyimplybiogenicitybut,giventhatnoneoftheavailableobservationsfalsifiesthe
ichnofossilhypothesis,VeraRubinRidgeanditssedimentaryfeaturesarehererecognizedasa
privilegedtargetforastrobiologicalresearch.
Keywords:palaeontology;ichnology;ichnofossils;pseudofossils;astrobiology;biosignatures;Mars;
VeraRubinRidge;GaleCrater;Haroldswick
1.Introduction
NewobservationsatVeraRubinRidgebytheMarsSpaceLaboratoryRoverCuriosityshow
millimetric,elongatestructurespreservedinsedimentaryrocksdepositedinfluviolacustrine
environmentswithinGaleCrater.OnEarth,suchsettingsareinhabitedbyanenormousdiversityof
macro‐andmicroorganismsproducingtracesoflifesubstrateinteractions[1,2]thataresimilartothe
structuresofVeraRubinRidge.
Eversincetheywereannounced,thestructuresfromVeraRubinRidgehavebeencontroversial.
Inablogpost,NASAscientistslabelledthestructuresasenigmaticsticklikefeatures,proposingan
abioticoriginaserosionresistantmineralveins[3].Bycontrast,DiGregorio[4]suggestedapotential
biogenicnatureasthefossilizedproductsoflifesubstrateinteractions(ichnofossils).Thisbiogenic
hypothesiswasinformallydisputedinasetofblogpostsincludinginterviewsofEarthandplanetary
scientists[5–7].Despiteofthisdebate,morphology,sizeandtopologyofthesticklikestructuresare
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yetincompletelyknown,andtheirpotentialbiogenicityremainsuntested.Todate,thereisnopeer
reviewedpaperdiscussingtheabiotichypothesisforthesticklikestructures.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
anironbearingenvironmentappropriateforbiosignaturepreservation[13].
ThesticklikestructureshavebeenimagedbytheMarsScienceLaboratoryroverCuriosity
duringMartiandays(Sols)1905,1921and1923.Theirhostrockpertainstothefluviolacustrine
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.ImageswithsticklikestructureshavebeenanalysedwiththeimageanalysissoftwareFiji
1.51w(http://fiji.sc),animageprocessingpackagebasedonImageJ.Resultsofwidth,lengthand
angleanalysisarelistedintheAppendixA,FiguresA1–A3andTablesA1–A3.Sizeofthestructures
havebeenderivedfromsourceimagesusingtheMAHLIequations[21].
3.Results
11sticklikestructuresarepreservedaserosionenhancedreliefswithinasiltstoneoutcrop
(Figure1:specimenss1tos11).Theyconsistofhorizontal,straightsegmentsthatarefrequently
connectedbycurvedturns(Figure2a).Structuresareunlined.Contactwiththehostrockissharp,
buttherearenomanifestgrainsizedifferencesbetweenthesticklikestructuresandthehostrock.
OurinvestigationsusingimageanalysisandMAHLIequations[21]showanaveragewidthof0.7
mm.Theendsoffewstructures(s1,s11)appeartaperedbutitisunclearwhethertaperingresults
fromprimary(i.e.,structureproducing)orsecondary(e.g.,weathering,fragmentation)processes;as
such,endsofthesestructureshavebeenexcludedfromimageanalysis.Accordingly,eachstructure
retainsaconstantwidthalongitslength(averageperspecimenvariationinwidth:0.1mm).Widths
tendtobeconstantalsobetweenstructures(widthrange:0.5–0.9mm).Resultsofimageanalysisshow
anaveragelengthof4mm,butthisvalueunderestimatestheoriginallengthbecausesticklike
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.ThesticklikestructuresatVeraRubinRidge,Mars.Labelsindicateindividualspecimensof
sticklikestructures(s1s11)andbulbousstructures(b).OutcropimagedbyroverCuriosityusing
MAHLI(image1922MH0001520010703174C00_DXXX).
Figure2.SticklikestructuresfromMars.(a)Sticklikestructures4intersectingtheSshapedforms5,
whichchangesitscurvatureatshortdistancefromspecimens1.(b)Tjunctionformedbyspecimens
s8ands9.(c)Intersectionbetweenspecimenss10ands11.SeeFigure1forspecimennumbers.
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4.Discussion
Inordertotestthenullhypothesis(abioticorigin),thesticklikestructuresarecomparedwith
similarnonbiologicalstructuresfoundonEarthandknowntooccuronMars,accordingtotheir(a)
morphology,(b)size,(c)geologicalcontextand(d)topologicalinterrelationships.
Themorphologicalfeaturesofthesticklikestructuresaresharedwithcrackfills.Desiccation
cracksareacommoncomponentofcontinentaldepositsonEarth(Figure3a),whereastheyhavebeen
reportedonMarsfromtheMurrayFormation[14]andMeridianiPlanum[22].Martiandesiccation
cracksmeetorthogonally,formingTjunctions[14];thiscomparesfavourablywiththeTjunction
observedinapairofsticklikestructures(s8,s9;Figure2b).Subaqueoussedimentarycracks,also
knownassynaeresiscracksorintrastratalshrinkagecracks[23,24],havebeendocumentedonMars
intheSheepbedmudstone[25].SimilarlytotheVeraRubinRidgestructures,subaqueous
sedimentarycrackscanformincompletepolygonsconsistingofcurved(curlicuelike;Figure3b),at
timesbranchedelements;unlikethesticklikestructures,theyareusuallyspindleshapedandcan
presenttaperedtips[23](Figure3b–d).OnEarthtaperingisanimportantfeatureofichnofossillike
structuresbecauseitarguesagainstanichnologicalorigin[26].MostoftheVeraRubinRidge
structuresarenottapered;theonlyexceptionisrepresentedbyspecimenss1ands11,butitisunclear
whethertheirnarrowtipsresultedfromprimary(i.e.,structureproducing)orsecondary(e.g.,
weathering,fragmentation)processes.
Thecommonlysteepflanksofthesticklikestructures(Figure2a–c)areconsistentwithcrack
fills(Figure3b–d).Becauseofmorphologicalsimilaritiesandgeologicalcontext,sedimentarycracks
areaplausibleanalogueforthesticklikestructures.OnEarth,formationofcracksiscommonly
mediatedbymicrobialmats,whereasonMarscracksareassociatedtostructuresresembling
microbialites[27].Itshouldbehowevernotedthatabioticprocessescanexplaintheproductionof
cracks[23,24].
Nonsedimentarycrackingisanadditionalcandidateprocesstoexplainthesticklikestructures
becauseoftheabundantfracturesobservedonMars,wheresulfatemineralizedfracturesattributed
tohydraulicfracturingarefoundintheMurray[14]andYellowknifeBayformations[16].The
immediatesurroundingsofthesticklikestructuresshowabundantnonsedimentarycracks,but
theirsize,lackoffillandorientationaremarkedlydifferentwithrespecttothesticklikestructures.
Filledfracturescanalsoberejectedbecauseoftheirtypicaltabularshape,whereassticklike
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)andTjunctions
(Tj).PenhaGarciaIchnologicalPark,Portugal(LowertoMiddleOrdovician).
Thesticklikestructuresmightbeexplainedassinglemineralcrystalsbecauseoftheirelongate
shape,thecommonlypolygonalcrosssectionandtheflatfacesmeetingat~90°.Thesefeaturesare
shared,forinstance,withgypsumcrystalsgrowninsabkhaenvironmentsonEarth(Figure4a,b;see
alsoFigures9aand11ain[28]).Crosscuttingbetweens4ands5remindsofpenetrationtwinning,
i.e.,theprocessbywhichmineralsintergrowandresultininterpenetratingindividuals[29].Curved
morphologyofthesticklikestructuresdoesnotdisprovethenull(abiotic)hypothesissincecurved,
needlelikegypsumcrystalsofcomparablesizehavebeenreportedfromterrestrialevaporitic
settings[30,31].Sulfateminerals,includinggypsum,havebeendocumentedontheMartiansurface
withdatafromsatellites,landers,androvers[32].Amongotherprocesses,curvatureofmillimetre
sizedgypsumcrystalscanresultfromimpurityincorporationfollowedbycrackformationand
mechanicaltwinning[33].Nevertheless,euhedralmonocrystalsandtheirpseudomorphsarenot
perfectanaloguesofthesticklikestructures.Infact,thetipsofmoststicklikestructuresdonotshow
thesmoothflatfacesandthesharpcrystallikeoutlinesthattypifyeuhedralmonocrystals[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],canberejectedasanexplanationfortheVeraRubinRidgestructuresbecausethesticklike
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.Crystalsshowapolygonalcrosssection(Po).Largearrowsshow
pointedspursclosetosomevertices,whichareatypicalfeatureofaragonitecontacttwins.3.48Ga
oldDresserFormation(PilbaraCraton,Australia).Imagefrom[36].(d)Glendonitemouldsfromthe
shallowmarineRurikfjelletFormation[37]showingsharpcontactwiththehostrock(Sh).Early
CretaceousofSvalbard.
Althoughthenullhypothesis(abioticorigin)cannotbereadilydisproved,itshouldbenoted
thatthemorphologyofthesticklikestructuresresemblesthatofhorizontalburrowssuchas
Helminthoidichnites(Figure5a)andPlanolites,whicharecommoncomponentsoffluviolacustrine
settingsonEarth[38–40].ConstantwidthisatypicalfeatureofburrowsonEarth(Figure5a–d)since
tracemakerstendtominimizeenergyexpenditure,producingburrowsaswideastheirwidth[41].
SticklikestructuresarewithinthesizeandshaperangeofichnofossilsonEarth(seeFigure5).Most
specimensfromVeraRubinRidgepresentpolygonalcrosssections,assuggestedbyfragmented
specimensandtheprevailinglyflatuppersurfaces(Figure2a–c).Polygonalcrosssectionis
apparentlynonadaptiveforburrowingbehaviour,buttherearefewexamplesofichnofossilson
Earthwithsquarishsections,e.g.,Arthrophycus[42],(Figure5e,f)Bulbichnnus[43]andOblongichnus.
Subcylindricalspecimenswithroughlyellipticalcrosssectionarealsofound(e.g.,s2),althoughthey
arerarerthanpolygonalones.
Figure5.Ichnofossilsandtheirsalientfeatures.(a)Fossilburrows(Helminthoidichnites)showing
constantwidth(Co),curvedmorphology(Cu)andcolorcontrastwiththehostrock(Cc).Upper
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CarboniferousofthePramolloBasin,ItalyAustria[39,40].(b)Fossilburrows(Taenidiumisp.)
displayingconstantwidth(Co)andintersections(In).CortePereirosection,TocadaMouraComplex,
OssaMorenaZoneinsouthernPortugal(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,squarirshcrosssection(Po).Mação,Portugal(LowerOrdovician).
Specimen13,184oftheGeologicalMuseumofLisbon[44].(f)Detailofthesquarecrosssection(dashed
lines)ofArthrophycusalleghaniensis.ThemagnifiedareaisindicatedbyPoinFigure5e.
Moststicklikestructuresdonotgeometricallyinteractwitheachother,butsomepairsare
coalescingandintersecting.AprominentexampleconsistsofaSshapedstructure(s5)thatabruptly
changesitscurvatureatshortdistancefromanothersticklikefeature(s1),stayinginclosecontact
withitbutwithoutintersecting(Figure2a).Thisindicatesthatthedevelopmentofsticklike
structurescanbeinfluencedbyexisting(syngenetic)ones.OnEarth,thisisatypicaloutcomeoftaxes,
i.e.,theorientationandturningofanorganisminresponsetoaperceivedexternalstimulusfield[45].
SimilarlytotheichnotaxaHelminthoidichnites(Figure5a),HelminthorhapheandNereites
(=Helminthoida)irregularis(Figure5d),theSshapedstructurefromVeraRubinRidgecanbemodelled
byusingathigmotaxisrule,compellinganagenttostayclosetoanexistingentity[46,47].
Nevertheless,evaporiticmineralscangrowveryrapidly,henceweexpectthatacrystalcanactasa
physicalconstraintforthegrowthofsuccessive,almostsyngenetical,ones.Thisphenomenoncould
replicatethigmotaxisbehaviour,posingsignificantproblemsindisprovingthenull(abiotic)
hypothesis.
Softsedimentdeformation,fluidescapingandsedimentarycrackingmayaccountforthe
genesisofichnofossillikesedimentarystructures[48]butsuchabioticexplanationsoftheobserved
thigmotaxisliketopologicalrelationshiparedifficulttoproveforthoseprocesses.Verticalfluid
escapecanproducetinyverticaltubes(pillars)slicingsteeplythroughmassiveorlaminatedsand
[49].Alternatively,sandfilledfractures(sanddykes),commonlyrootedinastructurelesssandbed,
canbeformedbytheintroductionofmaterialfromabove,eitherunderpressureorbysimplefilling
ofpreexistingcracksorfissures.Sanddykesarestraighttosinuousinverticalview;theirsinuosity
canbeincreasedbypostdepositionalcompaction,withthedevelopmentofptygmaticfolds[50].
However,theseprocessesarenotknowntoproducethetopologicalrelationshipsobservedin
Martianspecimens.Similarly,toourknowledgetherearenoexamplesofsedimentarycracksthat
changetheircurvaturewhenencounteringothers.
TheSshapedstructureiscrosscutbyanothersticklikefeature(Figure1:s4),showingthatthe
herestudiedstructurescanalsointersectduringtheirdevelopment.Thisfeatureissharedwith
ichnofossils(Figure5b).Activedisplacementofsediment,includingcrosscutting,isconsidereda
strongindicatorofanichnologicalnatureforterrestrialstructures[26].Itshouldbehowevernoted
thatcrosscuttingisalsosharedwithcracksandsyngeneticalcrystalintergrowth.Ourimageanalysis
revealsanaverageintersectionangleof63.1°(intersectionanglerange:39.8°–85.6°).SomeoftheVera
RubinRidgestructurespresentangularterminations(e.g.,s5),whichisexpectedfrombreakageof
anorganismwithamodulargrowth[26].Similarsizeofdifferentstructuresandsinuousorientation
arealsocompatiblewithbodyfossilsofflexibleorganisms,whichhowevercannotbepreservedas
crosscuttingstructures.Crosscuttingprecludesthehypothesisofsticklikestructuresasbody
fossils.Inaddition,carbonizedremains,whichareacriterionforrecognizingthebodyfossilorigin
ofsedimentarystructures[26],arenotdocumentedfromtheVeraRubinRidgestructures.
Thestickshapedfeaturesareunlined;contactwiththehostrockissharpandwelldefined,
althoughthereisapparentlynodifferenceingrainsizebetweenthesticklikestructuresandthehost
rock.Sharpcontacts,andtheelevationofthesticklikestructuresrelativetothehostrock,suggest
thatthehostrockweatheredatafasterratethanthesticklikestructures(differentialweathering).
Hence,thesticklikestructuresarelikelytobemoreresistantthanthehostrock,possiblybecauseof
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moreintensecementation.Resistancetoerosionisconfirmedbythepresenceofanisolatedfragment
(Figure1:s2)thatexperiencedshortdistancetransport.
SticklikestructuresfromVeraRubinRidgearedarkertonedthanthealteredhostrockbutshow
similarhueswithrespecttotheunalteredsubstrate.Unlikethesubstrate,brokensegmentsoften
displaymetallichues(s5inFigures1and2a).Thisdoesnotprecludeanichnologicaloriginofthe
sticklikestructuresbecauseichnofossilswithmetallichuescanresultfromearlydiagenetic
processes,e.g.,onEarthpyriteandbaritefilledburrowshavebeendocumentedfromavarietyof
depositionalsettings,includingpostglaciallacustrineclays[51,52].Thesticklikestructuresarefound
overanareaofapproximately15cm2.Suchclustereddistributionimpliesthattheprocesses
responsibleforthedevelopmentand/orpreservationofthestructuresshouldhavebeenactive
locally.Thisiscompatiblewithbioticprocesses,e.g.,onEarthstressedenvironmentsare
characterizedbylowdiversityassemblages[53]withunevendistributionofichnofossils[54,55].
However,thisdoesnotprecludeanabioticoriginforthesticklikestructures.Forinstance,in
sedimentaryrocks,crystalsmaygrowinaggregatesdependingontheunevenlyhighconcentration
ofdissolvedions.
Thesticklikestructuresoccurinageologicalcontextthatfitswithevaporiticcrystalsand
sedimentarycracks.Thegeologicalcontextalsofitswiththeacceptedcriteriafortheidentificationof
ichnofossilsonEarth[56],i.e.,theyaresyngeneticwiththeprimaryfabricofthehostrockwhichis
sedimentaryandwellunderstoodintheregionalgeologicalcontextofGaleCrater;sticklike
structuresarecrosscutbylaterstagefracturesandarefilledwithmaterialcapabletosurvivetothe
diagenetichistoryoftherocks.Inaddition,sticklikestructuresarerelatedtoanancienthabitable
environment[14–18].Bulbousstructures(e.g.,structurebinFigure1)areembeddedinthehostrock
ofthesticklikestructures.Theyarehereinterpretedasconcretions,intendedasconfinedbodiesof
clasticsedimentlithifiedbyauthigenicminerals[57].OnEarth,formationofconcretionsiscommonly
mediatedbybiogenicprocessessuchasmicrobialactivityandthepresenceoforganicmatter
includingmucusonburrowwalls[58]—butfullyabioticprocessescanalsoexplaintheoriginofsome
concretions[59].Nevertheless,toourknowledgetherearenoexamplesofabioticconcretionssharing
themainmorphologicalfeaturesofthesticklikestructures,whichareroughlypolygonalshape,with
constantwidthandcurvedorientation.OnEarth,curvedburrowsrepresentthemain
physicochemicalsettingforthedevelopmentofcurvedconcretions[58].Therearenoconclusive
evidencesshowingacommonoriginoftheconcretionsfromHaroldswickandthesticklike
structures.Infact,concretionsshareasimilartexture,sizeandcolourwiththesticklikestructures,
buttheylackthemetallichuesthataresometimesobservedinsticklikestructures.Inaddition,they
differfromthesticklikestructuresinhavingapredominantlyellipticalcrosssectionwithalarger
size(majoraxisistypically~1.5mm).OtherpostdepositionalstructureswithintheMurray
Formationincludeenhancedrelieffeatures,darkraisedridgesandveins[60].Amongthese,
enhancedrelieffeaturesshareseveralfeatureswiththesticklikestructuresofHaroldswick,i.e.,they
areembeddedinthehostrock,showtopographicreliefabovethesurface,theyarenotclearlyrelated
tofracturepatterns,theyaredarkerthanthealteredhostrock[60].Unlikethesticklikestructures,
enhancedrelieffeaturestendtobedendritic,theirwidthisnotconstantand,mostimportantofall,
thecontactswiththehostrockshownoevidenceofdistinctboundaries.
Thehypothesistestingapproachweappliedfindsastrongparallelintheprotocolproposedby
BrasierandWacey[61,62]fordemonstratingbiogenicityofcandidatebodyfossilsandstromatolites.
Followingthisapproach,candidatefossilstructuresshouldnotbeacceptedasofbiologicalorigin
untilallplausibleexplanationsoftheirnonbiologicaloriginhavebeentestedandfalsified,thatis,
untilthenullhypothesisofanabiogenicoriginisrejected.Ourresultsshowthatnoneoftheplausible
nonbiologicalexplanationsofthesticklikestructuresisfullysatisfactory.Itmaybetherefore
temptingtoclaimthebiogenicityofthesticklikestructures,alsobecausenoneoftheavailable
observationsfalsifiesaneventualichnofossiloriginofthem.
Nevertheless,ourresultsdonotallowtoclaimthebiogenicityofthesticklikestructures.Infact,
availableevidencecannotdisprovethenullhypothesis,thatis,theabioticnatureofthesticklike
structures.CarlSagan’stenet(“Extraordinaryclaimsrequireextraordinaryevidence”;[63])doesnot
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onlysuggestcautionwheninterpretingextraterrestrialichnofossillikestructures,butitalso
encouragesthesearchforextraordinaryevidence(e.g.,regularlymeanderingichnofossils)onMars.
Insum,ourobservationsdonotnecessarilyimplythatthesticklikestructuresarebiogenicbut,
giventhatnoneoftheavailableobservationsatVeraRubinRidgefalsifiestheirichnofossilorigin,the
sticklikestructuresarehereidentifiedasaprivilegedtargetforastrobiologicalresearch.Assuch,the
sticklikestructuresfromVeraRubinRidgefollowtherecommendationofNASA’sMars2020Science
DefinitionTeam[64]inthattheymighthaveabiologicaloriginandthuscompelresearcherstogather
moredatabeforereachingaconclusionastothepresenceorabsenceofpastlife(seealso[65]).
Ourresultsalsohighlighttheurgentneedofestablishingarobustsetofquantitativecriteriafor
demonstratingthebiogenicityofichnofossillikestructures,microscopicandmacroscopic.Inother
words,thereistheneedofdefiningthemorphospaceofichnofossilsandabiogenicichnofossillike
structures.Thiswillallowtorejectthenullhypothesisofanabiogenicoriginforcandidate
ichnofossilsviamorphospaceanalysis.Thisclearlyparallelsthecellmorphospaceapproach[61].
Theestablishmentofbiogenicitycriteriawouldbenefitnotonlythesearchforextraterrestrial
life,butalsothesearchforPrecambrianlifeonEarth.Infact,thePrecambriangeologicalrecord
comprisesseveralproblematicstructuresthathavebeenrejectedastracefossilsandreinterpretedas
eitherabiogenicstructuresorbodyfossilsinsomecases[26,66].Ichnofossillikepseudofossilshave
alsobeenrecognizedinmorerecentdeposits[67].Ametazoanlikebodyplanisnotessentialfor
producingmacroscopictraces,asevidencedbymicrobialrelatedstructuresinOrdovicianlow
oxygenenvironments[68].Microorganismscanevenproducemacroscopictrailsorburrows,as
evidencedby2.1Gaichnofossilsattributedtomulticellularorsyncytialorganismabletomigrate
laterallyandvertically[69].Thesenonmetazoanmacroscopicichnofossilsencourageresearchof
extraterrestrialanalogues,i.e.,ichnofossilsproducedbytheaggregationofamoeboidcellsintoa
motilesluglikephase,similartothatofterrestrialslimemolds.
Inaddition,fivepropertiesofichnofossilshighlighttheastrobiologicalpotentialofthese
biogenicstructuresandthereforeencouragetheestablishmentofbiogenicitycriteriaforichnofossil
likestructures[70].First,theypreservetheactivityofsoftbodied(e.g.,[71])orpoorlymineralized
(e.g.,[72])organisms,togetherwithskeletonizedforms.Second,theyareresilienttoprocessesthat
obliterateotherbiosignatures,asevidencedbytracefossilspreservedinmetamorphosedandhighly
tectonizedrocks[73–75].Becauseoftheirpreservationpotentialandabundance,ichnofossilsare
amongthemostabundantevidenceofmicroscopicandmacroscopicpastlife,asexemplifiedby3.7
billionyearsoldstromatolites[76],2.1billionyearsoldburrows[69],andca.551millionyearsold
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
Thenewobservationalandmorphometricdatareportedherehavefarreachingimplicationsfor
thesearchofextraterrestriallife.Ourmorphometricandtopologicdataareuniquetothesticklike
structuresamongMartiangeologicalfeaturesandshowthatichnofossilsareamongtheclosest
morphologicalanaloguesoftheseuniquefeatures.
Infact,thesticklikestructuresshowmorphological(elongateshape;presenceofcurves;
constantwidth;clustereddistribution)andtopological(crosscutting,coalescingrelationships)
featuresthatarecommonlyobservedinichnofossils(Table1).Geologicalcontextandsizearealso
compatiblewithichnofossils.Bycontrast,steepflanksandthesomewhatpolygonalcrosssectionare
Geosciences2020,10,3910of18
rarelyobservedinichnofossilsonEarth,withfewexceptions(Arthrophycus,Oblongichnus,
Bulbichnus).
Table1.Characteristicsofthesticklikestructuresandtheiroccurrenceintheirclosestmorphological
analogues.Fiveoccurrenceclassesareused(fromthemostfrequenttothelessfrequent:always,
usually,often,rarely,never).Colorscalereflectsfrequencyofoccurrence.
StickLikeStructuresSynaeresis
Cracks
EuhedralSingle
Crystals
Bioturbational
Ichnofossils
Geological
context
syngeneticwiththehostrockalwaysalwaysalways
fluviallacustrineoftenoftenoften
Morphology
elongateshapeinbeddingplane
viewusuallyusuallyusually
curvedusuallyrarelyusually
constantwidth(notapering)rarelyusuallyusually
Tjunctions,incompletepolygonsalwaysoftenoften
polygonalcrosssectionusuallyalwaysrarely
notipswithsmoothflatfacesusuallyrarelyusually
notendencytosplitalongdefinite
planesofweaknessalwaysrarely(cleavage)often
sharpcontactwiththehostrockalwaysalwaysusually
steepflanksalwaysalwaysrarely
Topology
crosscuttingrarelyusually
(twinning)
usually(false
branching)
coalescing(changetheircurvature
whenencounteringothers)rarelyrarelyoften
straightsegmentsarenotparallel
betweeneachotheralwaysusuallyusually
Other
metallichuesneveroftenrarely
millimetricwidthrarelyoftenoften
clustereddistributionoftenoftenoften
Followingtheinterpretationofotherextraterrestrialbiogeniclikestructures[65],theabsenceof
unequivocalevidenceforlifeonMarsistheonlyevidencethatfavoursanabioticoriginofthestick
likestructures.Nevertheless,availabledatacannotfullydisprovetwomajorabiotichypotheses,that
aresedimentarycrackingandevaporiticcrystalgrowthasgeneticprocessesforthestructures.The
polygonalsectionandthesteepflankssupportthecrystalhypothesis,althoughthesticklike
structureslackthesmoothflatfacesandthesharpcrystallikeoutlinesthattypifythetipsofeuhedral
monocrystals.TheTjunctionsandincompletepolygonsformedbythesticklikestructuresremind
offilledsedimentarycracks,althoughshrinkagecracks,unlikethesticklikestructures,areusually
spindleshapedandcanpresenttaperedtips(Table1).Accordingly,theavailableobservationsat
VeraRubinRidgedonotallowtoclaimthebiogenicityofthesticklikestructures,buttheyencourage
thecollectionoffurtherdata,especiallygeochemical,aboutthiskindofsedimentarystructureson
Mars.
Consequently,thiscasestudyraisesthequestionofhowtodisprovetheabioticoriginofan
ichnofossillikestructureinancientsedimentarydepositsonEarthandbeyond.Theanswertothis
questionnecessarilyliesinthestudyofmorphologyandgeologicalcontext,sincethechemical
compositionofpassivelyfilledichnofossilsisnotnecessarilyrelatedtobiologicprocesses.This
question,anditsconverse(howtoprovebiogenicityofichnofossillikestructures)areoffundamental
importancenotonlybecauseichnofossilsarethemostpersistentmacroscopicevidenceforlifeon
Earth,butalsobecausetheyareindependentfrommorphology,sizeandbiochemistryofthe
producer.Assuch,ichnofossilswouldallowtorecognizelifethatdiffersfromknownterrestrial
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,projectsALANX,
PALEOGIANTS,CURIOSITY.
Acknowledgments:Twoanonymousreviewersarethankedfortheirconstructivecommentswhichgreatly
improvedthepaper.DerekBurgess(MartinMarietta,Raleigh,NC, USA)andDavidLoope(Universityof
NebraskaLincoln,Lincoln,NE,USA)isthankedfordiscussiononconcretions.WethankBarrydiGregorio
(UniversityofBuckingam,Buckingham,UK)fordiscussiononthebiogenicityofthestructuresandwe
acknowledgehimasthefirsttomaketheparallelbetweenichnofossilsandthesticklikestructures.Lorenzo
Bonini,AngeloDeMin,RomanaMelis,StefanoFurlani,MaurizioPonton,FrancescoPrincivalle,NevioPugliese
(UniversityofTrieste,Trieste,Italy)arethankedfordiscussiononthemorphologyofthestructures.Six
anonymousreviewersarethankedforrevisingapreliminaryversionofthemanuscript.AramBayetGoll(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).JuanManuelGarcíaRuiz
(CSIC,UniversidaddeGranada,Granada,Spain)isthankedforprovidingapseudomorphpicture(Figure4c).
MadelineVickers(UniversityofCopenaghen,Copenaghen,Denmark)isacknowledgedforprovidinga
glendonitepicture(Figure4d).AlexanderE.S.VanDriessche(ISTerreCNRS&Univ.GrenobleAlpes)and
MaciejBąbel(UniversityofWarsaw,Warsaw,Poland)arethankedforcrystalimages.
ConflictsofInterest:Theauthorsdeclarenoconflictofinterest.
AppendixA
FigureA1.Widthanalysismap.Numbers1–34refertothemeasurementIDinTableA1.
Geosciences2020,10,3912of18
FigureA2.Lengthanalysismap.
FigureA3.Angleanalysismap.
TableA1.(followingpage).WidthmeasuresofthesticklikestructuresfromVeraRubinRidge,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.LengthmeasuresofthesticklikestructuresfromVeraRubinRidge,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.AnglemeasuresofthesticklikestructuresfromVeraRubinRidge,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.;BrighamGrette,J.Sedimentology,claymineralogyandgrainsizeas
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.StickShape,RiceSizeFeaturesonMartianRock“Haroldswick”.Availableonline:
https://www.nasa.gov/imagefeature/jpl/pia22213/stickshapericesizefeaturesonmartianrock
haroldswick(accessedon8October2019).
4. DiGregorio,B.IchnologicalevidenceforbioturbationinanancientlakeatVeraRubinRidge,GaleCrater,
Mars.InProceedingsofthe3rdInternationalConventiononGeosciencesandRemoteSensing,1920
October2018,Ottawa,ON,Canada,2018;pp.1–7.
5. Howell,E.No,ThoseAren’tAnimalTracksonMars.Availableonline:https://www.space.com/39894
marsrockfeaturesnotanimaltracks.html(accessedon8October2019).
6. Howell,E.DidCrystalsfromAncientLakesonMarsFormTheseTiny,WeirdThings?Availableonline:
https://www.space.com/39687ancientmarslakesmadeweirdcrystalfeatures.html(accessedon8
October2019).
7. David,L.CuriosityRoverSpotsWeirdTubeLikeStructuresonMars.Availableonline:
https://www.space.com/39294marsrovercuriosityweirdtubestructures.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.TheroleoflargescalediagenesisintheformationofVeraRubinRidge
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.AhematitebearinglayerinGalecrater,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,andevidenceforlargescalediagenesisinVeraRubinridgebedrockasimpliedbyChemCam
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.AhabitablefluviolacustrineenvironmentatYellowknifeBay,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. AlYoussef,M.GypsumCrystalsFormationandHabits,UmmSaidSabkha,Qatar.InSabkhaEcosystems
VolumeIV:CashCropHalophyteandBiodiversityConservation.PartoftheTasksforVegetationScienceBookSeries
(TAVS,Volume47);Khan,M.A.,Böer,B.,Öztürk,M.,Zahran,T.,Abdessalaam,A.,ClüsenerGodt,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.;Franchiniangela,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:WileyBlackwell,Hoboken,2011;Volume43,pp.183–204,ISBN9781444339109.
36. Otálora,F.;Mazurier,A.;GarciaRuiz,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.Animprovedmodelfortheikaiteglendonite
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;ItalyAustria).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
theUpperCambrianLowerTremadocianofNorthwestArgentina:ImplicationsfortheArthrophycid
LineageandPotentialinIchnostratigraphy.Ichnos2005,12,179–190.
43. Monaco,P.Bulbichnusgiorniin.ichnogen.andn.ichnosp.:Adeepwaterdomichnionpraedichnionmade
byaneunicidpolychaete(MarnosoArenaceaFormation,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.Syndepositionaltectonicsrecordedbysoftsedimentdeformationandliquefaction
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.;SchultzEla,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(PermianTriassic,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.MicrobiallydrivenformationofCenozoicsiderite
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.Microbialrelatedbiogenicstructuresfrom
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.;RollionBard,C.;Bankole,O.;etal.Organismmotilityinanoxygenatedshallowmarine.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.Organismsubstrateinteractionsandastrobiology:Potential,models,
methods.EarthSci.Rev.2016,171,141–180.
71. Seilacher,A.;Buatois,L.A.;Mángano,G.TracefossilsintheEdiacaranCambriantransition: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
distributionofcrustaceanichnofossilsintheLowerJurassicUpperCretaceousofAtlanticoccidental
marginbasins,Portugal.ActaGeol.Pol.2010,60,19–28.
73. NetoDeCarvalho,C.;Baucon,A.Gianttrilobiteburrowsandtheirpaleobiologicalsignificance(Lowerto
MiddleOrdovicianfromPenhaGarcia,Portugal).Comun.Geol.2016,103,71–82.
74. NetoDeCarvalho,C.;Baucon,A.;Gonçalves,D.DaedalusmegaichnositefromtheMuradalMountain
(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
shownbydiscoveryof3,700millionyearoldmicrobialstructures.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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... Numerous cocoon-eggs photographed on Sol 1032 are also embedded in substrate that appears to consist entirely of a coral-microbialite-like mass of coiled and worm-like forms ( [1][2][3][4][5]; including specimens that appear to have multiple appendages and resemble insecta (Arthropoda) [6][7] whereas others are similar to ovoid-shaped lichenizedfungi topped with a single hole and which may be sporing. Moreover, some of the specimens resemble dead or fossilized puffballs with stems attached. ...
... In other previous reports elongated tubular specimens photographed in Gale Crater have been interpreted as ichnofossils of burrowing worms [1,[3][4][5]. This interpretation was subsequently rejected by a NASA-controlled journal [21] which also failed to review or acknowledge the comparative statistical and quantitative morphological evidence that favors a biological interpretation [3][4][5]22]. ...
... In other previous reports elongated tubular specimens photographed in Gale Crater have been interpreted as ichnofossils of burrowing worms [1,[3][4][5]. This interpretation was subsequently rejected by a NASA-controlled journal [21] which also failed to review or acknowledge the comparative statistical and quantitative morphological evidence that favors a biological interpretation [3][4][5]22]. Tubular forms, interpreted as "tube worms" and "tube worm casings" as well as specimens resembling crustaceans and mass groupings of coiled worm-like forms have also been photographed in the dried lake-beds of Endeavor Crater. There were photographed adjacent to holes and upon soil and matrix the chemistry and mineralogy of which is consistent with an ancient lake or inland sea heated by thermal vents [3,17,22]. ...
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White and dark gray oblong-ovoid cocoon-egg-like forms (“cocoon-eggs”), less than a mm in diameter, many with a single hole in one end, and some with unidentifiable specimens protruding from (exiting or entering) these holes, were photographed on mudstone in Gale Crater, Mars, on Sol 1302. If biological some of these cocoon-eggs may be frozen, fossilized or have already “hatched” or undergone metamorphosis. Specimens resembling segmented and tubular worms and rhizoids and those with multiple appendages (Arthropoda) were also photographed on these and other substrates on Mars, including on Sols 130, 132, 302, 551, 553, 808, 809, 868, 906, 1280, 1921, in Gale Crater. Many Sol 1302 “cocoon-eggs” are embedded in substrate that appears to consist entirely of a coral-stromatolite-like mass of worm- and larvae-like forms; and similar worm and segmented larvae forms appear in the interior of three semi- transparent ovoid structures. Embedded in the soil surrounding these “cocoon-eggs” are numerous concave patches reminiscent of dried “egg-yolk,” cocoon remnants or fungus, and that may represent the final stage of “cocoon” collapse and disintegration; or they are fungal in origin. Some of the segmented and tubular worm-like forms may include rhizoids and tubular fungi. Ovoids with open apertures may be evidence of fungal sporing. The possibility that fungus and diverse species and their “cocoons” share the same substrate is supported by what appears to be cyanobacteria, worms, insecta (arthropoda), and stalks topped by ovoids or ovoid-shaped microbialite-like sediment. Fungi, lichens, cyanobacteria, stromatolites and arthropoda have been previously reported in other areas of Gale Crater, whereas fungal “puffballs” and specimens attached to rocks with stalks and topped with mushroom-like caps have been observed in Meridiani Planum. The possibility these specimens are abiogenic cannot be ruled out. The evidence, in our view, arguably supports the hypotheses that various organisms colonized Mars in the ancient or recent past.