BenLloyd
3/31/16
TheKeystoSurvivalforBirdsDuringtheCretaceous-TertiaryExtinction
Sixty-sixmillionyearsago,asixmile-wideasteroidslammedintothe
earthscrustatasitethatisnowatthe
northerntipoftheYucatanPeninsula.
Theinitialimpactwipedoutmany
species,andtheworld-widecoatingof
ashandclimate-alteringdebriscloud
createdmoredestruction.Over80%of
speciesaliveatthattimewentextinct.
Figure1.BirdsweretheonlyspeciesofdinosaurtosurvivetheK-TExtinction.
https://www.sciencerecorder.com/news/2015/06/05/cretaceous-bird-fossiloffers-clues-on-dinosaur-evolution/
Notablyresilientspecies,suchasthesmall,burrowingmammalsthatgaverisetoallmammals—
includinghumans—madeitthroughtheeventduetotheirabilitytoliveundergroundandeatalmost
anything.Anothernotableclassofsurvivorsarebirds,thoughtheirsurvivalisnotnearlyaseasyto
explainasthesurvivalofancientmammals.Unlikemammals,prehistoricbirdsweredirectrelativesof
thedinosaursthatwerecompletelywipedoutduringtheextinction.Infact,birdsaretechnicallytheone
classofdinosaursthatmadeitthroughthebolideimpact.Whatsetbirdsapartfromtheirdinosaur
counterparts?Whywerethey,alone,abletoescapeextinction?
Thereasonforthesurvivalofbirdsremainsagreatmystery.Therehasn’tbeenasingular,large
factorthatcanbeattributedtotheirsuccesswhentherestofthedinosaursdiedout.Rather,the
generalconsensusinthescientificcommunityisthatbirds’abilitytopersevererestedonmultiplesmall
adaptations.Thelargestofthoseadaptationsisthefactthatmostbirdswerenotaslargeastheir
dinosaurcounterparts.From230millionyearsago
allthewaytheK-Textinction,164millionyears
later,dinosaurbodysizehadbeensteadily
trendingupwards.EarlyTriassicdinosaurs
weighed,onaverage,nomorethanamedium
sizeddog.Theendofthecretaceouswashometo
suchfamouscreaturesastheTitanosaur,which
Figure2.Artist'srenditionofaMicroraptor,oneoftheEarly
Cretaceousancestorsofmodernbirds.
http://kids.nationalgeographic.com/content/dam/kids/photo
s/animals/Dinosaurs%20&%20Extinct/HP/microraptor.jpg.adapt.945.1.jpg
was122feetlongandweigheduptoseventytons.
Onlyonegroupofdinosaursstayedsmall:the
maniraptorans.Thisgroupoffeathereddinosaurs
includedvelociraptorsandeventuallyevolvedintobirds.Whenthatfatefulasteroidimpactedearth66
millionyearsago,theonlydinosaursurvivorsweremaniraptoransthathadevolvedtoweighunder1
kilogram.Theseanimalswerebirds.Large-bodiedanimalswerenotasadaptableasthesmall
maniraptorans;theyweren’tasmobileandrequiredmuchlargerquantitiesoffood.
Whilebodysizeislikelythebiggestfactorinunravelingthemysteryofbirds’survival,thereare
anumberofotherfactorsthatallowedbirdstoadapttoearth’snewpost-apocalypticclimate.There
wereotherflyinganimalsaround,suchaspterosaurs,thatwould’vebeenabletoevadetheinitial
destruction.Therewerealsoplentyofothersmallanimalsthatperished.Smallbodysizeandtheability
toflywerenotagoldentickettoescapingthe5thmajormassextinction.Twofossilizedseabirdswere
recoveredfromtheLondonClayFormationontheIsleofSheppey,andwerefoundtohave
exceptionallylargebraincavities.These55-million-year-oldbirdshadbrainsthatwereeverybitas
developedasmodernbirdsintheareasofsight,flight,andhigh-levelfunctionssuchastheabilityto
learnandrememberthings.Allmodernbirdbrainshaveanareaofextremelydensebraintissue,known
asthe“wulst.”Thisareaallowsmodernbirdstopackmorebrainpowerintoamorecompactandlight
package,whichisimportantforflying.
Everygramcounts.Unlikepreviously
thought,theseprehistoricbirdshad
wulststhatwerenearlyasdevelopedas
modernbirds.Theincreasedbrain
powerprovidedbythewulstswould’ve
suggestedthatthebirdsaroundatthe
endofthecretaceousweremuch
Figure3.OdontopteryxToliapica,thefossilizedPaleocenebirdsfoundon
theIsleofSheppey.
http://vignette3.wikia.nocookie.net/prehistrico/images/9/90/Osteodontor
nis_orri.jpg/revision/latest?cb=20150619142824&path-prefix=es
smarterthanpreviouslythought.This
increasedadaptabilitycould’vebeen
thedifferencebetweenlifeordeathduringtheEnd-CretaceousExtinction.
Beyondbrainandbodysize,theeggshapeofMesozoicbirdscontributedtotheirsurvivalofthe
initialbolideimpactduringtheK-TExtinction.Bird’slargercounterpartshadmuchmoreelongatedeggs
withsignificantlymoreporousshells.Theseporousshellscould’vebeenmoresusceptibletothe
contaminantsthrownintotheatmospherebythebolideimpact.Ancientbirdeggswerealso
proportionallylargerthantheeggsofotherdinosaurs,suggestingthatbirdhatchlingshadashorter,less
vulnerablechildhoodthanotherdinosaurs.Slowreproductiveperiodsmayhavehadalargepartinthe
extinctionofmostdinosaurs.Inadditiontoeggshape,incubationhabitsofancientbirdsmayhave
contributedtotheirsurvival.MostdinosaursintheLateCretaceoussimplyburiedtheireggswhilethey
incubate,whileancientbirdslikelyusedthecontactincubationmethodthatisprevalenttoday.If66million-year-oldbirdssatontopoftheireggsandprotectedthem,theywould’vehadalargerchanceof
survivinginthestrainedenvironmentthatfollowedtheimpact.
ThesurvivalofbirdsduringtheEnd-CretaceousExtinctionisoneofthemostremarkable
outcomesofthatevent.Birdswereuniqueintheirsurvival;alloftheirclosestrelativesdiedout,butfor
somereason,theydidnot.Aseriesofsmall,luckyadaptationsprovidedthenecessaryresistanceagainst
theasteroidthathittheearth66millionyearsago.Theearthisfacinganewcatastropheinthepresent
day;the6thmassextinction.Thistime,it’snotanasteroid;it’samixofhuman-causedactivities,ranging
fromearthsurfacedisturbancetointerspeciesmixingtolarge-scalegreenhousegasemissions.The
adaptationsthatgotbirdsthroughthe5thextinctionmayproveadvantageousduringthe6th,
butaswehaveseenwithNorthAmericanbats,centralAmericanamphibians,andtheextinct
GreatAuk,survivaltacticsthathaveworkedforhundredsofmillionsofyearsmaysuddenlyfail
inthepresenceofhumans.
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