Birds evolved from archosaurian reptiles and the oldest known bird fossil is Archaeopteryx lithographica from 150 million years ago. While the dinosaurian origin of birds was widely accepted, alternative theories emerged but were later disproven. Mesozoic birds diversified in the Early Cretaceous with the evolution of feathers likely beginning in non-flying dinosaurs. Modern flight capabilities evolved later. The mass extinction at the end of the Cretaceous wiped out many bird lineages but some survived to diversify after, resulting in over 10,000 living species divided into paleognaths and neognaths. Current evolutionary trends include higher extinction rates than new species emergence and increased hybridization due to human activities.

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EVOLUTIONARY TRENDS OF BIRDS
PRABHU PRASAD MANDAL
EGE052225

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INRODUCTION
Birdsareoneof themostconspicuousgroupsofanimalsinthemodernworld.
 Theirfeatheredbodiesareoptimizedforflight,theirsuperchargedgrowthrates
andmetabolism,andtheirlargebrains,keensensesmakethemsomeof themost
intelligentorganismsontheplanet.
Birdshavebeencalledglorifiedreptilesbecausetheydescendedfrom
archosaurianreptiles.
TheoldestknownbirdfossilisArchaeopteryxlithographica,whichhasamixof
reptilianandavianfeatures
(Archaeopteryx lithographica)

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(Archaeopteryx
lithographica)

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The origin of birds: is the debate already over?
• Thestudyofbirdoriginisover150yearsold.Thedinosaurianoriginof birdshadgainedbroadsupport
sincetheresemblancebetweenbirdsandtheropodswasfirstrecognizedbyHuxley(1868)andother
palaeontologists.
• Thistheoryheldswayuntilthepublicationoftheclassicbook‘Theoriginofbirds’ byHeilman(1926).
• Heilmanshowedthatdespitethesimilaritybetweenbirdsandtheropodsdinosaurswereprobablytoo
specialized(e.g.,he suggestedthatthereducedclaviclesofdinosaurscouldnothaveevolvedintothe
furculainbirds)tobethedirectancestorsofbirds.
• Insteadheproposedthatbirdsanddinosaursprobablyevolvedfromacommonancestorinagroupcalled
Thecodontia.Heilman'sproposalwassoauthoritativeandinfluentialthatthethecodontoriginofbirds
becametheprevalenttheoryfornearlyhalfacentury.

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Mesozoic birds
 The First100Million Years ofAvian History Birds haddiversified bytheEarly
Cretaceous, evolving intoanumberof groups ofvarying anatomyandecology.
 This diversification isrecorded bythefossils of the Jehol Biota of northeastern
China.
True modernbirds — membersofthecrowngroup Neornithes — areamostlypost-
Cretaceous radiation, although thereissomefossil evidencefor Cretaceous
species.
 features, suchasrapid growth, akeeled sternum,pygostyle, andbeak,areabsent
intheearliest birds andevolved inmore derivedbirds duringtheCretaceous.
The bipedal posture,hinge-like ankle, hollowed bones, andlong S-shapedneckof
birdswere inherited from deepdinosaurian ancestors,thewishbone (furcula) and
three-fingered handsof birdsfirst appearedin primitive theropods.
Perhapsthesingle mostrecognizable feature of birdsisfeathers, whichareused
toconstructanairfoil for flight (thewing), andalso for display, thermoregulation,
andeggbrooding.

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 The evolution of featherslikely beganintheearliest dinosaurs. A varietyof primitive theropods, suchas Sinosauropteryx anda
growing numberofplant-eating ornithischian dinosaurs arenow knownfrom spectacularlypreservedfossils coveredinsimple,
hair-like filaments called ‘protofeathers’thatarewidely considered tobetheearliest stageof feather evolution.
 theearliest feathersevolved innon-flying dinosaurs, likely for display and/orthermoregulation, and only later weretheyco-
opted intoflight structuresintheearliest birds.
The earliestbirds lackedmanykeyfeatures relatedto powered flight inmodernbirds,andprobably hadprimitive flight
capabilities thatvaried substantially betweengroups.
For exampleArchaeopteryx lackedabony sternumandevenacompensatoryspecialized gastralbasketfor anchoring large
flight.
 Itwasonly inbirds muchmore derivedthanArchaeopteryx thatthefully modernstyleofavian flight developed, enabledbya
keeledsternumsupporting enormous flight muscles,atail ,andacomplete triosseal canal intheshoulder.
 Theseinnovations thencombined withfeatures evolved earlier inbirdsandtheirnon-dinosaurian relatives toproduce highly
efficient, muscle-driven flight seenintoday’sbirds, whichallows somespeciestofly ataltitudes of9,000metersandover
distancesof hundredsof kilometers.
 This modernstyleof flight developed withorneartheorigin of Ornithuromorpha.
Otherdistinctive anatomical features of modernbirds, relating tothesensoryandrespiratory systems,first evolved intheir
dinosaurian ancestors.
 Living birdsarehighly intelligent withkeensenses,enabledinpartbyaforebrain thatisexpandedrelative tobody.

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Birds Dealt with a Crisis at the End of the Cretaceous
The courseof avianhistory wasaffected bythemassextinctionattheendof theCretaceous, 66million yearsago, which
wipedout all non-avian dinosaursandmanyothergroups .
The extinctionwasgeologically rapid andmostlikely causedbytheimpactof alarge asteroid orcomet.
Ittriggered aglobal cataclysmofclimate andtemperaturechange,acid rain,earthquakes,tsunamis,andwildfires .
Birdswere diverseintheLate Cretaceous, withmanyof thecharacteristiclineages of ‘archaic’birds fromthe JeholBiota
(speciesoutside ofthe neornithine crown) living alongside whatwasprobably amoderatediversityof early neornithines, as
indicatedbyrare fossils andmolecular phylogenetic studiestracing somemodernlineages intotheCretaceous.
 Noneof these‘archaic’ non-neornithine birds, however, survivedpasttheCretaceous andintothePaleogene.
This also indicatesthatbirdswere strongly affected bytheend-Cretaceous extinction, withmanymajorearly groups going
extinct.
 Multiple lineages ofearly neornithines haveendured theextinction, leaving themtheonly surviving membersof theinitial
Mesozoicradiation of birds.

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Birds after the Cretaceous
The morethan10,000 speciesof birdsliving intoday’sworld are dividedintotwomajor groups: the Palaeognathae, includes
flightless forms, suchaskiwis,ostriches, emus…)andthe Neognathae, thatincludes theremainderof bird diversity.
The Neognathae, iscomposedof thesubgroup Galloanserae (the‘fowls’, including ducks,chickens,andgeese)and Neoaves
(whichincludes everythingfrom pigeons andowls tofalcons andparrots).
 Atleast four tosixof thesebasal Neoaves lineages andseveralmembersof Palaeognathaeand Galloanseres are predictedto
havepassedthroughtheend-Cretaceous extinction.
 The newphylogeny helpsto betterunderstandtheevolution of mostintriguing traitsofsomeliving birds:vocal learning,
including theability ofsomespeciestoimitate humanspeech.This isaveryraretrait, seeninonly insongbirds, parrots, and
hummingbirdsamong birdsandveryfew mammals(e.g. dolphins, bats,elephants,andhumans)butnot non-humanprimates.
 All thevocal-learning bird lineagesandhumanswerefound tohaveevolved convergentmutationsandchangesingene
expressionin theregions of thebrainthatcontrol song(bird) andspeech(human).
Agreatamount of diversityandconvergencethatoccurred amongbirds during thepost Cretaceous revolution.

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 Evolution is aslowprocess.
 Bird speciesarecurrently going extinctata far greaterratethananypossible speciationorother generation of
newspecies.
 The disappearanceofapopulation, subspecies,orspeciesrepresentsthepermanentlossofarange ofgenes.
 Anotherconcernwithevolutionary implications isasuspectedincreasein hybridization. This mayarisefrom
humanalteration of habitatsenabling related allopatric speciestooverlap.
 Forest fragmentation cancreateextensiveopen areas,connecting previously isolated patchesof openhabitat.
 Several speciesof birdshavebeenbredincaptivityto createvariations onwild species.Insomebirdsthis islimited
tocolor variations, while others arebred for larger eggor meatproduction, for flightless-ness orother
characteristics.
Current evolutionary trends in birds

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