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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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