Phylogeny of Birds – Class Aves • Birds first appear, unambiguously, in the fossil record in the Jurassic ( ~ 150 mya). First bird is Archaeopteryx. • Transitional form between reptiles and modern birds. • Possessed reptilian skull with teeth, long bony tail, and claws on digits, but had feathers just like modern
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Phylogeny of Birds – Class Aves Birds first appear, unambiguously, in the fossil record in the Jurassic ( ~ 150 mya). First bird is Archaeopteryx. Transitional.
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Phylogeny of Birds – Class Aves
• Birds first appear, unambiguously, in the fossil record in the Jurassic (~150 mya). First bird is Archaeopteryx.
• Transitional form between reptiles and modern birds.
• Possessed reptilian skull with teeth, long bony tail, and claws on digits, but had feathers just like modern birds.
Archaeopteryx
Origin of Birds• Two schools of thought on bird ancestry:• Coelurosaurian Theropod (Dinosaur) Ancestry =
contends that birds derived from dinosaurs because they share many unique skeletal characteristics. – Main problem = coelurosaurs are contemporaneous or later
than early birds and “you can’t be older than your ancestors.” But, coelurosaurs and birds may share common ancestor.
• Thecodont Ancestry = contends that birds are derived from thecodont ancestor, probably in late Triassic period. – Main problem = thecodonts are primitive archosaurs and not
many unique skeletal features are shared between them and Archaeopteryx.
Thecodont
Coelurosaur
Fig 3.27 – Phylogeneticrelationships among the Amniotes
Note that birdsare included within the dinosaurs in thisphylogeny
Archaeopteryx and Bird Origins• Archaeopteryx was capable of powered flight– Shows features of modern flying birds (asymmetric
feather vanes, acute scapula-coracoid angle), although flight apparatus was primitive.
• So, because early birds flew, all subsequent birds were derived from flying ancestors.
• Because of adaptations for flight, birds are anatomically very uniform, moreso than any other vertebrate Class.
Angle in flightlessbirds
Classification of Birds• Class Aves divided into 2 Subclasses:– Subclass Sauriurae
Class Mammalia• All have hair and nurse young• Possess heterodontous teeth specialized for specific
functions• Other unique traits = sweat and sebaceous glands,
anucleate red blood cells• Divided into 2 Subclasses (about 5500 total species)– Subclass Monotremata – includes platypus and spiny echidna
• Lack nipples and external ears• Embryos develop in shelled eggs
– Subclass Theria (all bear live young and nurse through nipples)• Infraclass Symmetrodonta• Infraclass Metatheria – Marsupials • Infraclass Eutheria – Placental Mammals
Living Monotremes
Duck-billed Platypus
Spiny Echidna
Subclass Theria• Infraclass Symmetrodonta– Basal group of Mesozoic Mammals (Triassic to late
Cretaceous)– Molars are triangular when viewed from above – Perhaps not a monophyletic group
• Infraclass Metatheria – Marsupials (1 Order)– Young born very immature, develop in marsupium (pouch)– Includes kangaroos, koalas, opossum– Main current diversity in Australia
• Infraclass Eutheria – Placental Mammals (includes 4 Orders and 18 Suborders)