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

• Infraclass Archaeornithes – Archaeopteryx• Infraclass Enantiornithes – “opposite birds”; adaptive radiation

in Cretaceous, but extinct by end of Cretaceous

– Subclass Ornithurae• Infraclass Odontornithes – Cretaceous toothed birds, extinct at

end of Cretaceous.• Infraclass Neornithes

– Superoder Paleognathae – ratites and tinamous– Superorder Neognathae – includes most modern birds

• Approximately 30 Orders of living birds and 10,000 species.

Avian Phylogeny based on Feduccia (1995, 2003)

Enantiornithes

Odontornithes

HesperornisIcthyornis

Paleognathae

Ostrich - Africa Rhea – South America

Cassowary – Australia Emu – Australia

Kiwi – New Zealand

Class Mammalia• Derived from Therapsid (Cynodont) ancestors in

Triassic (~200 mya).– Early forms very similar in appearance to some

Therapsids – fossil mammals distinguished by squamosal-dentary jaw joint (quadrate-articular joint in therapsids and reptiles)

• First mammals likely small and nocturnal• After extinction of dinosaurs at end of Cretaceous,

mammals became dominant land vertebrates.– Endothermy and high capacity for activity (like birds)

probably assisted in rise to dominance.

Thrinaxodon

Cynognathus

Oligokyphus

Examples of Cynodont Therapsids

These possessed turbinates innasal passages suggestive ofendothermy (note the hair in thesereconstructions)

Cynodonts as a group were extinctby the end of the Triassic, but gaverise to the mammals

Fig 3.42 –

Phylogeneticrelationships among the Synapsids

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)

Symmetrodonts

Fig 3.45