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 – Phylogenetic relationships among the Amniotes Note that birds are included within the dinosaurs in this phylogeny

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 flightless birds

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

Icthyornis Hesperornis Odontornithes

Paleognathae Ostrich - Africa Rhea – South America Cassowary – Australia Emu – Australia Kiwi – New Zealand Paleognathae

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 Examples of Cynodont Therapsids These possessed turbinates in nasal passages suggestive of endothermy (note the hair in these reconstructions) Cynodonts as a group were extinct by the end of the Triassic, but gave rise to the mammals Oligokyphus

Fig 3.42 – Phylogenetic relationships 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

Spiny Echidna Duck-billed Platypus Living Monotremes

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