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
Hesperornis
Odontornithes
Icthyornis
Ostrich - Africa
Cassowary – Australia
Rhea – South America
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)
Oligokyphus
Cynodonts as a group were extinct
by the end of the Triassic, but gave
rise to the mammals
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