THE ORIGIN OF BIRDS

1.  BIRDS ARE DINOSAURS!

This statement is no more radical than saying humans are mammals.

Birds have the characteristics that are specific to dinosaurs.

If birds really do have the characteristics that are unique to dinosaurs, then by diagnosis, they are dinosaurs.

Where do birds come from?

Where do feathers come from?

How did avian flight evolve?

There are about 9000 species of living birds.  Birds are the most diverse group of vertebrates, other than bony fishes.

Birds belong to the class Aves in the Linnaen system.

Birds are obviously vertebrates - they have a dorsal nerve cord and backbone.

Birds are clearly tetrapods and amniotes.

They are diapsids (thus they are diapsid reptiles) and are also archosaurs.  Birds have a unique set of highly evolved features superimposed on the basic archosaur body plan.

Birds do not resemble crocodiles and alligators very much, the other great living group of reptilian archosaurs.

Finally, birds are unique in the possession of feathers.

Fingernail-type material (keratin).

Hollow central shaft.

Radiating from the shaft are barbs - barbs linked together with small hooks (barbules).

Barbs and barbules form the vane of the feather.

Feathers evolved only once - so, if an animal has feathers, it is a bird.

Thus, birds (all animals that possess feathers) are a monophyletic group.

Bird Skeleton from:  Bird Skeleton Printout- EnchantedLearning.com at http://www.enchantedlearning.com/subjects/birds/printouts/Skeleton.shtml

• The limb bones of birds are hollow and have thinner walls than nonavian theropod dinosaurs.  Braces and struts provide support.
• From: A Bird's Skeleton at http://www.comptons.com/encyclopedia/ARTICLES/0975/09781760_L.html
• The long bones are pneumatic - air sacs pass through a small opening (pneumatic foramen) into the bones.  Thus air passes into these air sacs and lungs when the bird respires.  The air sacs help lighten the body, supplement the lungs, and help cool the fast avian metabolism.
• In the bird skeleton, many bones are fused to form more rigid structures.  The caropmetacarpus (semilunate carpal bone fused to metacarpals; all metacarpals fused) in the forelimb and thetarsometatarsus (distal tarsals fused to metatarsals; all metatarsals fused together in the hindlimb) are fused limb structures.  The astragalus and calcaneum are fused to the tibia.  The pelvic bones are fused to form the synsacrum (sacrum is fused to the pelvic bones and the pelvic bones are fused).  In the shoulder region the distal portions of the clavicles are fused to from a furcula (wishbone; this also is the case in some theropod dinosaurs).  The distal caudal vertebrae are all fused to form a pygostyle.
• There are modifications of the forelimb to form wings.  The sternum is large and keeled.  The coracoid is long and forms a brace with the sternum to anchor large muscles for flight.
• There is an absence of teeth in living birds.  The jaws are covered with keratin to form a beak (rhampotheca).
• Birds have relatively large brains and advanced sight.  The larger brain may be the result of evolution of endothermy (complex controls to maintain requires a larger brain) and because of complex motor control for flight.  Well developed vision may also be associated with motor activites of flight.

• Late Jurassic - transitional form - a missing link (between nonavian dinosaurs and more modern birds).
• Found in Solnhofen Limestone in Bavaria, Germany. (SEE THIS LINK:  The Solnhofen Limestone of Germany at http://www.ucmp.berkeley.edu/mesozoic/jurassic/solnhofen.html) and THIS LINK:  Lecture 19 - Late Jurassic: Solnhofen at http://rainbow.ldeo.columbia.edu/courses/v1001/15.html
• First named for a feather impression - 1860.
• Solnhofen Lagoon - poorly oxygenated bottom conditions (anoxic) - retards decompositon by bacteria and scavengers.  Lagoons have low current flow, so not ripped-up.
• 1861 - feather impressions and bone - London specimen
• 1877 - Berlin specimen.
• Five other specimens have been found, for a total of seven skeletons and 1 individual feather impression. (GO TO THIS SITE TO SEE A SKETCH OF ALL SEVEN SPECIMENS AND WHERE THEY WERE FOUND: Reference - ARCHAEOPTERYX: Solnhofen Map at http://www.netpets.com/birds/reference/fun/map.html

7.  Features of Archaeopteryx

     Archaeopteryx shared many features with small theropod dinosarus (like Compsognathus).

• Size of a crow
• Skull had teeth
• Very few bones fused (dinosaur features rather than avian features)
• Had hollow limb bones - as do theropod dinosaurs (but theropod limb bones had thick walls and no pneumatic foramens)
• No sternum - lacked the main anchor for powered flight, but the forelimbs indicate it was capable of powered flight.
• Had a furcula - also present in some tentanuran dinosaurs
• Opisthopubic condition - as are some coelurosaurs (Deinonychus, Velociraptor, etc. - dromaeosaurids)
• Forelimb theropod-like
• Hindlimb bird-like - fibula reduced, proximal tarsals fused to tibia, distal tarsals fused to metatarsals, long metatarsals partly fused to each other.
• Pes has three slender forward-facing digits (2,3, and 4) and a fourth (digit 1) that faced backward.
• So, Archaeopteryx could be described as a feathered dinosaur capable of flight.
• Other features include:
• Stiff tail - zygopophyses elongate and interconnected (locked-up).
• Gastralia (belly ribs) - this is a primitive feature.
• Lacks a synsacrum - has a reptilian pelvis.
• Long arms - greater than 70% the length of the legs.
• Wrist - contains a semilunate carpal bone (characteristic of birds)
• Manus is long, digits 1,2, and 3 unfused.  Claws on end of digits.

• H. T. Huxley (Darwin's Bulldog) - First to strongly argue the connection of birds and dinosaurs.  He outlined 35 characters, 17 of those still recognized.
• Early 20th century - Huxley's ideas were questioned.  Many said similarity due to convergent evolution.
• Today, because of the similarity of so many characters, most think convergence seems unlikely to explain all the character similarities.
• Re-awakening - John Ostrom, Yale University - 1970s - Documented the relatioship between Archaeopteryx and coelurosaurian theropods.
• Bakker and Galton (1974) - Birds should be in a new class - Dinosauria.  Most paleontologists would not accept this.
• Gauthier (1986) - Applied cladistic analysis to the origin of birds.  He outlined 100 characters that indicated Archaeopteryx (and thus other birds) are coelurosaurian theropod dinosaurs.
• Archaeopteryx has an antorbital fenestra (as do all birds) - thus an archosaur.
• Fully erect posture, shaft of femur at 90 degrees to head.
• Ankle displays the modified mesotarsal condition.
• Ornithodiran archosaurs have four toes, three point forward (1, 2, and 3; symmetrical around digit 3) and the fourth is reduced.  Birds have the same type of foot, thus birds are ornithodirans.
• Within Ornithodira, Dinosauria have 3 or more sacral vertebrae (Archaeopteryx has 5 sacral vertebrae as do coelurosaurian theropods), shoulder more backwards than other archosaurs, reduced 4th finger on the hand, semiperforate to perforate acetabulum, etc. Archaeopteryx is a dinosaur.
• Thus birds bear the diagnostic features of theropod dinosaurs.
• Birds are also maniraptorian theropods (based on hand structure and fused clavicles).
• Birds are dromaeosaurid theropods - semi-opisthopubic pubis, stiff tail, interconnected zygopophyses on caudal vertebrae, etc.  So Archaeopteryx (and other birds) is closely related to Deinonychus and Velociraptor.

• Arboreal Hypothesis versus Cursorial Hypothesis (much debate on this, remember the video)

• Evolved very rapidly after appearance of Archaeopteryx.  By early Cretaceous much more modern.  Big jump.
• Sinornis - Early Cretaceous of China.
• Many new finds in China recently of early Cretaceous birds and so-called feathered dinosaurs.