Earliest known fossil bird: Archaeopteryx lithographica (Late Jurassic; 150 mya); Discovered in Germany
(Click here for pictures from the Univeristy of California Museum of Paleontology)

• preserved in limestone deposits so feather impressions visible
• long tail
• much less fusion of bones/reduction of bone number than modern birds
• no keel
• small braincase
• teeth
• evidence for flight capabilities:
• flight feathers with typical contours for flying birds
• arrangement of flight feathers on ulna & metacarpal

Question: where did it (and all birds) come from?

Hypotheses:
1. (Most widely accepted) Theropod dinosaur that evolved feathers for insulation; subsequently evolved flight
Support: Structural similarity to theropod dinosaurs, including:

• anisodactyl foot structure
• wrist structure: unique fused carpal bones in some theropods and Archaeopteryx
• fossil theropod discovered in 1997 has laterally facing shoulder socket, like Archaeopteryx and other birds (allows wing folding in birds)
• theropod-like skeleton discovered in 1998 has feathers, but not flight

2. (Not well accepted, but some ornithologists and paleontologists argue for it): Birds originated as early Archosaurs (group of reptiles that includes dinosaurs and crocodilians), but not a dinosaur group.
Support: based primarily on reasoning

• argued that theropods are adapted to running, and a runner was unlikely to give rise to a flying species
• runners flattened side to side for running; birds are flattened top to bottom, for flying
• flying argued most likely to have arisen from gliding; gliding occurs in tree-dwellers, not ground runners
• feather structure too complex to have evolved just for insulation; modern birds that use it only for insulation do NOT have vaned feather structure
• argue that theropod similarities result from convergent evolution (evolution of similar traits independently) and that the recent feathered skeleton that is flightless is a flightless bird, not a theropod
• Protoavis fossil -- 80 million years older than Archaeopteryx -- considered by some to be a bird; as old as earliest dinosaurs.  If a bird, would support this, but not feathered; other paleontologists consider it to be a composite or result of convergent evolution.

Early Cretaceous birds (about 135 mya) from China

Sinornis : (Early Cretaceous; 135 mya) shows development of many modern bird traits related to flight and perching

• decrease in number of back vertebrae
• decrease in number of tail vertebrae; pygostyle
• increase in diameter of flight-feather bearing bones: ulna, 2nd metacarpal
• wrist joins modified to allow tight wing folding (reduces drag on recovery stroke in flight)
• feet modified so that hind toe is on the same level as front 3 toes; in many modern birds this structure is related to perching ability
• retains primitive pelvic girdle, lack of fusion of hand bones, teeth, small braincase

Late Cretaceous birds:
Toothed oceanic birds (about 100 mya)
Apparently went extinct at Cretaceous-Tertiary boundary along with dinosaurs

 Hesperornis

• lacked keel
• diving bird superficially similar to loon

• had keeled sternum
• superficially similar to a tern

• major period of diversification of birds; origin of modern orders
• origin of some other avian types that are now extinct  (ex: huge carnivorous birds)

• Pleistocene glaciation, habitat changes have resulted in speciation, some extinction, to give rise to currently extant species
• speciation can occur when populations of a species are separated by external barriers; these would be created by glaciers, and habitat changes associated with extreme weather changes

Geological Time Scale

Era	Period	Epoch	Time (mya; approximate - different tables vary)
Cenozoic	Quarternary	Recent	0.01
		Pleistocene	1.5
	Tertiary	Pliocene	12
		Miocene	25
		Oligocene	34
		Eocene	56
		Paleocene	63
Mesozoic	Cretaceous	Late	100
		Early	135
Jurassic		Late	155
		Middle	170
		Early	180
Triassic			230