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One of the most fascinating examples of macroevolution is the path by which birds evolved from reptiles. Birds have many specialised adaptations that make flight possible. These include features like feathers (obviously), hollow bones (for reduced weight), a keel on the sternum (to attach big flight muscles to), and a shortened trunk.

Since the discovery of Archaeopteryx in 1861, more and more transitional fossils have been found. We’ve found a lot more different species with discoveries in Western China and Spain. There are now thousands of specimens available mapping out this transition through the geological column.[1]

But while Archaeopteryx is from the late Jurassic we have to go all the way back to the late Triassic almost 85 million years (my) earlier to start this evolution. It begins with a group of bipedal dinosaurs called the Theropods [1].

The first significant changes are that all long bones became hollow and one digit on the foot was lost (birds have 4 digits on their feet, not 5). Ceratosaurus (L Triassic) is an example of this [1].


This Jacana shows the 4 digits on the feet common to modern birds- an adaptation originating from the Late Triassic
The next was the evolution of a rotary wrist. While for theropods this led to a large grasping hand, the rotary wrist would later become crucially important for wing strokes in birds. Allosaurus (L Triassic) is an example of this [1].

By the early Jurassic the Coelurosaurs had evolved two more features would later be useful for flight.

First, the coracoid and sternum (in the chest) became bigger. This gradual enlargement would be necessary to attach the larger and stronger wing-muscles of birds. Second, body feathers make an appearance. Sinosauropteryx is a representative of these changes. These had no flight function but appeared millions of years before Archaeopteryx. Feathers would precede rather than accompany the evolution of flight [1].Sinosauropteryx Fossil
Figure based on image from Wiki Commons

By the middle of the the Jurassic, vaned feathers with differentiation between primaries, secondaries and rectrices were making an appearance. Again, the theropods concerned still had no ability to fly or glide. The maniraptor Caudipteryx is a transition species showing these accumulated adaptations [1].

These animals were still unmistakeably reptilian in appearance with their long bony tails, lengthy trunks and toothed bony jaws.

At this point in the Jurassic we have hollow bones, differentiated feathers and a broadening sternum, but not any ability to glide. None of these adaptations occurred to make flight possible, but rather produced traits that could be co-opted later.Body length is far too long to permit flight, so the next transition fossils show more fundamental changes to the body.

The next theropod fossils in the transitions start to depict both a shortened trunk and a stiffened long tail. The famous Velociraptor shows this transition [1]. The recently discovered Anchiornis huxleyi (Late Jurassic 151-161mya) adds to this transition by including elongated forelimbs and of course, feathers [2].

Anchiornis fossil
Link from National Geographic

It is only then after all of these changes have accumulated, that flight first makes its appearance in the fossil record. This is with Archaeopteryx in the late Jurassic, about 150mya.\

Archaeopteryx Fossil
(Figure based on image from Wiki Commons)

(Part II tomorrow)


References

[1] Sereno, P.C. (1999) The Evolution of Dinosaurs, Science 284:5423, pp2137-2147


[2] Hu, D., Hou, L., Zhang, L. and Xu, X. (2009). A pre-Archaeopteryx troodontid theropod from China Nature 461, 640-643