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The significance of Archaeopteryx is that it is the first fossil in this sequence that is capable of basic flight. With its laterally facing shoulder joint and split propulsion lift wing with asymmetric feathers, basic flight was now possible. The hallux (the 4th digit) also appears reversed which would give a basic perching function [1].

The postcranial skeletal pneumaticity also shows that Archaeopetryx had the modern bird air-sac system. This is not unique to Archaeopteryx and is found in other non-avian theropods (e.g. Majungatholus) [3]

Nonetheless, the long bony tail of Archaeopteryx, the simple shape of its sternum, the fact it still had bony jaws with teeth, lacked a furcula (wishbone) all showed its transitional status [1].

The evolution of birds then refines an improves these flight ability in a more rapid radiation. The thorax deepens (Confuciusornis, Sinornis) until the deep sternal keel appears in the Euornithes birds in the early Cretaceous [1]. This deep sternal keel would make the attachment of large flight muscles possible.

The hallux lengthens so that advanced perching becomes possible (the hallux in Archaeopteryx is too short). The long bony tail evolves into a truncated, stiff pygostyle (Confuciousornis, Sinornis). More specialised feathers appear (the alular) and the clavicles form an elastic furcula (wishbone).

Birds then undergo an explosive radiation in the late Cretaceous giving rise to the Neornithes (or new birds). In effect, bird evolution from the theropods in the late Triassic took 170my to reach the modern birds we see today. Many gradual transitions can be mapped out by following single traits, such as feathers, the digits, the sternum and the tail.

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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

[3] O’Connor, P.M., Claessens, L.P.A.M. (2005). Basic avian pulmonary design and flow-through ventilation of non-avian theropod dinosaurs, Nature, 436, 253-256.