Evolution of Aves

Getting to be a modern bird

For all of its limitations, the Mesozoic record provides for us insights into the transition from primitive theropods, through eumaniraptorans, to Avialae (including Archaeopteryx), and finally to Aves. The first part of the long evolutionary sequence - the part that ran from primitive theropods to avialans - was detailed in Chapters 9 and 10. The second part, the evolution of Aves from the primitive avialan condition, represented by Archaeopteryx, through the remarkable avian discoveries recently made in China, can be read from the cladogram in Figure 11.2.

Our best understanding of the sequence of evolutionary events is as follows:

1. Development of the perching adaptation in the foot, in combination with limited flapping flight capabilities.

2. Development of a pygostyle.

3. Reduction in number of trunk vertebrae; and development of a flexible furcula, strut-like coracoid, carpometacarpus, and fully folding wings.

4. Further reduction in the number of trunk vertebrae, loss of gastralia, final rotation of pubis to lie parallel with ilium and ischium.

5. Reduction of number of trunk vertebrae, decrease in size of acetabulum, patellar groove, a groove at the distal end of the femur to accommodate the patella (knee cap).

Steps 1-5 occurred in the Mesozoic; step 6 may have occurred after the Mesozoic was over, because all Mesozoic avialans, for which a skull, is known, had teeth. Skulls are unfortunately

Dinosauri Stampare

Figure 11.7. Ichthyornis, a gull-like bird from the Late Cretaceous of the USA.

10 cm

Figure 11.7. Ichthyornis, a gull-like bird from the Late Cretaceous of the USA.

10 cm

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