Updated July 09, 2015 with a new image of Archaeopteryx.
Archaeopteryx, one of the most primitive birds, was among the smallest of all adult dinosaurs. But what followed Archaeopteryx was even smaller (Fig. 1).
Size reduction in early birds
Early maturation produces smaller adult birds. They cycle through life and reproduce quickly, before reaching the size of their parents and grandparents. And if their chicks mature even more quickly they will have smaller hips to produce smaller eggs. Smaller eggs produce smaller chicks that reproduce quickly. On and on, well, you get the idea.
The wings lost their individual fingers in these early birds. The tail shortened, developing a pygostyle (fused tail bones). The rostrum was shorter. The orbit was relatively larger. The sternum deepened, developing toward its modern shape. These little guys were smaller, lighter and stronger flyers. They were probably very hard for a predator to catch. They could lay eggs in otherwise inaccessible places. They may have had a higher, more bird-like metabolism, with greater needs for food, and more rapid reactions. They weighed only a fraction of Archaeopteryx, so the little birds could jump and fly easier.
Evolution happens more quickly in small taxa because they grow and reproduce at a faster rate. More generations appear in less time. By flying, birds can access more environments, from trees to seashores, which also has selective effects.
Parallels in pterosaurs
Earlier we talked about various pterosaur lines that shrank and evolved into new forms prior to ultimately producing larger forms. However, pterosaur ancestors, like Cosesaurus, were not larger than early pterosaurs. They did not develop wings through serial size reduction.
Juveniles vs. Adults
Gobipteryx was considered an embryo. If so, it would have grown to the size of Archaeopteryx. Not sure about the possible juvenile status of the others. Since birds grew up so quickly and are all about the same size, the odds are the rest represent adults.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Dames W 1884. Ueber Archaeopteryx. Palaeontologische Abhandlungen, 2 (3):119-198.
Heller F 1959. Ein dritter Archaeopteryx Fund aus den Solnhofener Plattenkalken von Langenaltheim/Mfr. Erlanger Geologische Abhandlungen, 31: 1-25; Erlangen
von Meyer H 1861. Archaeopteryx litographica (Vogel-Feder) und Pterodactylus von Solenhofen. Neues Jahrbuch für Mineralogie, Geognosie, Geologie und Petrefakten-Kunde. 1861: 678–679.
Owen R 1863. On the Archaeopteryx von Meyer, with a description of the fossil remains of a long-tailed species from the lithographic stone of Solnhofen. Philosophical Transactions of the Royal Society, London 153: 33-47.
Paul G 2002. Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Johns Hopkins University Press. 460 pp.
Wellnhofer P 1974. Das fünfte Skelettexemplar von Archaeopteryx. Palaeontographica Abt. A Vol. 147 S: 168-216.