Earlier a paper (Lee et al. 2014) demonstrated the well understood concept of phylogenetic miniaturization in birds (Fig. 1). We’ve seen this pattern often in the origin of major clades. Perhaps overlooked in birds, the behavior of tree clinging is key to their reduction in overall size, the increase in forelimb length and the evolution of flight feathers.
During this time some pre-bird dinosaurs became arboreal quadrupeds
while remaining terrestrial bipeds. Smaller lighter taxa with longer forelimbs find it easier to climb trees. The smallest taxa can perch bipedally on slender branches (Fig. 2), eliminating the need to use the forelimbs for clinging. As a consequence, forelimbs can be modified for flight.
Archaeopteryx was not the smallest of basal birds.
As early birds continued to evolve, becoming ever more bird-like, taxa continued to shrink in size (Fig. 3). Some were as small as hummingbirds and the smallest adult pterosaurs.
The act of tree clinging
builds up those all important pectoral muscles over several hundred generations and finds a likely analogous behavior (based on a similar morphology) in the arboreal non-flying fenestrasaur ancestors of pterosaurs, like Longisquama (Fig. 4).
The perching ability of birds
finds a convergent ability in basal pterosaurs, with the exception that pterosaurs use pedal digit 5 rather than pedal digit 1 to serve as a universal wrench. (Fig. 5, Peters 2000, 2002, 2010). Even so, most pterosaurs (ctenochasmatids and nyctosaurs not included) continued to retain large, tree-clinging fore limb claws.
Lee MSY, Cau A, Naish D and Dyke GJ 2014. Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds.
Peters, D. 2000. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos, 7: 11-41
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.
Peters D 2010. In defence of parallel interphalangeal lines. Historical Biology iFirst article, 2010, 1–6 DOI: 10.1080/08912961003663500