Friday, October 3, 2014

Arboreal theory and the pterosaur to bird theory

The arboreal theory of bird evolution ("trees down") is held by a number of researchers and academics. The core of the theory is that the lineage leading to birds began in the trees (not on the ground).
The pterosaur to bird theory is a different idea. Pterosaurs were already flying in the trees, by flapping their skin membrane, and they later developed feathers.
The ancestor of birds was not a dinosaur, nor was it a thecodont. It was a pterosaur.

The trees down theory begins with a gliding arboreal creature. This is closer to being correct except that the gliding phase took place earlier, in the evolution of the pterosaur. This is a key point.


  1. See Figure 1A
    Fig. 1. Representative aerodynamic measurements for pitching stability and control effectiveness.
    Longtailed taxa (A) have a stable equilibrium point at 1025 (yellow line) and the tail is effective in generating pitching moments at low angles of attack (pale yellow box).

    In shorttailed taxa (B), including extant Larus, the equilibrium point at 05 is unstable (red line) and the tail control effectiveness is reduced. One example (Rhamphorhynchus) drawn from pterosaurs illustrates similar possibilities in a phylogenetically distant taxon.



  4. For later reference:

    Now, this is not to say that basal pterosaurs were locomotory inept from the moment they landed. They may, however, have spent more time running around trees and cliffs than over floodplains and tidal flats. Basal pterosaurs typically have deepened, highly recurved manual and pedal claws with comparatively large flexor tubercles compared to the relatively slender claws of pterodactyloids. These claws are extremely thin despite their depth and would make excellent crampons to provide purchase when climbing, especially when combined with the antungual sesamoids and elongate penultimate phalanges that characterise the hands and feet of many basal forms. Furthermore, the orientation of the femoral head in basal pterosaurs means that the femur is projected forward, upward and laterally from the acetabulum, thereby causing the sprawling gait for the hindlimbs that acted in concert with the relatively short metacarpals to bring the bodies of these pterosaurs close to any surface they happened to be climbing over. These are all excellent adaptations to climbing (Fig. 5), and we should expect early Mesozoic environments to be covered with pterosaurs hanging from cliff faces, tree trunks and branches.

    Today we’ll start a short series on pterosaurs and trees demonstrating how some pterosaurs (not all) could interact with a tree trunk. If anyone finds this convergent with Archaeopteryx and kin, you’re probably right.