Tuesday, October 25, 2011

Pterosaur to bird - hand development

The pteroid is composed of two fused phalanges.

The radius, ulna and pteroid bones of FHSM VP-2183. The pteroid bone was once considered to be the 'thumb' (e.g. Digit I) in pterosaurs. The two other pieces of bone in the upper left of the photo are the proximal and distal syncarpals (wrist bones) that would connect to metacarpal IV.  

"The wings of many diurnal birds of prey have a vestigial claw located at the end of the thumb bone."


The alula (thumb wing) consists of one phalanx (occasionally two, sometimes even with a nail in some bird species). The alula is present in all birds except the penguins, where it is fused to the carpomatacarpus. It is a freely movable digit with a few small feathers attached. The function of the alula is not yet fully understood. Ornithologists suggest that one function among others is in the landing because it can make a change in the airflow and turbulence along the wing edge. The alula articulates to the carpometacarpus near the wrist.
There has been discussion about whether the alula is really a thumb or an index finger remaining after an evolutionary adaptation. Recent genetic research has shown that the alula is a real thumb and the other two the 'index finger' and 'middle finger'. The outer two fingers are lost (Kaiser 2007).

Arm section

1. Upper arm - humerus
2. Sesamoid bone - os sesamoides.
3. Ulna - ulna
4. Radius - Radius

Hand section (manus)

5. Wrist - radiale and ulnare
6. Metacarpal - carpometacarpus
7. Thumb - alula
8. Digits - phalanges

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