Freed from their primitive function: hands become wings, typing tools
When forelimbs rise during the transition to a bipedal configuration, they become available for other functions. The hands are free to change into something other than terrestrial chest supports and steering drivers.
Earlier we looked at four Triassic lineages that became bipedal (fenestrasaurs, dinosaurs + basal crocs (like Scleromochlus) and tropidosuchids (including Lagerpeton). In the Permian we can add some basal diapsids including Milleropsis and Eudibamus.
Today there are 19 species of lizards capable of rising to the bipedal configuration and humans are among several mammals capable of doing so. Bats are upside-down bipeds as they cling to cave walls and tree branches. Kangaroos and kangaroo rats hop bipedally. Primates from lemurs to apes occasionally rise to their feet. Some hang by their hands in a novel form of locomotion called brachiation.
All birds are bipeds, descending from bipedal dinosaur ancestors. Early crocs, like Scleromochlus, and were bipeds, but never did anything interesting with their hands.
Most of the time the hands of bipeds just hang there. Sometimes they help grasp objects including tree limbs and trunks.
Occasionally extradermal membranes extend posteriorly beyond the hands. In Cosesaurus extradermal fibers enhanced the flapping behavior as a secondary sexual character, adding to “the show.” Descendants enhanced this with an elongated fourth finger and a membrane tying those fibers together to become wings that became so large a folding mechanism at the base of the axially rotated wing finger was needed whenever the wings were not needed.
In theropods leading to birds much the same thing happened, only the fingers didn’t get much longer, the stiff feathers did. And the folding mechanism was at the wrist, not the base of the wing finger.
The ancestors of bats likewise had a rotating wrist joint which can be seen in the pen-tailed tree shrew (pre-colugo), Ptilocercus. This tree shrew rises to its haunches to hold insect prey while eating it, the way a squirrel holds a nut while eating it. No known bat ancestors demonstrate bipedality or inverted configuration. The closest ones, like Protictis, are known only from jaw fragments than include bat-like teeth. But we do know that modern civets and kin like Nandinia, don’t care what their orientation is on a tree trunk. And this taxon actually enjoy jumping out of trees to land in leaf litter.
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.
2012-11-24 02:01:14
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