Re: Bird bipedalism (was A layman's question...)

Thomas R. Holtz, Jr. (
3 Jul 1996 13:50:05 GMT (ORDOVER) wrote:
> Okay, than what evolutionary presures made the ancestors of birds get up
> on two legs?

Here are some of the scenario options, and the background:

The whole group of archosaurs (birds, crocs, and their extinct relatives)
and lepidosauromorphs (lizards, snakes, tuataras, and their extinct
relatives), together the Diapsida, are characterized by hindlegs longer
(by at least 30%) than their forelimbs, an adaptation for facultative
bipedality when they want to haul tail (diapsid hindlegs are powered
by femur-tail muscles, not femur-hip muscles, so diapsids don't "haul

Several times in diapsid history, a fully erect posture of the limbs
was assumed. In the case of bird ancestry, this would have been with
the ancestral ornithodiran (i.e., the ancestor to dinosaurs (incl. birds)
and pterosaurs (flying reptiles, incl. the pterodactyls). Such an
animal was small (<~50 cm long, half of which is tail) and predatory
(small vertebrates and arthropods). An erect posture gave an advantage
to vision (the higher up one is, the further distance they could see),
which would have been advantage to hunting (see the prey earlier than
your sprawling competitors) and predator evasion (see the predator
earlier than your sprawling competition, and haul tail before they

Within the Dinosauria, the ancestral habit was facultative bipedality.
Based on their anatomy and trackways, ancestral dinosaurs used their
forelimbs to grasp food items (small animals and/or plants) and to rest
hands on the ground when they came to a full stop. Probably, all
these early dinosaurs used a fully bipedal posture while running.

Within the Theropoda (carnivorous dinosaurs), the forelimb became more
and more specialized for grasping. In most dinosaurs, this grasping
ability was useful for clutching prey items, but this may have also
been used to climb in smaller forms. The transformation of the forelimb,
however, left it unsuited for locomotion, and theropods became obligate

Birds thus descended from, and remain, obligate bipeds.

And, pb, that's Dr. Tommy to you :-) (But at least he didn't call me