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Re: polarity of bipedality in dinosaurs



[See... I told you it went through! -- MR ]

In a message dated 96-09-19 17:40:22 EDT, jkane@dcn.davis.ca.us (Jack Kane)
writes:

>         I have been reading with great enthusiasm all of the different
> strands but I have a question. I the discussion about forelimbs it was
> mentioned several times about the move toward bipedalism. In my Dinosaur
> class here at UC Davis it was mentioned that dinos primative condition was
> bipedal and they were secondarily quadrupedal. Is this no longer the case,
> was I not listening that day ? Please explain which is the primative
> condition and the significance of that with the return to bipedalism in
> therapods. 

Dinosaur bipedality has >no< explanation in the standard hypothesis of
dinosaur evolution; dinosaurs simply "inherited bipedality from their bipedal
thecodontian ancestors," to paraphrase current thought. So--how and why,
then, did bipedality evolve among pre-dinosaurian thecodontians? You may be
interested to know that there is >not one< known pre-dinosaurian thecodontian
that was unequivocally, demonstrably bipedal. Many authorities cite
_Hesperosuchus_, _Ornithosuchus_, _Poposaurus_, _Postosuchus_, and so forth,
but these genera are >not< known from complete enough material to establish
bipedality, and there is considerable danger that material from individuals
of varying sizes has been used to argue for bipedality in these forms. When
the smoke clears, I'm pretty sure that all the so-called "bipedal
thecodontians" will vanish with the smoke.

In my BCF (birds came first) hypothesis, bipedality arises >naturally< in
dinosaurs when I assert that dinosaurian ancestors were quite small,
lightweight, habitually arboreal animals (which I call "dino-birds") in which
the forelimbs became modifed for grasping and climbing (at first), then
leaping through trees, gliding, and later flying. The various groups of
cursorial, non-arboreal descendants of these dino-birds that evolved from
time to time include all the dinosaurs from sauropodomorphs to theropods;
those dino-birds that did not give rise to ground-dwelling descendants
eventually evolved into modern birds.

Those dinosaurs that diverged earliest from the dino-bird lineage had
forelimbs that were least modified for an arboreal lifestyle, and many of
those dinosaurs successfully transitioned to a secondary quadrupedality (in
all cases, however, with quite substantial anatomical and postural
differences between fore and hind limbs, clearly the result of some forelimb
modification that occurred during the short but significant arboreal period
in their ancestry). These include the sauropodomorphs and the ornithischians.
The dinosaurs that diverged from later dino-birds, in which the forelimbs
became more substantially modified for grasping, climbing, gliding, and
powered flight (in more or less that order), are what we now call theropods.
In theropods, the forelimbs had become too highly modified into grasping,
gliding, and perhaps even flying organs for there to be an evolutionarily
likely reversion to quadrupedality, which is why theropods always remained
obligatorily bipedal.