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HADROSAURS SHEDDING TAILS



Zenlizard wrote...

> As a throwaway  (no pun intended, i didn't even notice until I typed
> that),  could hadrosaurs have had an analog to the tails of geckos, etc,
> where the tails are detachable, thus distracting predators and allowing
> the animal to escape?

Not a chance. Caudal autotomy is very easy to spot in reptiles as those that do
it have a vertical break line running down the body of the caudal centrum. No
archosaur has any such thing. 

Caudal autotomy in reptiles today is restricted to certain squamates and
_Sphenodon_. However, it's primitive for the group and is known from various
fossils diapsids. I *think* there's a _Petrolacosaurus_ tail vert that reveals
signs of it (petrolacosaurs are the first known diapsids) and it has also been
reported in _Mesosaurus_. This is rather suspect though, seeing as mesosaurs
were tail-propelled swimmers! _Tanystropheus_ is (according to Rupert Wild)
known to have been able to shed its tail and it is a prolacertiform, one of the
sister-groups to Archosauriformes. Whether caudal autotomy was lost in
Archosauriformes or in their ancestral group is not yet known.

This primitive ability to autotomise the tail has been lost many times. A good
rule of thumb is that, if a tail is required for locomotion, the option of tail
shedding becomes lost. This has been demonstrated widely in lizards (papers by
Hamley, Zani and Russell and Bauer 1996) where those groups that have evolved
elongate caudofemoral muscles (these grow along the sides of the tail vertebrae)
have lost the ability to autotomise. These elongate caudofemorals operate in
retraction of the hind limb and enable their owners to be powerful, often
bipedal, sprinters. 

As a result, there is a correlation between bipedality and caudal autotomy.
Bipedal lizards (e.g. some agamids, the iguanian family whose name I forget that
includes _Basiliscus_ etc. [Polychro--??) are never naturally tail-shedding
forms, and tail-shedding forms (e.g. most gekkotans, lacertids) are never
bipedal (there is actually one report of a bipedal gecko but it is regarded as
suspect by most herpetologists). Dinosaurs have obviously carried all of this
to the extreme in becoming full bipeds with an non-expendable tail. Little known
is that at least one lizard, _Chlamydosaurus kingii_, has gone the same way -
 this animal is a true biped (Shine and Lambeck 1992 and pers. obs!!). Dave
Peters notes that adaptations like those of bipedal lizards are also seen in
pterosaurs and he therefore argues that they were capable of facultative
bipedality as well. 

Dinosaurs (and pterodactyloid pterosaurs too??) have gone even further than this
though, and have switched the job of femoral retraction to hip-based muscles
with the caudofemoralis longus then being reduced or lost. Add to this a
requirement for weight reduction and alterations in anatomy that allow the
centre of balance to move toward the thorax (rather than near the hips), and
it's goodbye to the tail and hello pygostyle. This pattern of alteration has
been discussed extensively in the literature (I hope you know who by). 

"This is your last chance Jabba - free us, or die"
"Heh heh heh - checum a leyr"

DARREN NAISH