Re: More on Digit Loss in Theropods

["David Marjanovic" <david.marjanovic@gmx.at>]

> A few weeks back I picked up a nice book (The Shapes of Time, by Ken
McNamara) that discusses heterochrony and the evolution of certain body
shapes.  There was a short passage discussing the evolution and loss of
digits. McNamara and his colleagues Emily Gale and Peter Alberch tried to
correlate the relationship between digit number and body size in certain
mammals (especially dogs), finding that in larger dog breeds more digits
were often present.
>
> This is great,

No. Giraffes have only 2 digits per limb and no trace of others. Some
elephants (forgot which ones) have lost finger V. *Tyrannosaurus* is the
biggest tyrannosauroid and has retained least of mcIII (AFAIK). And so on.
It may be correlated in dogs, but not everywhere else.

> [...] at least in some animals, developing extra digits may not be that
difficult.  However, in theropods there is no trend to re-evolve digits.  I
believe George Olshevsky said that this was due to the evolution of the
wing.  I can't say that I support that viewpoint 100 percent, but I still
must say that something was likely holding the re-evolution of digits back
in theropods.

This kind of polydactyly, also seen in ichthyosaurs*, involves _duplicating_
a digit in the middle of the hand or foot and Y-shaped metacarpals/-tarsals.
That's totally different from the extra digits in *Acanthostega*,
*Ichthyostega*, *Tulerpeton* and frogs which have identities of their own.

> After all, at least in many modern animals, it isn't too hard to develop
extra digits.

The praehalluces of frogs might be such a case -- if so, the only one known,
and maybe even a real reversal that appeared while the necessary genes were
still present (I'm speculating). (These toes are very short, so I can't see
an advantage in their presence; however, they are not duplicates of the
halluces and have separate tiny metatarsals.) Seems like it is very hard
IMHO.

* Some ichthyosaurs have an additional row of bones that starts directly
from the humerus -- a total neomorph that shows that we still know little
about hand development.