[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Reversed Hallux in Theropods: a query



>W But a vestigial pedal digit, which adds unnecessary
>weight to the foot and which interferes with the animal's day-to-day walking
>and running, is a positive liability. I'm pretty sure (correct me on this,
>bird experts) that all extant ratite birds (ostriches, emus, cassowaries,
>rheas, etc.) have actually _lost_ the hallux entirely as an external digit
>(some might still have it as an internal splint metatarsal)

A number of birds, as I said in another message, have lost the hallux
entirely on becoming cursorial; of course the ostrich has retained only two
digits.  You are correct about ratites though I believe at least some of the
extinct moas may have retained the hallux.

I have been thinking more about this issue to try and think of uses for a
fully functional opposed hallux that do not require an arboreal habit; there
are several.  Some birds (including terrestrial birds like the Purple
Swamphen Porphyrio porphyrio) are quite adept at using their feet to
manipulate objects such as nesting material (I have observed this).  Others
indeed perch - but on rocky outcrops, not in trees.

But it has occurred to me that we may be missing the boat by thinking solely
of a terrestrial vs arboreal dichotomy.  As I have said terrestrial birds
show quite a lot of variation in the extent of the hallux and its claw, from
complete loss to exaggerated development.

But remember that the first terrestrial bipeds in the therapod-bird line
were not just terrestrial or cursorial - they were probably hunters.  Once
you realize this a use for an opposed hind foot that does not involve
perching becomes immediately apparent - grasping and subduing prey.  In fact
the closest living analogue to such  a creature - the Secretarybird of
Africa - uses its  feet in precisely this way.  An opposable foot can be
used to subdue small reptiles or even extract them from burrows (as the
living Harrier-hawks of Africa and Crane-hawk of the New World Tropics do
very efficiently).

It therefore seems at least possible that the original development of this
character relates not to an arboreal lifestyle but to a diet of active small
vertebrates.  Even if it did evolve for an arboreal habit it could have
served this purpose for any number of small terrestrial dinosaurs (and I
note that, unless I am mistaken, the character is absent in small
herbivorous dinosaurs).  In any case such a condition could have been highly
functional for small terrestrial predators, and might have been retained or
even enhanced for that purpose.

If this is correct then the hallucal position may not be a safe guide to
whether or not ancestral forms were arboreal (something I admit I have had
at least one problem with - I cannot quite see how bipedality would arise in
conjunction with arboreal habits (primates are very different because apes
evolved this condition, to the extent that they did, in conjunction with
brachiation)).  Instead, it seems to me that hallucal reversal is consistent
with a number of hypotheses, such as:

1.  Reversal evolved as a grasping adaptation in small predatory terrestrial
dinosaurs (or their ancestors); this line gave rise (in any way you like) to
therapods and birds, with either all lines remaining terrestrial or one (or
perhaps more) lines possibly becoming arboreal and using a grasping foot for
perching (ie the condition evolved first making arboreal habit easier to
achieve).

2.  Reversal occurred as a perching adaptation; descendants became
terrestrial but retained the condition (at least initially) for a functional
purpose such  as prey capture; one line of this lineage gave rise to birds,
either with or without a secondary return to the trees.

3.  Reversal evolved as a perching condition:  the line divided into a
terrestrial lineage which gave rise to therapods and an arboreal one giving
rise to birds.

4.  Reversal evolved as a perching condition; at various points branches
from the arboreal line convergently developed terrestrial forms that
developed into the various therapod lineages.

Intermediates and other variations are of course possible.

I cannot see that the condition of the hallux in therapods allows us to
eliminate any of these possibilities; therefore I question whether it is a
useful character in determining this issue.
--
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
International Wildlife Coalition              Fax/Modem: (905) 569-0116 (home)
Home: 1825 Shady Creek Court                  Messages: (416) 368-4661
Mississauga, Ontario, Canada L5L 3W2          Internet: ornstn@inforamp.net
Office: 130 Adelaide Street W., Suite 1940    
Toronto, Ontario Canada M5H 3P5