Differentiation of Sloths [was Re: T-Tip]

["Jaime A. Headden" <qilongia@yahoo.com>]

Berislav wrote:

<Another question:is the number of digits really
crucial for defining close relationship of two animal
species/genera? Aren't there for example, two living
species of sloths (closely related): one with
two-clawed hand the other with three-clawed hand?>

  *Bradypus tridactylus* is a bradypodine?
megatheriid, and *Choloepus didactylus* is a
choloepodid, both sloths, but different suprageneric
taxa (okay, "families," but I think it goes farther
than that). Bradypus is closer to megatheres like
*Eremotherium*, giant monster sloths, whereas I
believe Choloepus is closer to nothrotheres, smaller
quadrupedal sloths. I may be wrong, and I'm not
entirely up to date on my sloth phylogeny.

  The manal cannon bones of the respective sloths
differ only slightly, with mc's II-III fused along
their midline to some degree, whereas in tridactylus,
the mc I is almost as long as II-III, with IV splint
like, and in didactylus, both I and IV are short and
splintlike. As in the development of unidactyly in
horses, reduction and vestigialization of digits
follows lack of use of those respective digits. Both
sloths have three pedal digits. Other differences
between the two taxa are dental and physiological
features.

  How this relates to *Compsognathus* and
Tyrannosauridae, read previous posts. However, I
disagree with George and Ostrom on the matter of
vestigialization of digit three based on the lack of a
cohlear distal articular surface for a third manal
digit, as the most advanced ornithomimosaurs (more
advanced than *Gallimimus*) possess rounded distal
articular surfaces on all metatarsals. A rounded
articular surface could still bear a phalanx, whether
cartilaginous or osseous; for such an apparently
fragile and tiny element, preservation would not
neccesarily favor a fragment that size, even in
laggerstaetten like that in Canjuers or Bavaria. Lack
of some fine detail in some *Archaeopteryx* specimens
(the London and Teyler specimens come to mind) such as
feather imprint argues in favor of there possibly
being _something_, not neccesarily that there _was_
something.


=====
Jaime "James" A. Headden

"Come the path that leads us to our fortune."

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