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

A bit about BCF theory



Darren Naish writes:

>(Ronald Orenstein writes):
>>5. There are extremely strong similarities between
>>_Archaeopteryx_ and various maniraptorian dinosaurs, though
>>most of these postdate Archaeopteryx by a considerable period.
>
>Padian and Jensen, as many of you may well know, described
>maniraptoran material from the Morrison, so maniraptors were
>perhaps fairly widespread in late Jurassic times. The Bristol
>palaeontology team, Benton et al, described maniraptor fragments
>from their BATHONIAN site here in the UK. Thus
>pre-_Archaeopteryx_ maniraptors _are_ known.
>
>BCF seems at least appealing to many of those I have spoken to,
>but the consensus remains that it lacks the evidence, and,
>despite what George says, nothing convincing has appeared to
>make BADD (I think I prefer TH's term... what was it again?)
>appear at all sterile. I wonder how many of those interested are
>aware that similar, 'alternative' lineages have been
>hypothesised in the literature - for taxa other than archosaurs
>- , but - because they lack the material evidence - do not win
>significant support. I do not want to offend George by putting
>his ideas in the same boat as those who believe that whales are
>derived ichthyosaurs, or that mammals are direct descendants of
>amphibians, but the principles seem much the same.
>
>I post this here as I'm hoping that people generally like to see
>discussion of it, hope you don't mind George.

Well, now that I have a bit more time--having returned from my
in-laws' Thanksgiving Day feast--I can compose a somewhat longer
and less hurried response than my earlier reply.

As an aside: Jensen's Dry Mesa small-maniraptoran material
includes at least two taxa: _Paleopteryx thomsoni_, which is the
actual maniraptoran (?archaeopterygid) and _Mesadactylus
ornithosphyos_, which is supposed to be pterosaurian (according
to Jensen & Padian, 1989). Dan Chure has found lots of
distinctive maniraptoran tooth crowns a millimeter or two high;
he has named one taxon _Koparion douglassi_. Not even the experts
agree on whether these taxa are valid or are validly classified;
the material is just too incomplete. The big problem is, when we
finally do start finding good skeletal remains of small, arboreal
maniraptorans, will we know what we are looking at?

Now for the main event.

BCF predicts that at least some of the smallest Jurassic and
earlier maniraptorans had relatively large, grasping forelimbs.
Considerable diversity is to be expected, since most lineages of
small maniraptorans never gave rise to large carnivorous forms
but simply evolved into other small maniraptorans. We would be
truly fortunate to find a form close to the ancestry of carnosaurs
(for example), and we are truly fortunate in having found a form
(_Archaeopteryx_) close to the ancestry of dromaeosaurids. But
I would hope that whatever small maniraptorans (and small
tetanurans, carnosaurs, ceratosaurs, herrerasaurians, and so forth)
we do find preserve enough characters from their ancestries to allow
cladistic analyses to show their relationships to the larger theropods.

As a byproduct of the above, BCF notes that the character
"reduced forelimbs" and its many variants, e.g., "humerus less
than 50% of femoral length" is not a theropod synapomorphy. This
feature recurs in dozens of theropod lineages, as indeed it
recurs in later avian lineages throughout the Mesozoic and
Cenozoic right up to date, whenever a lineage of flightless,
cursorial birds branches away from a lineage of volant birds. The
plesiomorphic condition of the forelimb in Dinosauria would be
"humerus greater than 50% of femoral length."

Did it never strike anyone as strange that, despite their
supposed evolution as consummate grasping organs, theropod hands
usually had only three digits? Dinosaurs plesiomorphically had a
five-fingered (pentadactyl) manus, as shown by the preservation
of this primitive condition in brontosaurs and ornithischians
(i.e., phytodinosaurs). Why would the dinosaurian hand _lose two
fingers and even three fingers_ on the theropod/avian side of the
group? Surely five fingers are better than three for grasping and
capturing prey. And why would the theropod hand lose those two
fingers from the _caudal_ side of the manus: IV and V, not the
more symmetric I and V (embryological studies of modern birds
notwithstanding)? And note that digit III is generally the
slenderest and most dispensable of the remaining digits, as if it
were next in line on the way out.

BADD theory provides no explanation; it merely accepts this
situation ("Well, what they heck, why _not_ lose a couple of
fingers?") as a _fait accompli_. But if one tries to visualize
what a manus might look like if it were a grasping organ being
modified into a _feathered_ wing, then the peculiar digital loss
in the manus begins to make some sense: Roughly speaking, _the
caudal digits were lost because they were in the way of the
feathers._ Imagine a small, arboreal form caught on the horns of
an evolutionary dilemma: should it retain a grasping forelimb, or
should the forelimb become more winglike and lose its grasping
ability? The theropod manus represents a compromise between these
two mutually exclusive evolutionary characters.

This compromise stablized for many millions of years (until the
wing became well enough developed in the archosaur-to-bird
lineage that its grasping function could be abandoned
completely), because theropod after theropod shows up in the
fossil record with the tridactyl manus. Then, all of a sudden, we
have didactyl manus (tyrannosaurids), monodactyl manus
(_Mononykus_), and of course volant birds, in which the three
digits are fused into a unit. It is as if some kind of
evolutionary threshold were crossed sometime in the Late
Jurassic, as enough improvements occurred to the wings to allow
the animals to forego grasping hands entirely.

How about the peculiar nature of the large-maniraptoran forelimb?
If you articulate the forelimb bones of _Deinonychus_ naturally,
you find that the forelimb's motion is rather restricted: The
hands fold with the palms toward each other, and the arm bones
are generally incapable of pronation and supination. The
semilunate carpal keeps the hands "flat." How is this an
_improvement_ in the manual grasping function? BADD theory does
not say: it accepts this as another _fait accompli_ in
maniraptoran/theropod evolution. But BCF notes that this is
precisely the kind of hand and forelimb one would expect in an
animal descended from a form in which the forelimb was at least
as long as the hind limb and was developing the efficient power
stroke that birds presently have: forelimb/wing motion is
restricted so that contraction of only a few precisely placed
muscles are required to move it just the right way for flight.
The large maniraptoran predators (dromaeosaurids, troodontids,
etc.) that evolved from such halfway-to-flight ancestors did not
_develop_ the maniraptoran forelimb; they were "stuck with it" as
part of their evolutionary legacy from their nearly volant
Jurassic ancestors (which resembled _Archaeopteryx_).

How about the retroverted first digit of the foot? BADD theory
notes reduction of the hallux as an adaptation for cursoriality.
Indeed, this is the case: in cursorial forms the tendency is to
symmetrically _lose_ the outer digits of the foot. The evolution
of ornithischians provides a good example of this. But the
operative word here is "lose": This does not mean "articulate
with the back of the adjacent digit and sit there." If you keep
in mind the arboreal dino-bird, in which the forelimb is becoming
less and less a grasping organ and more and more a gliding or
flying organ, you realize that there arrives a real need for the
feet to assume some of the animal's grasping function when it is
moving, climbing, leaping, and/or gliding from tree to tree. The
hallux did not retrovert in large cursorial theropods so that 50
million years later it would be there to allow their bird
descendants could perch: it retroverted ancestrally, to convert
the foot from a strictly locomotor organ to a perching organ. In
the arboreal dino-birds, the retroverted hallux was a large and
powerful digit, and so it remained in enantiornithan and other
Mesozoic birds. But in each of the cursorial theropod lineages,
it became reduced: another arboreal adaptation that theropods
were "stuck with." A few theropod lineages (e.g.,
ornithomimosaurians) eventually lost digit I, but in most of the
lineages pedal digit I found a use as a dew claw (for
intraspecific combat?) and hung around despite its uselessness in
locomotion.

How about the simple rpesence of huge claws on most theropod
hands and feet? In what other vertebrate predators are the claws
so enormous and powerful with respect to the other phalanges?
Granted that they were nice to have--marvelous killing tools--but
does that really explain how they got there in the first place?
Now imagine how useful such claws would be to small, arboreal
forms clambering about, constantly under the threat of falling.
Feduccia has shown that comparatively large, trenchant claws are
a hallmark of arboreal birds; so why stop at birds? They would be
useful to any arboreal vertebrate--and they make a dandy pre-
adaptation for the predatory descendants of such arboreal forms.

The whole point of BCF is that once you get into it, more and
more characters among theropods (hollow bones, pneumatic
skeletons, stiff tails, the furcula and its selective loss in
various groups, etc.) that BADD theory doesn't explain in
functional terms acquire quite natural and even sensible(!)
explanations. The central archosaur-to-bird lineage itself
undoubtedly followed lots of evolutionary blind alleys, throwing
off descendant forms, with all kinds of "almost volant but no
cigar" adaptations, most of which became mercifully extinct but
some of which found new roles and niches and diversified into the
various theropod groups. But one thing the lineage did do is
(somehow) evolve into modern birds. And as long as we continue to
project the larger, cursorial theropods into this lineage instead
of regarding them as divergent forms, we will _not_ gain a true
picture of avian evolution.