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

Re: Bird Origins



In a message dated 95-10-07 22:23:31 EDT, martz@holly.ColoState.EDU (Jeffrey
Martz) writes:

>     Are you saying that Archaeopteryx is not descended from a theropod, 
>but from a separate lineage derived from a common ancestor with theropods?

Yes, but only certain theropods, not the entire group. Like all later birds,
_Archaeopteryx_ arose within Theropoda; but the particular theropod group
with which _Archaeopteryx_ shares the closest common ancestor is
Dromaeosauridae.

>    These dino-birds must have had all five digits to be sauropod ancestors.
>
>According to your cladogram, in what order did the various dinosaur 
>groups and birds branch off? 
>

Indeed, the very earliest dino-birds, situated below and slightly above the
divergence of Phytodinosauria from the central lineage, did have five pedal
digits. And five manual digits were still present in _Longisquama_, which is
the earliest-known archosaur to have possessed a furcula, or wishbone, as far
as we know, a unique theropod-avian feature. Wishbones are unknown in
phytodinosaurs (but separate clavicles might be known in some
sauropods--difficult to confirm), so it is likely that _Longisquama_ was a
dino-bird that had diverged from the central lineage after the
phytodinosaurs. Continuing up the central lineage, pedal digit V was lost by
the time the lineage reached the lagosuchian and herrerasaurian "grades,"
probably as a cursorial adaptation. The next pedal transformation, in which
digit I retroverted, defines (for me at any rate) the clade Theropoda, at a
point even higher up the central lineage. Very soon thereafter, the clade
Ceratosauria (retaining a small digit IV in the hand but no digit V) diverged
from the central lineage, leaving the central lineage itself as the "core" of
the clade Tetanurae. Then there was one or more (hard to tell because of
incomplete specimens) divergences that may collectively be lumped together as
Carnosauria. They are the most primitive theropods to have lost manual digit
IV as well as V--the result of an improved wing design in the central-lineage
dino-birds (though some of the more primitive carnosaurs may have retained a
tiny manual digit IV). As dino-birds continued to improve their wings by
elongating the digits and making the wings foldable, they became ancestral
coelurians, then ancestral maniraptorans, and then ancestral dromaeosaurids.
At this point the typical dino-bird resembled _Archaeopteryx_, or at the very
least, a slightly more primitive version of Archy.

Above this point, the central lineage dino-birds became ever more birdlike,
as their pectoral girdle and sternum expanded, their wing fingers started to
fuse, their tail shrank, and so forth. Cursorial theropod descendants from
these last dino-birds (before they became "true" birds) are very rare in the
fossil record, first because the world's ecosystems were pretty much full up
with legions of more primitive theropods that had diverged earlier from the
central lineage, and second because their wings were nearly useless as
grasping organs and would have been something of a maladaptation.
_Hesperornis_ is one such flightless dino-bird or primitive "true" bird
descendant.

As I noted earlier, although there is only one central lineage that leads
from the first archosaur to the common ancestor of extant birds, the lineage
had scores, perhaps hundreds of descendant groups. Any of those groups, at
any time, could have given rise to a subgroup of cursorial, flightless forms
that we would recognize as (probably aberrant) theropods: _Avimimus_,
_Mononykus_, you name it. That's why theropod taxonomy is still so riddled
with problems, unlike, for example, phytodinosaurs, whose taxonomy is rapidly
becoming a classroom exercise. (One major descendant group, only recently
recognized, is Enantiornithae, which were not cursorial, flightless forms but
were the most diverse volant group--powerful fliers that commandeered the
Cretaceous skies.)

The tendency to generate cursorial, flightless forms continued into the
Cenozoic, where we have forms such as _Diatryma_, _Phorusrhacos_, _Dinornis_,
the dodo, and the ratites. But with the world filled with mammals, these
"theropods" could not diversify to the extent that their Mesozoic
counterparts did. Imagine if the fossil record of Cenozoic birds were limited
only to those giant, ground-dwelling birds, all adaptively similar to but
actually not very closely related to one another. What would we say about the
avian family tree then?