A bird's tale/tail. (was Re: Greg Paul is right (again); or "Archie's not a birdy"... but Jeholornis is (not!))

[Tim Williams <tijawi@gmail.com>]

 <GSP1954@aol.com> wrote:


> Oh, piddle paddle. If it's a dinosaur with wings (or whose ancestor had
> them) and a short tail its a bird. Sapeornis, Confuciusornis, Ichthyornis and
> Hesperornis were all boids.


Of these four birds/boids, only one of them was incontrovertibly
capable of true powered flight: the basal carinatan_Ichthyornis_.
_Sapeornis_ and _Confuciusornis_ would have been extremely poor
fliers, if they could fly at all.  Even _Sapeornis_ has the appearance
of a glider.  Outside of Ornithothoraces, _Sapeornis_ is one of the
few avialans that shows evidence of good perching ability (although
_Changchengornis_ probably wasn't bad).  _Sapeornis_ probably had a
lifestyle that involved gliding from tree to tree, using rudimentary
thrust-generating motions to gain height during the glide.


The presence of a pygostyle does not indicate powered flight, so
simply saying "if it has a short tail, it's a bird" is potentially
misleading.  It implies that short-tailed avialans employed the tail
in the same manner as modern birds, when in fact there is no evidence
that the pygostyle initially played any significant role in
aerodynamic locomotion - apart from getting that damn tail out of the
way.  Compared to the pygostyles of more derived birds, the first
pygostyles were fairly long and rod-like, and it's been suggested that
they were associated solely with tail reduction (e.g., Gatesy, 2001;
Clarke et al., 2006).  This applies to all non-euornithean birds, as
it does for non-avian theropods with a "pygostyle", such as _Nomingia_
or _Beipiaosaurus_.  In crown birds and (probably) their basal
relatives (Euornithes) the pygostyle took on a sophisticated
aerodynamic function, with the advent of the bulbi rectriculum, and
associated rectricial fanning.  All known members of the Pygostylia
had a pygostyle (at least ancestrally), but only a derived subset used
this pygostyle in the same manner as modern birds.


> Whether they were in Aves is another matter, but
> they were birds. If it looks like a bird then its a bird. So I'll be damned
> if I am not going to call them protobirds that being appropriate for long
> tailed fliers and long tailed neoflightless taxa.


Sorting avialans into "short-tailed" and "long-tailed" is a false
dichotomy, because there is a whole bunch of post-jeholornithid,
non-euornithean birds that had intermediate tail morphologies.
Avialans like sapeornithids_, confuciusornithids, _Zhongornis_ and
enantiornitheans are "short-tailed" in the sense that the tail is
truncated and distally fused (forming a pygostyle) compared to the
primitive theropod condition; but the tail is still long compared to
the highly specialized pygostyle of modern birds, where it has an
important role in flight.


We're still a long way from establishing that any non-avialan
maniraptoran could fly.  IMHO, the evidence is trending in the
opposite direction.  Alas, there is a temptation to link bird-like
features that appear in maniraptorans to an aerodynamic function
(including powered flight), just because it serves an aerodynamic
function in modern birds.  But for a whole raft of characters
(pygostyle; quill knobs; strongly bowed metacarpal III;
carpometacarpal fusion, etc) being a flight-related character is just
an hypothesis.  Powered flight (as exhibited by modern birds) might be
limited to Ornithothoraces, meaning that Avialae (and Paraves too)
underwent an extended period of experimentation in aerial locomotion
before "achieving" true powered flight.  For pterosaurs and bats, by
contrast, this phase might have been brief.


> Now usually when a subject drifts away from the original topic I object to
> the heading retaining the same title. But for some reason I cannot quite put
> my finger on I like that this particular subject heading has not changed.
> Exactly why eludes me but its is such a superb title. May it enjoy a long
> life.


I find your first argument regarding on-topic subject headings
persuasive, so I feel duty-bound to change the subject heading.





Cheers

Tim