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RE: Giant birds

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> dbensen
> I'm not sure what I'm asking.  It is in the nether-regions of
> cladisitics that
> language starts to break down.  At what point to we distinguish between
> dinosaurs and birds?

We NEVER distinguish *between* dinosaurs and birds, any more than we
distinguish between dinosaurs and ceratopsians, or between dinosaurs and
brachiosaurids, or between dinosaurs and ornithischians.

Birds are a KIND of dinosaur, just as ceratopsians, brachiosaurids, and
ornithischians are.

Given that, I take it your real question is "when do you call something a
bird?"  This is tricky, because the word "bird" is an English word, and
actual science uses Neolatin terms.  So, in response I ask: what do you
consider the word "bird" to mean?  Do you mean it to be "feathered
dinosaurs"?  Do you mean it to be "flying dinosaurs"?  Do you mean it to be
"dinosaurs sharing all the features common to the living animals we call

As it turns out these three definitions of birds are entirely different
groups of animals.

Bird = "feathered dinosaur": Based on current information, 'bird' in this
sense would encompass *at least* Coelurosauria, and quite possibly more of
Theropoda (Avetheropoda? Tetanurae? Neotheropoda? all theropods?).  The
absence of feathers in adult _Carnotaurus_ suggests that Ceratosauria was
not feathered: if no ceratosaur was feathered than "feathered dinosaurs" was
restricted to some clade between Tetanurae and Coelurosauria.  The absence
of feathers in both pre-hatching and adult sauropods strongly advocates that
no stage of the sauropod life cycle had feathers: because of this it is
likely that basal saurischians were unfeathered too.

Bird = "flying dinosaur": This represents Avialae sensu Gauthier, Aves sensu
most everyone else, the clade of _Archaeopteryx_, living birds, their most
recent common ancestor, and all descendants.  My feeling is that MOST people
would use "bird" in this sense.  Although Greg Paul has advocated the
position that several of the maniraptoran groups (troodontids,
oviraptorosaurs, etc.) are closer to modern birds than was _Archaeopteryx_,
phylogenetic analyses have not yet supported this position.  If they do,
then troodontids, oviraptorosaurs, etc. would be "birds" in this sense just
as much as ostriches are birds.  Current phylogenies, however, would exclude
all traditional dinosaur groups from "birds" in the sense of "flying

Bird = "creatures with all the characteristics of modern animals we call
'birds'": This would be Aves sensu Gauthier, Neornithes sensu most everyone
else, all descendants of the most recent common ancestor of ratites,
tinamous, and neognaths.  _Archaeopteryx_ lacks many of the derived features
which characterize modern birds (pygostyle, alula, carpometacarpus,
tarsometatarsus, synsacrum, toothless beak, triosseal canal, etc., etc.,
etc.): these characters were acquired step by step so that various extinct
avian groups have some but not all of these.  Few people would use "bird" in
this sense alone; however, as Gauthier points out, many aspects of bird
biology studied by ornithologists are (strictly speaking) known ONLY for
this clade.

Incidentally, it is because of this sort of conclusion over informal
(vernacular) words that scientists have adopted a more formal language of

> When is something a flightless bird and when is it a
> dinosaur.

Something is a flightless bird when it is a) a bird (see above), b) does not
fly, and c) is descendend from flying ancestors.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-314-7843