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Re: Coelurus a maniraptoran (for how long?)



HP Jaime A. Headden wrote:

> My
> personal conclusion is that compsognathids and tyrannosaurids
> are closely related, even if compies represent the plesiomorphic
> state that tyrants dinos and are not their closest allies within
> the coelurosaur paradigm.

I have thought this for a certain time (for _very_ compelling reasons --
both share 2-fingered hands, which has turned out to be wrong for
compsognathids, both have relatively big heads and short arms, and
compsognathids fit the "new" idea of tyrannosaurs being derived from small
coelurosaurs instead of carnosaurs). When the Arctometatarsalia stuff was
published, and I read The Dinosauria only to find out that it takes lots of
imagination to declare the feet of *Compsognathus* arctometatarsal, I
dropped that. Compsognathids have AFAIK lots of plesiomorphies
(superficially they look like coelophysoids). So I'd be very interested in
your reasons! :-)

This may be the place to mention my own hypothesis :-) :

Coelurosauria
  |--Dryptosauridae
  |--Compsognathidae
  | inc. sed. *Scipionyx*
  `--+--Ornitholestidae
       |--Coeluridae
       `--Eumaniraptora(/Aves)
             |--+--+--*Archaeopteryx*
             |    |    `--?*Rahonavis*
             |    `--Dromaeosauridae sensu lato
             `--Maniraptoriformes ([node]/Avialae [stem])
                   |--Arctometatarsalia
                   |    |--Tyrannosauroidea
                   |    `--Bullatosauria
                   |         |--Ornithomimosauria
                   |         `--Troodontidae
                   `--Maniraptora
                         |--+--Oviraptorosauria sensu lato
                         |    `--Segnosauria
                         | inc. sed.*Avimimus*
                         `--Metornithes
                               | inc. sed. *Protoavis* if it is (partly) a
bird
                               |--Alvarezsauridae
                               `--+--*Yandangornis*
                                     `--Pygostylia

Regarding Segnosauria -- I surrender. :-) Until yesterday, I had terribly
misinterpreted Fig. 1 e & f in the description of *Beipiaosaurus* which show
that its first metatarsal looks just as in normal theropods.

I was too lazy to look up *Bagaraatan* =8-) . Off the top of my head it's
outside Eumaniraptora.

Possible synapomorphies:
DISCLAIMER: I lack knowledge of the relevant data matrices, I haven't seen
_any_ of the fossils except a very good specimen of *Confuciusornis* --
though at a distance, in a museum -- and I wouldn't recognize lots of
characters -- trochanteric features, all those prongs on vertebrae, muscle
attachment sites... -- if the fossils were put in front of my eyes. So I
have largely concentrated on what I can gather from the literature
(including this list's archives) and what is easy to see from low-resolution
skeletal restorations. And I haven't run anything through a computer, I
can't say if the above cladogram is the most parsimonious that the following
may produce:
-+- Unnamed node Ornitholestidae + Coeluridae + Eumaniraptora: arms reaching
at least the level of the ankles when fully extended downwards (may be a
plesiomorphy, but compsognathids have much shorter arms), distal carpal 1 +
2 enlarged and semilunate (compsognathids have small carpals and relatively
immobile wrists like coelophysoids), metacarpal I reduced in length and
metacarpal II long and slender to improve the opposability of the thumb.
This node might collapse into Eumaniraptora because the following character
states are AFAIK unknown for ornitholestids and coelurids:
- nearly all cervical and dorsal vertebrae (at least) pneumatised (to an
extent often comparable to very large carcharodontosaurids and sauropods),
furcula boomerang-shaped.
-+- Eumaniraptora (*Deinonychus* + Neornithes): tail reduced in length and
more lightly built (compsognathids have very long, rather heavy tails),
hindlimbs longer than usual (reversed in Dromaeosauridae sensu stricto, in
compsognathids relatively as short as in coelophysoids IIRC), probably wing
and tail feathers (if not earlier).
-+- Maniraptoriformes: more than 5 sacral vertebrae (is that right for
tyrannosauroids?), separate exit for the cranial nerve V1. I have forgotten
whether the character "articulars and quadrates pneumatised" is
applicable... that's in the archives, however. :-]
-+- Arctometatarsalia: Hm. Gaps in my knowledge. I think it's more
parsimonious to evolve the namegiving condition 3 or 4 times
(Arctometatarsalia, Caenagnathidae, Mononykinae; *Avimimus* may be an
oviraptorosaur, according to
http://www.cmnh.org/fun/dinosaur-archive/2000Mar/msg00310.html, so its
arctometatarsus may be synapomorphic with the ones of Caenagnathidae -- I
can't wait for the Mesozoic Bird book and the June issue of JVP) instead of
6 or 7 times (3 times separately in Tyrannosauroidea, Ornithomimosauria and
Troodontidae), though the fact that it did evolve several times might cast
doubt on this. Seems like I must get the Gaia theropod palaeobiology volume
too. $$$... :-(
It may tell something, however, that ornithomimosaurs and tyrannosaurs at
least have so similar proportions (stunning illustration in PDW), and that
troodontids and tyrannosaurs have small, pointed interdental plates.
-+- Bullatosauria: Well, the namegiving feature ("bulbous area at forward
region of the braincase"), overall similarities in proportions, and an
increase in the number as well as a decrease in the size of the teeth, along
with reduced space between the teeth (trend to a more cutting bite).
-+- Maniraptora: even shorter tail, tail not stiffened distally,
pneumatisation extending to mid-caudals, ornithoid eggshell, maybe a
double-headed quadrate (though the heads can fuse...), maybe some sort of
proto-pygostyle (but the vertebrae are not always fused...).
-+- Metornithes: Becomes hard to defend. Those alvarezsaurids with known
skulls have prokinesis (respectively loss of the jugular ascending process)
like AFAIK Ornithothoraces, and AFAIK no non-metornithine has this, but
confuciusornithids don't have it. I interpret this as a secondary
specialisation for a stronger (leaf-cutting?) bite, but it looks a lot like
the plesiomorphy... However, alvarezsaurids are maniraptorans, and this is
somehow a logical place for them :-) .
I have included *Protoavis* here because of its renal fossae... and I'll
write further Details On segments based on the Palaeontographica paper
_sometime_... :-)