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Re: Featherless _Mononykus_? (LONG response to long essay)



On Fri, 26 Apr 1996 Dinogeorge@aol.com wrote:

> Okay, here we go. I am not the first to come up with this >>earthshaking<<
> notion, but: {{{{_Mononykus_ is NOT a "bird"}}}}.

> One major problem is the TAIL. In the tails of _Archaeopteryx_ and all known
> later birds, the haemal arches, also known as the chevon bones, are greatly
> reduced or absent (so are the tails, for that matter!).

More on this below.


> _Avimimus_--which shares a number of features with _Mononykus_ and could
> well be closely related; compare the hind limb bones of the two genera, for
> example).

I agree.  See below.


> _Archaeopteryx_ is much
> more advanced toward the avialan condition in many important and major areas
> of the skeleton, including the skull, the pectoral girdle, and the vertebral
> column, than were any of the known dromaeosaurids.

Yes!!  Phil Currie would be proud.  Actually, the idea that dromaeosaurs
were very close to _Archaeopteryx_ is what initially led me to the unlikely
idea that arctomets are more birdlike than _Archaeopteryx_--I didn't see
how I could stuff the arctomets in between dromaeosaurs and
_Archaeopteryx_ without a lot of retro-evolution on the part of the
arctomets.  Upon reflection, I think that dromaeosaurs and archaeopts are
not close relatives and that arctomets are intermediate between the two
in the branching pattern leading to birds.


> My current best guess as to the phyletic position of _Mononykus_ is in a
> sister group to _Avimimus_.

The argument has been made that _Avimimus_ may be a chimera of parts from
several taxa.  I'll agree with you, though, that whatever animal
_Avimimus_'s legs come from is likely quite close to _Mononykus_.  The
ulna referred to _Avimimus_ looks to me like it has a fairly large
olecranon as well.


> This clade itself arose within Maniraptora,
> likely as a sister group to the clade Bullatosauria (Troodontidae plus
> Ornithomimidae), sometime before the divergences of the dromaeosaurids
> (suborder Deinonychosauria) and oviraptorids (suborder Oviraptorosauria) from
> the lineage joining the common ancestor of Dinosauria to Avialae.

[BUZZER]  Sorry.  Can't agree with you there.  Tyrannosaurids are
considerably closer to birds than are dromaeosaurs, and their feet, among
other things, are similar enough to those of the other arctomets that I
consider the Arctometatarsalia to be a natural group, very close to the
ancestry of the Avialae (incl. _Archae._).

Oviraptorosaurs are likely aberrant arctomets as well.  The more
primitive members (e.g. _Chirostenotes_, _Elmisaurus_) have arctomet
feet, and the feet of more advanced groups become progressively
broader:  the feet of _Oviraptor_ are more pinched than those of the
more advanced _Ingenia_.

Sorry, also, George and others who want therizinosauroids to be
phytodinosaurs, but these animals, too, are more birdlike than
dromaeosaurs.  Phil Currie informs me that _Erlik/cosaurus_'s cranial
nerves are wired in the avian pattern.  It is difficult to pin down the
exact phylogenetic affinities of the Therizinosauroidea, but since their
feet are so broad, I would tend to place them just below the
Arctometatarsalia.

Finally, since George mentioned haemal arches, there is the much-abused
_Jurapteryx recurva_, usually interpreted as a juvenile _Archaeopteryx
lithographica_.  The chevrons in this animal are rather better developed
than are those of _Archaeopteryx_, and the tail is considerably longer,
as are the legs.  The feather pattern on the tail is also different.  In
_Jurapteryx_, the feathers on either side of the tail are about the same
length all the way to the tip, creating a ribbonlike tail:

/|\
/|\
/|\
/|\
/|\
|||

In _Archeopteryx_, the feathers get longer, then shorter, towards the end of
the tail, creating a more aerodynamic, fanlike pattern I can't reprduce very
well:

   /|\
  //|\\
 ///|\\\
 ///|\\\
////|\\\\
|||/|\|||
  |||||

If you want to try to attribute this to a growth series, more power to
you, but it doesn't make much sense to me.  How many birds fledge while
their legs are still too long and their arms too short to fly?  Does
the *pattern* of feathers change much during a bird's lifetime?  GSP's
reconstruction, I'm afraid, seriously downplayed the differences between
the two taxa.  IMHO, this makes _Jurapteryx_ look like an outgroup to
_Archaeopteryx_ + birds (or, possibly, a very early flightless bird!).

To sum up:

Ornl-\
Comp-?--------\
Drom--------\ |
Thez------\ |-/
Ovir--\   | |
Tyra--|-\ |-/
Mono\ | | |
Avim/\/ |-/
Troo\/  |
Ornm/   |
Jura-\--/
Arch\/
Ornt/

Ornl = Ornitholestes; Comp = Compsognathus; Drom = Dromaeosauridae; Thez
= Therizinosauroidea; Ovir = Oviraptorosauria; Tyra = Tyrannosauridae;
Mono = Mononykus; Avim = Avimimus; Troo = Troodontidae; Ornm =
Ornithomimosauria; Jura = Jurapteryx; Arch = Archaeopteryx; Ornt =
Ornithothoraces.

I'm not sure what to do with _Alvarezsaurus_.  I'd have to see a foot to
be sure.

Thank you all for your time.

Nick Pharris
Pacific Lutheran University
Tacoma, WA 98447
(206)535-8206
PharriNJ@PLU.edu