dino tails

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

>If this is the case, it would seem to me that any dinosaur group that
>showed a diminution of the tail would be an important marker or signpost
>on the way to birds, since birds have very little tail under all the
>feathers.  How are bird muscles done?  Heaven knows I eaten a few, but
>a don't recall much of the tuchis musculature...  And what dinosaurs
>show smaller tails?

In birds (and probably to a certain extent, dromaeosaurids), the role of
the M. caudofemoralis became replaced with muscles attached to the hip,
although in a very different pattern than in mammals.

And it depends on what you mean by smaller.  Ceratopsians had tails with
fewer vertebrae and shorter length than most other ornithischians, but this
probably didn't have anything to do with birds.  In theropods, there are
two major trends in tail shortening:
1) reduction in the total number of caudal (tail) vertebrae, with major
drops in numbers from 50+ in many ceratosaurs and primitive tetanurines to
30-40 in coelurosaurs, and a drop again in birds.

2) movement of the "transition point" (essentially, the furtherst down the
tail the main part of the M. caudofemoralis reached) closer to the body.
The transition point is fairly indistinct in some primitive theropods, is
fairly far out in some ceratosaurs, comes closer to the trunk in
tetanurines (including most non-dromaeosaurids, non-birds), and comes very
close to the body in dromaeosaurids and birds.

The major result of trend #2 is to allow most of the tail to become a rigid
structure.  This is what happens in dromaeosaurids, Archaeopteryx, and
modern birds.  In fact, in most birds (including all modern forms), the
distal tail vertebrae fuse into a single solid structure, the pygostyle.

>I still like the idea that dinosaurs had _other_ uses for the tails,
>though - otherwise it seems to me they could still be a lot smaller...

Well, most of them probably did.  Counterbalance, as you mentioned before,
plus display, defense, maybe tripodal feeding, and more.

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.