Re: fourth trochanters and _Avimimus_

["John R. Hutchinson" <jrhutch@socrates.berkeley.edu>]

At 1:50 PM +0000 3/30/98, Darren Naish wrote:

>I'm unsure about the hind limbs of _Avimimus_. There is a fourth
>trochanter, thus the limb is not from a bird.

I'd be very careful about the use of the "presence/lack" of the fourth
trochanter. It is hard to dichotomize it, especially when you get within
Maniraptora. Based on the specimens I've seen of various maniraptorans, it
varies from a noticeable ridge (which we call the fourth trochanter) down
to a faint scar (which we call an "absence" of the fourth trochanter). The
scar may even appear absent depending on taphonomic filtering, and I'll bet
there's plenty of intraspecific variability. I've seen lots of latitude
even within dromaeosauridae, or basal avians (e.g. _Archaeopteryx_,
Mononykinae, _Patagopteryx_).

It's a continuum, not a dichotomy (note how it is scored as a 3-state
character in Forster et al's _Rahona_ paper; good move). Polarity is
difficult to clearly establish. What is clear is that it is reduced in
maniraptorans (and ornithomimids, whereve they fall out), and becomes more
reduced in birds, especially ornithurines. However, the mm. caudofemorales
(brevis and longus) muscle scar(s) remain(s) even in many neornithines, and
apparently has(have) some scaling-based variability; becoming much more
prominent (and could almost be called a fourth trochanter) in some large
paleognaths. The use of the fourth trochanter/mm.caudofemorales muscle scar
_alone_ as a diagnostic character for avian status is a bad idea; look to
the whole suite of characters.

I have heard that there are others currently studying the _Avimimus_
material for a revised description and discussion of it's nature(s). I
think dismissing it as a chimera may be premature, but still could very
well be true. Which parts are/are not avian is a difficult issue. I see no
problem with the hindquarters being avian, but could easily be non-avian
maniraptoran too.

                       --John R. Hutchinson