[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Sinosauropteryx tail colors



On Thu, Jan 28, 2010 at 5:44 AM, Thomas R. Holtz, Jr. <tholtz@umd.edu> wrote:
>
> And this presupposes that the ringed tail themselves are a feature being
> selected for, rather than an epiphenomenon generated by patterning on the
> rest of the body being wrapped aound the narrow part.

Very good point, although rings are pronounced enough in some species
(e.g., zebra-tailed lizards) that there seems to be more going on. But
that does sound like a good explanation for other species with less
prominent rings, and perhaps for how ringed tails evolve in the first
place before they become exaggerated in those species where the rings
are prominent.

On Thu, Jan 28, 2010 at 2:09 AM, TooTs <DragonsClaw@gmx.net> wrote:
>
> Speaking of *Microraptor*: wouldnt those leg-feathers qualify as "structures
> rather expensive to grow, showing the fitness of the individual" - display
> structures?

Now this, to me, seems like an excellent case for the epiphenomenon
idea. Suppose we have an ancestral form with feathers but no wings.
Wings, for some reason or another, become advantageous. But the
initial mutations which cause long feathers to develop on the arms are
mirrored on the legs (since all limbs share a lot of the same
developmental pathways). The advantage of having long feathers on the
arms is big enough to outweigh the disadvantage of long feathers on
the legs. (The disadvantage could be simply that leg feathers limit
mobility, but also that leg feathers are a waste of developmental
resources.) Later on, from this four-winged group, subsequent
mutations accumulate in one lineage that hamper development of long
feathers on the legs (or restrict the sphere of developmental
influence, or whatever), and two-winged aviremigians arise. These are
not hampered by long leg feathers, and so they go on to outcompete
their four-winged brethren.

Of course, that's a Just-So Story until someone does the
genetic/developmental legwork (pun intended). I will note, however,
that something very similar happens in (apo-)pterygote insects: the
earliest ones have six wings (the anterior ones being shorter), and
these are winnowed down to four before the appearance of the crown
clade. (And further down to two in dipterans.)

One potential problem with this idea is that, so far as we know,
oviraptorosaurs don't have four wings. But this could just mean that
they independently evolved the two-winged condition. So the
"epiphenomenon model" would predict the existence of basal,
four-winged oviraptorosaurs. (Or, there's also the possibility that
oviraptorosaur and eumaniraptoran wings are convergences, but somehow
that seems far less likely to me.)
-- 
T. Michael Keesey
Technical Consultant and Developer, Internet Technologies
Glendale, California
http://tmkeesey.net/