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Re: Dinosaur Revolution Review
----- Original Message -----
> From: Tim Williams <email@example.com>
> In short: It's complicated. The most likely explanation is that both
> kinds of integument (scaly + filamentous) co-existed in many taxa.
> The two were not mutually exclusive. For example, derived
> tyrannosaurids had scaly skin, whereas the basal tyrannosaurid
> _Dilong_ clearly had branched, filamentous integument
> ("protofeathers"), and almost certainly had scales as well. The same
> principal is exemplified by modern birds, in which scales are
> essentially limited to the feet, and almost the entire body is
> feathered. However, even on the feet, feathers emerge between scales
> in some birds - so feathers and scales can co-exist on the feet.
I still disagree with this assessment on the grounds that it does not seem to
be backed up by evo-devo studies, nor by what we see in the fossils. When we
get good preservation we see that filamentous integument covered everything
(even down to the tip of the snout, it's never localized. The only cases that
seem to challenge this are controversial in and of themselves. The stuff
associated with that _Psittacosaurus_ specimen might be filamentous quills, or
they might be plant debris; maybe even elongate scales (they are remarkably
thick), The argument for filaments on _Juravenator_ is based off of extremely
faint impressions that the authors interpreted as "filament-like." The
structures themselves are tiny (viewable only under high magnification) and --
at best -- represent the tips of something. They could be filaments, but they
could also just as easily be taphonomic, or preparatiional artifacts. Heck,
even the infamous collagen fibres from back in
the day could be possible here.
I'm also hesitant to use (relatively) highly derived birds as examples that
scales and feathers can "live in harmony." I think these particular birds
(well, really just the owls) would make for a fascinating developmental study,
and that results from that study would go a long way to determining just how
easy, or hard it is for feathers to revert back to scales. I strongly suspect
that this is an apomorphic trait for the owls, but without a good molecular
study one really can't say much either way.
As for _Dilong_ I eagerly await its redescription (which is, hopefully, on its
way). As it currently stands it is still a putative tyrannosauroid. I would not
be surprised at all if a rediscription of the taxon winds up pulling it out of
Tyrannosauroidea and closer to Maniraptora (as has happened before).
> There might also have been an ontogenetic component to the degree of
> filamentous body covering. Baby _Tyrannosaurus_ might have been
> covered in insulatory protofeathers, such as down. Adults, by
> contrast, were massive enough not to need an insulatory body covering,
> and so were entirely scaly - or perhaps retained proto-feathers on
> some parts of the body solely for display (such as behind the eyes).
The problem with the ontogenetic argument is that it assumes scaly skin to =
naked skin. As has been mentioned before, scales are a form of integument.
There are no extant vertebrates that move from one integument type to another
as they mature (I leave inverts out because arthropods can do pretty much
anything). A fitting example of this can be seen in ostriches. Ostriches are
large birds that live in hot environments. The young are born covered in downy
feathers from head to crus. As they mature, the feather type changes and the
birds lose the feathers on their legs and necks. The neck and the legs are
completely bare. No sc
w on this area. In fact, the scales never move past the tarsometatarsus, a
place they had been since birth. Given this lack of integument change with
ontogeny I find it highly doubtful that a tyrannosaur hatchling would start off
downy and then move to scales as it matured.