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Re: Bird reduce their "heating bills" in cold climates
Jura <firstname.lastname@example.org> wrote:
> You are oversimplifying here. "Saurischians" is really
> "maniraptors and a couple of non-maniraptoran coelurosaurs.
> The majority of known saurischian integument is scaly.
I don't see why this is a "one-or-the-other" situation. Scaly integument and
dino-fuzz could co-exist in coelurosaurs (and other dinosaurs). After all,
modern birds have scales on their feet and on part of the face.
There is no reason why feathers (and their precursors) should have replaced
scales. This what Xu et al. had to say about _Dilong_:
"Large, derived tyrannosauroids were reported to have scaled skin
(28), but the presence of two kinds of body covering is not
unexpected. However, current understanding of the integumentary
morphology in non-avian theropods is hindered by poor information
on distribution. Given the diverse morphologies of integumentary
structures in living birds, it is possible that non-avian
theropods had different integumentary morphologies on different
regions of the body, and derived, large tyrannosauroids might bear
both scale-like and filamentous integumental appendages.
Alternatively, the lack of filamentous integumentary structures in
derived tyrannosauroids is correlated with the large size, a
physiological strategy also adopted by some mammals such as
elephants, which lose most of their body hairs as they mature
Further, in many Yixian dinosaurs, the filamentous integument (feathers or
dino-fuzz) is preserved as a halo around the body. Scales are not preserved on
the body itself - even on parts of the body where we would expect to see scales
(such as the feet, or face). This also applies to _Archaeopteryx_. So the
absence of scales is a preservational artifact, and so should not be used as a
guide as to whether scales were present or absent in real life. This is what
Xu et al. were alluding to.
> Assuming that all these fossils are preserving rea
> filamentous integument evolved on four separate occasions
> (coelurosaurs close to Maniraptora, Hypsilophodontids,
> _Psittacosaurus_, Pterosaurs), rather than the 7+
> re-evolutions of a scaly covering that would be required if
> it was a symplesiomorphy.
The above scenario strikes me as highly *non-parsimonious*. It assumes that
scaly integument is lost when filamentous integuments appeared. However, the
evidence suggests that a scaly covering was never actually lost - instead,
filamentous integument first appeared alongside scales, and the former
subsequently spread over the body at the expense of the latter.