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Re: Dinosaur Revolution Review



Jura <pristichampsus@yahoo.com> wrote:

> Not to get this thing started again, but do keep in mind that the majority of 
> dinosaur skin impressions show that they were scaly. Filamentous integument 
> is the exception, not the rule.


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.


In the course of theropod evolution, filamentous integument first
appeared alongside scales, and the former subsequently spread over the
body at the expense of the latter in individual taxa.  The extent of
filamentous/feathery body covering in a given taxon would depend on a
number of factors, especially the need for insulation (and/or
ornamentation).  Smaller taxa would presumably have a greater need for
insulation than larger taxa, given the higher surface area-to-volume
ratios of smaller taxa, where internally-generated body heat is more
easily dissipated.  Local climates probably also had an effect.


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).






Cheers

Tim