RE: Epidexipteryx feathers

[Tim Williams <twilliams_alpha@hotmail.com>]

Alan Brush wrote:


> The long ribbon-like tail feathers may also have a precedence. In their
> chapter on Birds in the "The Jehol Fossils" Zhang and Zhou describe two
> such cases. One in the well known Confuciusornis of a pair of elongate
> tail feathers which have a racquet-like structure. Similar feathers exist
> in extant Motmots and hummingbirds. The mechanism by which they are
> produced is known and they are considered to be involved in display. The
> second is in the same chapter in the reconstruction of Protopteryx they
> indicate the long "central tail feather...may represent the ancestral
> type of feather" (pg 144). The implication is that a scale was somehow
> reconfigured into a feather. A similar scenario for the origin of
> feathers was proposed by P. Regal (1975). This scenario is unlikely as
> avian scales develop differently and contain different from extant
> reptilian scales. The statement itself disagrees with Zhang and Zhou's
> description (pg 123) of the 'current picture of feather evolution" which
> is essentially that proposed by Prum & Brush (2002, 2003).


I fully agree.  Similar elongated tail feathers are also seen in 
_Changchengornis_ and _Paraprotopteryx_.  These avian taxa have been described 
as close relatives of _Confuciusornis_ and _Protopteryx_, respectively (but see 
http://dml.cmnh.org/2008Apr/msg00448.html).

In their description of _Protopteryx_, Zhang & Zhou (2002) argued for a role 
for _Longisquama_ in the origin of feathers.  This is an idea that's been 
around for a long time, despite being refuted many, many times.  The proposed 
link between _Longisquama_ and bird feathers is a very poorly supported 
hypothesis, given that (a) _Longisquama_ is phylogenetically very distant from 
birds; (b) it contradicts the evolutionary developmental scenario proposed by 
Prum & Brush;  and (c) _Longisquama_'s superficially feather-like integumental 
structures are unconvincing feather homologs.  The latter was recently 
discussed in depth...

Voigt, S., Buchwitz, M., Fischer, J., Krause, D., and Georgi, R. (2008). 
Feather-like development of Triassic diapsid skin appendages.  
Naturwissenschaften. doi: 10.1007/s00114-008-0453-1.


> It seems unnecessary to coin new terms to describe the feathers on
> Epidexipteryx. All the features occur in extant feathers and are
> produced by a common mechanism based on the geometry and ontological
> processes of the follicle. Fusion of parts (barbs, barbules, etc) not
> uncommon is a variety of taxa implying that it is a mechanism capable of
> producing a great deal of morphological variety. Apparently these vary
> early form are no exception.


Zhang &c's description of _Epidexipteryx_ ends with the statement that "Unless 
_Epidexipteryx_ is interpreted as secondarily flightless, the absence of 
pennaceous limb feathers in this taxon suggests that display feathers appeared 
before airfoil feathers and flight ability in basal avialan evolution."  This 
is probably true - and not all that surprising.  Many non-avian theropods show 
bony structures that appear to have been used for ornamentation, such as the 
assorted cranial crests, horns, and bosses seen in certain coelophysoids, 
ceratosaurs, basal tetanurans, carnosaurs, tyrannosaurs, and some basal 
coelurosaurs (e.g., _Proceratosaurus_).  But these cranial structures are not 
as prevalent in the maniraptorans (though some oviraptorosaurs had casques), so 
it may be that the "medium" for sexual display was shifted from the skull to 
the integument.  _Epidexipteryx_ and certain primitive birds took this a step 
further by evolving elongated tail feathers that were specialized for
 display.  As Alan says, there is no reason to believe that this was 
mechanistically distinct from modern avian feathers that are dedicated to 
display.


In short, it appears likely that the immediate precursors of feathers evolved 
originally for insulation, and then separate subsets of these structures took 
on novel functions (e.g., display, aerial locomotion).  _Epidexipteryx_ 
indicates that display may have preceded aerial locomotion.


Thanks for the post, Alan - very, very interesting.


Cheers

Tim
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