Re: [dinosaur] Avian tail ontogeny and interpretation of juvenile Mesozoic specimens (free pdf)

[Tim Williams <tijawi@gmail.com>]

David Marjanovic <david.marjanovic@gmx.at> wrote:

>> The avian tail played a critical role in the evolutionary transition from 
>> long- to short-tailed birds
>
> That's the most perfectly tautological sentence I've encountered in quite a 
> while. :-)


Heh heh.  Well, they're not wrong.  :-)

There's also the evolution of short-pygostyled birds versus
long-pygostyled birds, which is just as interesting.

"The data indicate that the Early Cretaceous confuciusornithids and
enantiornithines had larger pygostyles relative to body size than
other extinct or extant birds. _Sapeornis chaoyangensis_ and the basal
ornithuromorphs included in this analysis, however, had relative
pygostyle sizes well within the range of extant birds."

This makes sense, given that there is likely to be a functional
explanation behind this difference. In confuciusornithids and (most)
enantiornithines the tail/pygostyle did not have an aerodynamic
function; instead, the rectrices were used principally for display
(streamers, racket plumes, etc).  By contrast, in _Sapeornis_
(probably) and ornithuromorphs the pygostyle supported an aerodynamic
tail fan, and was important for flight.

To complicate things, certain enantornithines had a rectricial fan,
which suggests the tail was used for lift generation (_Chiappeavis_;
maybe _Shanweiniao_ - though not everybody thinks the latter had a
true rectricial fan: O'Connor et al. [2016
dx.doi.org/10.1016/j.cub.2015.11.036]).  Pengornithids like
_Chiappeavis_ have relatively shorter pygostyles compared to more
derived enantiornithines.  One hypothesis is that in
confuciusornithids and enantiornithines the dorsal surface of the
pygostyle supported an expanded caudal levator muscle for controlling
the display feathers (Wang and O'Connor, 2017 DOI:
10.19615/j.cnki.1000-3118.170118). So the relatively long pygostyle
might reflect the tail's ornamental function.