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Re: [dinosaur] Morphological coevolution of pygostyle and tail feathers in Early Cretaceous birds (free pdf)
> WANG Wei & Jingmai K. O'CONNOR (2017)
> Morphological coevolution of the pygostyle and tail feathers in Early
> Cretaceous birds.
> Vertebrata PalAsiatica (advance online publication)
I thought this was an extremely interesting study - especially
alongside the review on the function(s) of pennaceous feathers in
birds and their relatives, also pre-published recently (Sullivan et
). It's great
to have a paper dedicated to the pygostyle. This skeletal feature is
critically important to flight in modern birds, because it anchors the
fatty and muscular tissue that positions the rectrices as part of the
aerodynamic tail fan complex. But, as Wang & O'Connor emphasize, the
advanced aerodynamic purpose of the modern pygostyle is likely not
typical of its function in other theropods - including many basal
Wang & O'Connor argue that the fused distalmost caudal vertebrae of
non-avian maniraptorans (such as in _Beipiaosaurus_ and certain
oviraptorosaurs) should not be called a "pygostyle" at all, but should
simply be described as "pygostyle-like". This is because fusion of
the distalmost caudals appears to have evolved separately in various
maniraptoran lineages, independently of birds (Aves). Here they have
a point: calling the coossified terminal caudals of any theropod a
"pygostyle" tends to lead to over-interpretation of this structure -
such as restoring oviraptorosaurs with elaborate tail fans (e.g.,
Persons et al. 2014). (I'm not saying oviraptorosaurs *didn't* have
tail fans - only that this interpretation goes way beyond the current
evidence.) Basal avians show you can have a pygostyle without
necessarily having a tail fan. And even if the preserved rectrices
look like they form a "tail fan" (= they form some sort of radial
array), it doesn't necessarily mean the "tail fan" had the same
mobility or function as the highly refined rectricial fan of modern
Wang & O'Connor clearly regard a true pygostyle as a synapomorphy of
the avian clade Pygostylia. They seem to be defining Pygostylia based
on the presence of a pygostyle (as Chatterjee intended when he came up
with the name Pygostylia), rather than using the phylogenetic
definition of Pygostylia (Chiappe's node-based clade that includes
Confuciusornithidae and Neornithes). This seems to be confirmed by
Fig 9, which shows Pygostylia including only pygostyle-bearing avians:
sapeornithids, confucusiornithids, ornithothoracines (=
ornithuromorphs + enantiornithines).
The similarities between the pygostyles of confuciusornithids and
enantionithines lend support to the idea that a pygostyle is
synapomorphic for the Confuciusornithidae + Ornithothoraces clade.
But _Sapeornis_ complicates things. There are some superficial and
tantalizing similarities between the _Sapeornis_ pygostyle and that of
ornithuromorphs: low pygostyle lamina (putative) + fan-shaped radial
array of pennaceous tail feathers (rectrices). However, in other
respects the pygostyle of _Sapeornis_ is quite different to the
pygostyles of ornithuromorphs and (for other reasons) other birds, and
it's doubtful that that the tail had an aerodynamic function. It
remains a distinct possibility that _Sapeornis_ evolved its pygostyle
independently. Would the distalmost coossified caudals then be called
a true pygostyle, or just "pygostyle-like"? This may hinge on exactly
where _Sapeornis_ comes up in the Aves family tree - lower or higher
(crownward) than confuciusornithids. Some phylogenies actually
recover _Sapeornis_ as more basal than the long-tailed, pygostyle-less
_Jeholornis_ (e.g., Gianechini et al. 2017), which would certainly
favor independent acquisition of a "pygostyle" in _Sapeornis_.
A true "tail fan complex" appears to be limited to Ornithuromorpha:
rectrices forming an aerodynamic tail fan, controlled by the
rectricial bulbs (bulbi rectricium). Basal ornithuromorphs seem to
have this complex, not just neornithines. By contrast, the tail of
confuciusornithids and enantiornithines doesn't appear to have had any
aerodynamic function at all. In these birds, the pygostyle appears to
have been used to support specialized display feathers, but not an
aerodynamic tail fan. The only possible exception is _Chiappeavis_, a
pengornithid enantiornithine, which might have some form of
aerodynamic tail fan, but not an ornithuromorph-style tail complex.
(The "tail fan" of the longipterygid enantiornithine _Shanweiniao_ has
since been re-interpreted as a more typical enantiornithine-style
rachis-dominated arranegement of feathers; O'Connor et al., 2016.)
I like Wang & O'Connor' tentative hypothesis that the laterally
expanded processes of the pygostyle in confuciusornithids and
enantiornithines suggests the presence of enlarged caudal levator
muscle for raising their ornamental tail feathers (as in modern
phasianids). Time will tell if this interpretation holds up.
In general, the presence of a pygostyle (or pygostyle-like structure)
in a fossil theropod says nothing about what kind of tail plumage it
might have had. However, the precise morphology of the pygostyle (or
pygostyle-like structure) provides clues regarding its function
(display, flight, both, something else).
I'm drawn to the hypothesis of Gatesy (2001) that
coossification/fusion of the distalmost caudals into a pygostyle was
originally a byproduct of tail reduction, and that it was co-opted
much later for an aerodynamic function as part of the derived tail fan
complex (i.e., rectricial bulbs and a true aerodynamic tail fan).
Under this scenario, the pygostyle of birds would have begun as a
spandrel, rather than an adaptation in its own right.
Gatesy SM (2001) The evolutionary history of the theropod caudal
locomotor module. In: New Perspectives on the Origin and Early
Evolution of Birds: Proceedings of the International Symposium in
Honor of John H. Ostrom (eds Gauthier J, Gall LF) pp. 237-254. New
Haven, CT: Peabody Museum (Natural History).
Gianechini FA et al. (2017) The cranial osteology of _Buitreraptor
gonzalezorum_ Makovicky, ApesteguÍa, and AgnolÍn, 2005 (Theropoda,
Dromaeosauridae), from the Late Cretaceous of Patagonia, Argentina. J.
Vert. Paleontol e1255639
O’Connor JK et al.(2016). An Enantiornithine with a fan-shaped tail,
and the evolution of the rectricial complex in early birds. Curr.
Biol. 26: 1-6.
Persons WS et al. (2014). Oviraptorosaur tail forms and functions.
Acta Palaeontologica Polonica 59: 553-567.