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RE: Jeholornis and enantiornithines had one functioning ovary like modern birds



  When I suggested the massive size decrease in birds as a correlation to 
oviduct loss, Tim williams replied:

<I don't see why body size reduction is necessarily a factor in the switch to a 
single functional oviduct.>

  Two things:

  First, apparent oviduct reduction occurs within *Avialae,* as basal paravians 
show paired oviducts via paired laying. It also seems to be conserved, if it is 
persisting through long-tailed birds and short-tailed ones, and is the 
condition for virtually all extant birds.

  Second, merely because kiwis can retain paired functional ovaried, but a 
single functional oviduct, some biological factors exists: 1, The oviducts are 
_paired_, but merge shortly after leaving the ovaries; 2, non-crown birds may 
retain a nonfunctional, but retained second ovary, which may be useful for 
hormone production, but is atavistic; 3, high investment in eggs in kiwi may 
serve as a functional need to develop the second ovary, a need probably not 
required in other birds, and this may be related to the costs of possessing two 
ovaries -- one may be active, and receive more nutrients than the other at a 
time (I actually do not know for sure, but it would be akin to a "on/off" 
switch in which some sections are more "on" than others to allow a fully 
developed and healthy ovary to be present at all times, and most likely an 
ecological consideration).

  I wonder, then, if paired ovaries are retained until the neognath-paleognath 
split, and convergently lost in other birds, but the ovaries prior to then were 
monofunctional, with a second back up and single duct system. It is perhaps no 
difficult thing to atavize the second oviduct and produce a single, duct. Wild 
speculation.

  Body size in this manner thus would prioritize smaller organs and invest 
towards display, flight, etc; but these factors no in play, as in kiwi, would 
permit the birds to "turn on" the second ovary. Just wild speculation, as I 
said.

Cheers,

  Jaime A. Headden
  The Bite Stuff (site v2)
  http://qilong.wordpress.com/

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


"Ever since man first left his cave and met a stranger with a
different language and a new way of looking at things, the human race
has had a dream: to kill him, so we don't have to learn his language or
his new way of looking at things." --- Zapp Brannigan (Beast With a Billion 
Backs)


----------------------------------------
> Date: Tue, 19 Mar 2013 13:28:01 +1100
> From: tijawi@gmail.com
> To: dinosaur@usc.edu
> Subject: Re: Jeholornis and enantiornithines had one functioning ovary like 
> modern birds
>
> Matthew Martyniuk <martyniuk@gmail.com> wrote:
>
> > No more than the fact that they retained teeth, the loss of which is
> > also usually seen as a flight-related, weight reducing adaptation.
>
>
> Yes, this weight-reduction hypothesis has been popular for a very long
> time. But the prevalence of (1) toothless/beaked non-avialan
> theropods (e.g., _Limusaurus_, ornithomimids, oviraptorosaurs) and (2)
> toothed Mesozoic avialans (e.g., most enantiornitheans, many
> non-neornithean ornithuromorphs) muddies the waters. IMHO a
> better-supported hypothesis is that loss of teeth is related more to
> diet than to flight, in the line leading to modern birds.
>
>
> Thus, crown birds (all of which are toothless) would have evolved from
> a single stock of herbivorous/granivorous ornithuromorph birds that
> lost their teeth due to diet.
>
>
> > It does suggest that if basal paravians were volant, they were not as
> > well adapted for flight as avialans, but that would have been the
> > working hypothesis anyway.
> >
> > On the other hand, this could be seen as support for the hypothesis
> > that basal avialans themselves were volant, which is not universally
> > accepted.
>
>
> Yes, both good points. Although "weigh reduction" would also be
> useful for aerial gliding, not just powered flight.
>
>
> Jaime Headden <qi_leong@hotmail.com> wrote:
>
> > Not really, if body size reduction is also a factor. basal Avialae are all 
> > small, very small animals; it is only
> > much later that birds become much larger, in "troodont" size ranges or 
> > bigger.
>
>
> I don't see why body size reduction is necessarily a factor in the
> switch to a single functional oviduct.
>
>
>
>
>
> Cheers
>
> Tim