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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