Re: Birds and mosasaurs

["Augustus T. White" <augwhite@neosoft.com>]

Jeffrey Martz wrote:
> 
> > The gap between mid-Jurassic and early/mid Cretaceous is 50-60 Myr.  This is
> > not a "slight" extension of temporal range.  It is very similar to putting
> > cavemen with dinosaurs.
> 
>      A pretty extreme analogy; we aren't talking about a single
> species like _Homo_ sapiens, or even a genus like _Homo_, but an entire
> family that (unlike cavemen) already has an established temporal range of
> tens of millions of years.  Besides, I think the Bathonian age theropod
> teeth from England are still in the running for dromeosaur and troodontid
> identity.  If so, the range is already extended.

The point was simply to illustrate that a *lot* can happen in 60 Myr.  
Dinosaurs are, on 
the whole, a very rapidly evolving group.  One would expect to see considerable 
coming 
and going, even at the family level, in that length of time.  I agree that 
Bathonian 
teeth are still regarded as possibly dromeosaur or troodont or, with at least 
equal 
probability, something entirely new.  

> >Its interesting, too, that your examples of bird
> > relatives are Troodonts and Dromaeosaurs.  These groups are not particularly
> > closely related.
> 
>     That is beside the point; they are the two most commonly touted
> ancestors for bird ancestry (or at least closest relative), and either one
> (or both) could be extrapolated into the Jurassic with about equal
> plausibility.

That is the point: that the synapomorphies with birds are widely dispersed 
characters 
found (at a minimum) as far back as basal coelosaurs.  How else can one come up 
with a 
reasonably parsimonious explanation for the division of bird-like characters 
between 
troodonts and dromeosaurs?

> > Their LCA was probably something like Ornitholestes,
> > indicating that, at the very least, their synapomorphies with birds are
> > derived from fairly deep in the theropod line, if not even deeper.
> 
>       Their LCA is still within the Coelurosauria, so it isn't
> THAT deep.  Besides, general bird-like coelurosaur synapomorphies aside, I
> think that a lot of the very bird-like (or at least _Archaeopteryx_-like)
> features that have been pointed to in these families aren't homologous; in
> other words both families have different bird-like characters, or they at
> least derived some of the ones they DO have in common seperately (someone
> correct me if I am wrong), so we would still be looking at one group or
> the other as being more closely tied to birds, not thier LCA.

My point exactly.  If dromeosaurs and troodonts each have a different suite of 
avian 
characters, then one, high probability explanation is that there existed a more 
basal 
form that had both suites.

>       Getting back to the plausibility of extrapolating a Cretaceous group
> backwards into the Jurassic, a possibly innapropriate example might be the
> varanoids.  Pythonomorphs (mosasauroids and snakes) seem to show a lot of
> features suggesting they were derived from a varanoid lizard or something
> pretty close, and aigialosaurs are a perfect early-middle Cretaceous
> intermediate between late Cretaceous mosasaurs and varanoids.
> Unfortuntaley, varanoid fossils (I think) are known only from the
> later part of the Cretaceous, and certainly not the Jurassic.  Pushing
> them back far enough to make them ancestral to Pythonomorphs would involve
> a jump of tens of millions of years, and yet I don't think there is much
> opposition to the idea that varanoids probably extend back into the very
> early Cretaceous or Jurassic.

First, just so we're on the same page: I'm not saying that your families did 
not extend 
back to the Jurassic.  I'm not even saying they are not ancestral to birds.  I 
*am* 
saying that this possibility is not so large that we can effectively dismiss 
other 
derivations as unworthy of serious consideration.  

Second, I suspect that one reason your analogy doesn't hold is that 
Pythonomorphs, 
compared to dinosaurs, show considerable evolutionary stability.  Extrapolating 
varanids 
back perhaps 30 Myr is not equivilant to extrapolating a dinosaur family back 
twice that 
distance.  Not that it couldn't be right, mind you.  It could.  But its not so 
likely 
that it comes close to ruling out BASTA (Birds Are Some Type of Archesaur -- 
since it 
lacks another acronym).

>      So, the evidence comes from 1) the apparant phylogenetic connection
> between Pythonomorphs and varanoids, 2) the age of the earliest
> Pythonomorphs, and 3) the incomplete nature of the Jurassic
> fossil record.  Basically this same evidence (or lack-thereof, if you
> prefer for the third) used to put dromeosaurs and/or troodontids in the
> Jurassic, and yet extrapolating varanoids into the Jurassic based on
> morphological and cladistic evidence of a Pythonomorph-varanoid connection
> gets little flack.  Better evidence, or just a less trendy topic?

Both, I expect.  Moasaur taxonomy, for better or worse, does not receive the 
same level 
of scrutiny.

  --Toby White