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Birds and mosasaurs
> 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.
>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
> 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.
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.
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?