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Re: Sci. Am. - present.



LNJeff said:

>     I challange anyone who sees the absence of Jurassic dromeosaur
>and troodontid fossils as a huge problem for the idea that dinosars are
>not ancestral to birds to show me the Jurassic fossil that IS.  You can't,
>because it hasn't been found either.  Unless _Archaeopteryx_ popped out of
>nowehere, it is safe to say that the fossils of bird predescessors haven't
>been found, period, whatever they may have been. Dromeosaurs and
>troodontids are known from the early-mid Cretaceous, but
>we haven't found the transitional forms between then and thier earlier
>non-dromeosaur and non-troodontid ancestors either; but I think its safe
>top say that they existed.  The Jurassic is an almost blank slate for a
>lot of animals that pop up in the Late Jurassic and Cretaceous, and to say
>the very least, dromeosaurs and troodontids are no more unlikely to be
>present in the Jurassic then any other established group that apparantly
>pops out of nowhere in the Cretaceous.
>     The prospect gets more likely if you look at the anatomical evidence
>tying dromeosaurs and troodontids to birds. Your aquatic ape analogy
>applies more to advocates of ALTERNATIVES to "BFD"; the dinosaur-bird link
>doesn't NEED to invent a completely unknown intermediates, just slightly
>extend the temporal range of a very well documented one.  Look at the
>other candidates without more then VERY superficial resemblances linking
>them to birds; _Megalanocosaurus_, _Longisquama_, or hypothetical flying
>crocodylomorphs, and it becomes pretty clear which current idea on bird
>origins is the most plausible.             

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.  Its interesting, too, that your examples of bird
relatives are Troodonts and Dromaeosaurs.  These groups are not particularly
closely related.  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.
Certainly BADD is the leading contender ("BFD" being an unfortunate
acronym!), but it has genuine problems that can't be dismissed as, you might
say, no BFD.

  --Toby White