[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Birds and mosasaurs
> 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
> teeth are still regarded as possibly dromeosaur or troodont or, with at least
> probability, something entirely new.
It would make them a prety long-lived lineage, I admit. Howver,
ankylosaurs extend from at least the late Jurassic (and further?) until
the latest Cretaceous. I do not know if the Cretaceous ankylosaur
FAMILIES extend back to the Jurassic (someone who knows more might say
something), and I am at a loss to think up any other fairly exclusive
dinosaur groups that are KNOWN to be as long lived. However, given the
incompleteness of the Jurassic fossil record, how certain are you that, at
least for a lot of Cretaceous dinosaurs, you are not seeing a group that
extends back a lot futher then is currently known? Jurassic ankylosaurs
and Cretaceous diplodocids and camarosaurs are relatively recent
discoveries, if memory serves.
> That is the point: that the synapomorphies with birds are widely dispersed
> 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
> troodonts and dromeosaurs?
This reminds me of Fedduccia's argument that the undeniably
convergent bird-like features, like the beak in ornithomimids and other
coelurosaurs, proves that ALL the resemblances between birds and
dinosaurs are convergent. This is a thouroughly strange line of
reasoning; how does the REPEATED evolution of bird-like characters
in a group make it UN-likely that bird origens lie within that group? If
a group liked to evolve bird-like charatcers a bunch of times, doesn't
that increase the likelyhood that one eventually produced a bona-fide
When you say "at a minimum", you seem to be implying that
all dinosaur-bird synapomorphies are found in a more general theropod
group then Coelurosaura. This is not the case. Although Dinosauria,
Theropoda, and Tetanurae have several avain synapomorphies, Coelurosauria
has even more. In fact, as you go through these taxa from the most to
least inclusive, the number of characters goes up; just like you would
expect if bird-origens lay within the Coelurosauria. The dromeosaurs and
troodontids are the most bird-like in a VERY bird-like group.
> 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.
If I have been reading the evidence correctly, the implication is
that the different bird-like features were derived independantly, NOT that
all were present in the common ancestor and were lost differentially in
> 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
> derivations as unworthy of serious consideration.
There ARE no other derivations, unless you want to play the same
game as Fedducia and Larry Martin and invent COMPLETELY hypothetical
bird-like crocodylomoph groups and very late surviving thecodonts, a
guessing game a lot more extreme then extrapolating dromeosaurs and
troodontids into the Jurassic. At least we have evidence that there
were THEROPODS then for God's sake. As has been pointed out
in the past, even Chatterjee proposes that _Protoavis_ is a theropod, and
if he is right the number of bird-like convergences among
theropod groups goes up even more. I brought up the mosasaurs just to
point out that the morphological evidence for a coelurosaur-bird link is
at least as concrete as that proposed for Pythnonomorphs and varanoids,
and yet the absence of Jurassic varanoid fossils gets little flack. If
evidence comes to light some day that theropods are NOT ancestral to
birds, I don't think I will be able to feel svery stupid about it; it will
be the most absoutely incredible case of evolutionary convergence anybody
has ever heard of.