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Bird origins



Hi!

I'm not (yet) a professionnal, and I don't know much (yet) about dinosaur
(including birds) anatomy, or about evolution patterns, so this post is
entirely of amateur nature. As (almost?) anyone on this dino list, I try to
understand bird evolution and origin. Currently I don't believe in BCF or
in secondarily flightless dromaeosaurs but I'm open to every theory or
suggestion. Here is a summary of my current thinking on the subject, and
I'd like someone who has enough time  to review this post. I'd be glad to
discuss with some professionnal who does not find me and this post boring.

1. The evidence

     We know that derived structures looking much like feather rachis were
present in pterosaurs and *Sinosauropteryx*. There are two ways to
interpret these strutures:
          1) They are the result of convergence and are not shared by every
member (or at least the least advanced members) of the group including the
last common ancestor of pterosaurs and *Sinosauropteryx* and all of its
descendants (Ornithodira).
          2) They are shared by all Ornithodirans exept forms that
secondarily lost or reduced them (ornithischians, sauropodomorphs, maybe
some theropods). Pterosaurs had them, as well as *Lagerpeton*,
*Scleromochlus*, *Marasuchus*, *Pseudolagosuchus*, herrerasaurids,
*Eoraptor*, the most basal dinosaurs, coelophysids, etc...
I believe in the second hypothesis. By the way, could the possible lack of
rachis in sauropodomorphs and ornithischians be a diagnostic point for the
Phytodinosauria?
  
     We know that bird-like animals lived in the Jurassic and in the
Cretaceous. Some are very bird-like, such as *Caudipteryx* and *Unenlagia*.
Others, like dromaeosaurids and oviraptors, were more "primitive" compared
to birds. As far as I know, there have been three main ways to interpret
these taxa:
          1) They were relics of a transition that occured in the Triassic,
and they are secondarily flightless. Someone like George Olshevsky or Greg
Paul would explain this scenario much better than me so I'll not talk about
it any more.
          2) They were relics of this transition, but they were not
secondarily flightless (cf. later part of this post).
          3) They were IN the transition and they became always more
birdlike, and one day they became ______! (fill the blank). This hypothesis
has much stratigraphic problems, so I don't buy it.
Again, I prefer the second scenario.

     Recent discoveries show that derived theropods (coelurosaurs and maybe
even birds) were present back in the very early Jurassic, and even in the
Triassic! I see two ways of thinking about them:
          1) They were chimaeras, or they were not identified correctly. I
feel this hypothesis of a chimaera *Protoavis* and a badly identified
Hettangian therizinosaur as a bit too easy.
          2) They were real and they are strong evidence for some of our
theories. We should never try to adapt fossil to our personnal theories. We
must adapt our theories to new discoveries.

2. My ideas

     Here's my view of bird evolution. First ornithodirans (probably
creatures looking like *Scleromchlus* or *Marasuchus*, but without their
limb specialistions). They appeared during Lower Triassic and developped
primitive feather rachis. Then they diverged into pterosauromorphs and
dinosauromorphs. Dinosauromorphs became dinosaurs during the Middle
Triassic, and the main dinosaur clades diverged very quickly:
ornithischians and sauropodomrpohs were probably already distinct (or maye
they only existed as a basal phytodinosaurian if this clade isn't
paraphyletic). Theropods were already derived and the main theropod
lineages were emerging.
     I think in the beginning of the Late Triassic (or even at the end of
the Middle Triassic), basal ceratosaurs, megalosaurs, spinosaurs,
carnosaurs and coelurosaurs already existed and developed. Most of these
lineages probably remained modest for quite long, and it is likely that
other unknown groups were present but that they disappeared quickly (hey,
that's a second Cambrian explosion, isn't it!). Only one of these lineages
knew a large radiation: the Coelurosauria. Compsognathids were probably the
first of them. Then, ornitholestids, and then basal arctometatarsalians,
and then oviraptors and therizinosaurs, with always more specialised
"protofeathers" and forelimbs. then appeared dromaeosaurs, "protobirds",
and after that, and only after that, arboreality and flight appeared with
ancestors of *Archaeopteryx* and *Rahonavis*, and with *Protoavis*. Guess
what appened next!
     A last thing: I don't think we should search for the begginnings of
dinosaurs (or at least of theropods) in South America. I believe that birds
and other coelurosaurs  appeared in China or Europe, and after that
radiated everywhere else. That's because:
          1) Many primtive birds and "protobirds" come from China.
          2) Chinese dinosaurs often look like European dinosaurs
(*Metrianthosaurus*-*Yangchuanosaurus*, *Plateosaurus*-*Lufengosaurus*,
etc...).

     Yes, search for *caudipteryx* in Europe!

That's all! Please comment this post!

Best regards.


Félix Landry
e-mail: forelf@internet19.fr