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Re: New Burnham Book

OK, I have calmed down and I can speak more clearly now.

Burnham's hypothesis that forests thinned out in the Late Cretaceous can be 
tested by the paleobotanical record. I do not know what "thin" means in this 
context, maybe more space between trees, a smaller tropical zone on the earth, 
or more woodland as opposed to true forests. Let's just say, for sake of 
argument, that Burnham presents some paleobotanical evidence in his book that 
this did happen. 

It still can only have a bearing on the relative abundances of ground 
dromaeosaurs and advanced birds, on the one hand, and gliding dromaeosaurs on 
the other, in the Late Cretaceous. Those groups all evolved more than 55 
million years earlier. The hypothesis that their relative abundances changed is 
also testable, statistically.

But regardless of the outcome, it certainly is not evidence that dromaeosaurs 
are secondarily terrestrial, because that would have happened well before 
Deinonychus in the Early Cretaceous. It also has no bearing on whether birds 
evolved from dinosaurs, because that would have happened in the Middle Jurassic 
or earlier.

He may be arguing that we have more  abundant ground dromaeosaur fossils in the 
Late Cretaceous, but he would have to demonstrate why this is not just a 
preservation artifact (more fossils overall from the Late Creatceous, fewer 
small - bodied fossils). And, as Mickey points out, Hesperonychus and Rahonavis 
may have been gliders in the Late Cretaceous, even Bambiraptor could have been.

Regardless, it has no bearing on the evolution of ground dromaeosaurs, because 
they were abundant by the Early Cretaceous (Deinonychus), and diverged in the 
Jurassic, based on teeth from England.

It also has no bearing on the evolution of carinate birds, which are known from 
the Early Cretaceous.

And it has no bearing on basal paravians, at least three lineages of which seem 
present in the Jurassic.

Instead, what we DO know is that Anchiornis is tantalizingly close to the 
divergence of the three paravian clades, and that it occupied the well known 
Tiaojishan Biota. This was a typical Jurassic forest dominated by conifers and 
especially diverse bennetites and cycads. There is certainly no evidence of a 
cooling trend or thinning forests in this habitat. Moreover, this habitat is 
the closest we've ever gotten to the habitat where paravians could have 
diverged into birds, troodontids, and dromaeosaurs, it is before Archaeopteryx, 
and yet it seems to have escaped Burnham's attention.

Thus, the Tiaojishan is so much more relevant to these questions than Burnham's 
seemingly uninformed conjectures.

Maybe some sbtlety of his argument was missed by the editorial summary, but 
it's hard to see what difference cooling at the end of the Cretaceous (if any) 
makes. Perhaps it is an attempt to cobble together a new explanatory scenario 
of why birds were the only ornithodires that survived the KT event without 
going out and collecting any new data.

And, yes, Mickey's point is an important one. There are a few forms of possibly 
gliding basal paravians now with very long arms, but there are also several 
lineages with short arms that probably did not glide. The ancestral state is 
not obvious. Either long arms re-evolved multiple times or short arms did, and 
neither is more likely on the face of it. It would have to be analyzed 
carefully to see what ancestral state is optimal. I am inclined to guess that, 
since Mahakala is late Cretaceous, it probably underwent a reversal, but that 
is just a  guess until someone does an optimization study.


----- Original Message ----
From: Michael Mortimer <mickey_mortimer111@msn.com>
To: dinosaur@usc.edu
Sent: Mon, February 22, 2010 1:40:09 AM
Subject: RE: New Burnham Book

Tim williams wrote-

> Exactly. Plus, there were small, feathered, and possibly winged paravians 
> near the end of the Cretaceous - such as _Mahakala_ and _Rahonavis_. Why 
> couldn't these have been four-winged gliders? Or two-winged gliders? 
> Maniraptoran gliders might have persisted until the very end of the 
> Cretaceous. Even if maniraptoran gliders did go extinct in the Lower 
> Cretaceous, it does not necessarily mean that birds were to blame.

While I agree Burnham's book sounds terrible and your basic point stands, 
Mahakala itself had rather short arms.  Thus I doubt it was gliding, though 
there are other taxa like Hesperonychus that could fit the bill.

Mickey Mortimer