[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
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
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 <firstname.lastname@example.org>
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.