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Triassic dinosaur evolution



Posted for Steve Brusatte.
______
 
I'm glad to see that many on the list have been discussing my recent paper  
on the evolutionary radiation of dinosaurs.  I've been in touch with David  
Peters off list about some of his questions, but I'd like to make a few points  
so that everybody understands the specifics of our analysis.  As Bill  Parker 
said, this paper does not present a new phylogenetic analysis.  In  the paper 
we refer to a "new phylogeny": this phylogeny was the subject of my  MSc thesis 
and is currently under review.  The large dataset of skeletal  characters 
that we present (437 characters for 64 taxa) is NOT a cladistic  dataset.  The 
characters have been derived from cladistic studies, but the  way we score them 
is not ideal for a cladistic study.  For instance, the  way we treat 
inapplicable characters is meant to quantify overall similarity  rather than 
derived 
similarity, since morphospace is more of a phenetic concept  than a 
phylogenetic 
one.  

The tree we present in our supplementary  information is the first MPT 
recovered from a slightly outdated phylogenetic  analysis.  In the time that we 
wrote the Science paper and had it go  through review we have updated our 
phylogenetic analysis, and the "final"  version is now in review as a 
stand-alone 
phylogenetic study.  We present  the first MPT because we need a fully resolved 
tree for our methods.  Do  not put too much faith in this tree: most of the 
broad groupings are correct,  but over the past few months we've been able to 
fill 
in a great amount of data  on several dinosauromorphs and "rauisuchians."  I 
hope our phylogeny will  be published soon...but even when it is, Sterling 
Nesbitt and Randy Irmis' work  will probably quickly eclipse it!

In light of some of the press articles,  I also want to be specific about 
what we mean when we say that dinosaurs "lucked  out."  We're not saying that 
evolution is random or that natural selection  does not operate!  What we are 
saying is that this classic idea of  dinosaurs "outcompeting" other reptile 
groups, because of "superior" or  "special" adaptations, over a prolonged 30 
million year period cannot be  upheld.  This outdated idea is part of the 
antiquated 
notion of evolution  as a progressive force, where better groups outcompete 
lesser groups over time:  "reptiles" to dinosaurs to mammals to humans.  We're 
also not saying that  there is no reason whatsoever why dinosaurs made it 
through the TJ extinction  and crurotarsans were decimated.  There almost 
certainly WAS some reason:  perhaps physiological, perhaps to do with growth or 
reproduction.  You can  call this "competition" if you will, but it's not 
competition in the classic  Bakker and Charig sense.

In essence, the fundamental question is this:  would dinosaurs have "taken 
over" if not for the TJ extinction?  Classic  ideas would say yes: dinosaurs 
were "superior" and preordained for success, and  showed that superiority over 
millions of years of tooth-and-claw  competition.  We say no: there were no 
signs that the dinosaurs were doing  anything "better" than the crurotarsans 
over 
the 30 million years they  overlapped in the Triassic.  Dinosaurs were lucky 
in the sense that there  was a very rapid mass extinction that changed the 
rules upside down.  What  had been the norm for 30 million years was no longer 
the 
norm.  Dinosaurs  coped, crurotarsans did not.  Dinosaurs were "lucky" in 
that they had the  adaptations to cope with a sudden and unexpected mass  dying.

Steve



-- 
Stephen Brusatte, MSc
American  Museum of Natural History
Columbia University, New  York
brusatte@gmail.com
--
 



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