Re: the start

[John Alroy <jack@homebrew.uchicago.edu>]

Just a quick comment on John Erick Schneide's post (I'll get back to  
David Schwimmer when I get a chance). The "impact during the  
Triassic/Jurassic" to which Schneide refers is (I believe) the impact  
that created the Manicouagan crater in Canada. As I recall, recent  
dates on the crater show it to be far too old to relate to the  
Triassic-Jurassic boundary event. Additionally, there are plenty of  
dinosaurs around already in the late Triassic (Carnian-Norian). It's  
true that Michael Benton has argued for 1) a late Carnian extinction  
event, and 2) a rapid dinosaur radiation after that. However, 1)  
rests on shaky biostratigraphic data (the correlations largely rest  
on a priori assumptions about what "early" or "late" Carnian  
vertebrate faunas should look like) and has never been supported  
statistically (e.g., tests for above-background extinction rates or  
for the Signor-Lipps effect). Meanwhile, 2) suffers from similar  
data-quality and statistical problems. In the first place, you need  
to show that an evolutionary radiation is something that needs to be  
explained at all.  This means showing that the radiation proceeded  
more quickly than would a random diversification with constant  
speciation and extinction events. A statistically unusual pattern of  
this kind is abundantly clear for the early Paleocene radiation of  
mammals, for example, but I know of no such study for any group of  
Mesozoic vertebrates. I also should point out that our knowledge of  
late Triassic vertebrates is far better than out knowledge of, say,  
early Triassic vertebrates, but generally not that good and far worse  
than our knowledge of virtually any fossilizable group of marine  
invertebrates (molluscs, trilobites, forams, graptolites, ostracods,  
etc.) at any point in history. I think we should refrain from arguing  
about "why" the dinosaurs radiated until we have some solid evidence  
that there was anything unusual about their early history of  
diversification.