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Re: Question from a Dilettante about Avian dinosaurs surviving K-Pg boundary



________________________________________
From: Jason Brougham
Sent: Monday, September 16, 2013 2:18 PM
To: rbox@crateandbarrel.com; "dinosaur@usc.edu"@listproc.usc.edu
Subject: RE: Question from a Dilettante about Avian dinosaurs surviving K-Pg 
boundary

We don't understand extinction very well, but the best answer is that 
extinction is a stochastic process. That is to say, it has predictable elements 
but it also includes random, non - deterministic events.

An analog might be the way that organisms arrive and colonize new islands. You 
can predict that lizards can survive drifting on wood in the ocean longer than 
rodents can (because of their slower metabolisms, which endure starvation and 
thirst better), and that may partly explain why lizards are fairly common on 
distant islands, but there are plenty of island rodents, too, and that was true 
even before humans started spreading them. We use physiology as the explainable 
part, but the random part matters a lot in biology.

We know that, at the end of the Cretaceous, the global ecosystem underwent a  
huge disaster: the K/T event. The earth was struck by an enormous asteroid just 
off the Yucatan Peninsula. Forests were knocked down and covered in ash as far 
away as Antarctica.There were severe plant extinctions in North America and 
much milder ones in Australia and New Zealand. Thousands of years later many 
parts of North America had just one or two species of fern growing and nothing 
else.

In a setting like that you would expect widespread extinctions and that's just 
what we see. Moreover, survival in a collapsed ecosystem is difficult 
regardless of the animal's habitat or physiology. Even if they can survive the 
initial disaster they must reproduce in a world where primary productivity is 
starting over from near zero, so plant food or prey will be scarce. The only 
hard part is figuring out what the survivors found to eat!

Nonetheless physiological explanations have been advanced for the patterns seen 
in all big extinctions. To the limit of my knowledge they a
rom the same problems: there are many exceptions to the explanation suggested, 
and the lineages in question always survived similar disasters, usually 
multiple times. So maybe trilobites went extinct in the Permian because they 
sucked at moulting (Brandt, 2002), but why then did they flourish for 300 
million years and survive multiple mass extinctions? Maybe ammonoids laid too 
many eggs to survive the K/T event (Kennedy, 1989), but they survived several 
other such events and other cephalopods reproduced the same way and survived.

Then you have the non - avian dinosaurs. We have the urge to do what we do in 
the other cases: look for any physiological or habitat difference between them 
and birds, and then assign their extinction to that difference. The problem is 
that there is no difference in the physiology of dinosaurs and birds because 
birds are dinosaurs. Robertson et al (2004), to their credit, did think of one 
difference. Some birds sit in the water or in shelters like caves or holes in 
trees, while we do not have evidence that any non-avian dinosaurs did that at 
the end Cretaceous. We have evidence that some Ornithischians did build 
burrows, and some unpublished evidence that troodontids may have also, but none 
at the end Cretaceous. So that, they reasoned, must be why birds survived. The 
ejecta re - entering the atmosphere from space made the entire sky 
incandescent, and infrared radiation would burn anything that wasn't sheltered. 
They suggest that ducks would survive because they were in the water. But no 
one has tested whether ducks can really survive in a broiler for 30 minutes as 
long as they can dive into water repeatedly. I kind of hope no one ever does 
because I bet you the results will be pretty ugly (and dead). Then the singed 
ducks could survive by eating submerged aquatic vegetation.  That makes perfect 
sense, but no one knows how their cousins, the singed lithornithids, chickens, 
and the ancestors of all other modern birds, survived the broiler, or how they 
survived on a diet of ashes and char
hat the answer is more likely that the sky was not that hot in the southern 
hemisphere, plant communities recovered faster there, and land animals 
repopulated the earth by dispersing from there. We have few good fossil 
deposits from there at the K-Pg boundary, but that's where one would look if 
one wanted to find non-avian dinosaurs that survived for awhile after the mass 
extinction event. I would not be the least bit surprised if we do find them one 
day.

Then consider choristoderes. They did survive the K/T, in crocodile - like 
aquatic habitats, but then died out forever in the Oligocene, while 
crocodilians and monitor lizards sailed right on to today.

I submit that any explanation for mass extinction patterns  that does not 
include randomness and contingency is unrealistic. When a bomb hits a bunker, 
some of the soldiers inside die while others may be completely unharmed. The 
survivors start trying to figure out what qualities and factors separated those 
who lived from those who died, and often they get stuck in illogical thought 
patterns that can be painful and obsessive.
Maybe the ones standing on the east side of the bunker got hit harder, or the 
ones with helmets on did better, but the correct answer is that it was at least 
partly random, that is the meaning of the event, that is a hard lesson to 
learn, and that is an important part of how history always unfolds.