Re: what killed choristoderes?

["Timothy Williams" <twilliams_alpha@hotmail.com>]

Ken Kinman wrote:

>      Does anyone have any ideas what killed off the choristoderes
> (champsosaurs) in the Eocene (or Oligocene?)?

The youngest record for the Choristodera is Oligocene (the 
appropriately-named _Lazarussuchus_).  The oldest record AFAIK may be the 
poorly known _Pachystropheus_ from the Late Triassic.  More definite records 
of champsosaurs are of Middle Jurassic age.

They certainly
> weren't
> easily killed off, since they made it through the K/T extinction (perhaps
> due to a similar aquatic habitat that also protected the surviving crocs).

And, like crocs, they remained remarkably conservative through their long 
evolutionary history (apart from one or two oddballs, like the long-necked 
Japanese critter _Shokawa_).

>      It seems likely to me that the crocs just crowded them out, either by
> competition (perhaps for habitat, such as for nesting sites), or big crocs
> acquired a taste for choristodere eggs or young (and just ate them into
> extinction)?

The choristoderans were remarkably gavial-like (although at least a few 
mimic the placodonts), so possibly it was competition with crocs that wiped 
them out.  (Like competition with the radiating rodents has been suggested 
as a reason for the extinction of the multituberculates.)  I've also heard 
the extinction of the choristoderes may have something to do with the 
radiation of the Cetacea, again because of niche competition.  With such a 
spotty fossil record (they just come and go like summer clouds) and enormous 
ghost lineages it's impossible to gauge if the extinction of the 
champsosaurs was the culmination of a decline or not.

Unless the extinction of a group is sudden and worldwide (and sometimes not 
even then) it is generally very difficult to figure out why some groups die 
off and others prevail.  For why some groups survive mass-extinctions and 
others don't, I like Steven Jay Gould's idea that sometimes it's just dumb 
luck.


Tim

P.S. And now for something completely different.  I've been asked politely 
by a few list members for a please-explain for my elephant joke.  (Why do 
elephants have Big Ears?  Because Noddy wouldn't pay the ransom).  Here 
'tis:

Noddy was a little elvin boy created by author Enid Blyton, who wrote a 
series of delightful children's books centering around the adventures of 
Noddy in his home town of Toyland.  Noddy's best friend was a fellow elf 
named Big Ears.  (Hence the joke.)  The books were very popular in Britain 
and Australia, and I've seen them in the U.S., but I don't believe they 
really took off here.



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