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Re: what killed choristoderes?




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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