Daniel Smith
Posts: 970
Joined: Sep. 2007
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| Quote (mitschlag @ Feb. 14 2008,14:15) | | Quote (Daniel Smith @ Feb. 13 2008,19:37) | Sorry for the delay mitschlag, but my time is limited.
I apologize also for not being able (yet) to find you a good example from Schindewolf's chapter on ammonites, (he doesn't give, very often, the specific names of the ammonites he speaks of, so it's hard to find the type of specific example you want), but I did find a pretty detailed example in one of Schindewolf's stony corals examples:
It is found on pages 205 - 208 in the section entitled "The Origin Of Major Types". Look at the figure on page 207, and read the corresponding description of it on the pages already outlined.
What you are looking at here is Schindewolf's breakdown of the splitting off of the heterocorals from the pterocorals. Like the suture line in ammonites, he uses the septal structure in the corals to retrace their evolution. |
I appreciate your diligence. Regarding Ammonoid evolution, Schindwolf says on p 204, | Quote | | The examples cited of the transformation of type from the Triassic ceratites to the Jurassic ammonites or from the pterocorals to the cyclocorals show particularly well that each appearance of a new type signifies a radical break in the course of evolution. |
I went back to pp 125-145, looking for his argument, but couldn't find a clear statement. Such efforts are not helped by the author's reluctance to give page numbers for his backward- and forward-looking references and by a poor job of indexing by his translator-editors ("ceratite" isn't indexed, for example). |
Thanks mitschlag, for keeping this thread interesting, civil, and on-topic. I really appreciate that!
I think what you're looking for is on page 140, in his section entitled "Mesozoic Ammonoids": | Quote | This progressive evolution [Note: among Triassic ceratites - D.S.] from smooth shells to those with multibifurcate ribs takes place within a whole cluster of parallel lineages, not always absolutely simultaneously and in many conservative or prematurely extinct lines also not attaining the highest stage of sculpturing, but we can still speak, in general, of a directional, steady progression of sculpture. All these lines then died off conspicuously, after having reached their highest degree of sculptural specialization, at the boundary between the Triassic and the Jurassic--all except one, which remained smooth and undifferentiated and survived this critical juncture to become the starting point of a new cycle of ammonoid evolution, of a renewed and profuse proliferation.
Exactly the same course of sculptural evolution that we see in the Triassic is repeated in the Jurassic-Cretaceous era in the Ammonitacea: at the beginning, there are once more smooth, unsculptured forms; these go on to develop simple ribs, then bifurcate ribs, then multibifurcate ribs (figs. 3.32 and 3.152).
BQiP, pg. 140, (my emphasis) |
Does that help?
| Quote | Anyway, regarding the corals, I grasp his point, and I agree that a gradualistic intervening sequence of adult forms between the heterocorals and the pterocorals is hard to envision. His suggestion of an ontogenetic switch looks reasonable to me.
And that could have happened without divine intervention! |
Perhaps, but I'd like to believe that--taken altogether--the sheer volume of such evolutionary steps lends itself more to a 'preprogrammed plan' explanation than to 'trial and error' and chance.
That said, Schindewolf would agree with you, not me, about the need for 'divine intervention'.
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"If we all worked on the assumption that what is accepted as true is really true, there would be little hope of advance." Orville Wright
"The presence or absence of a creative super-intelligence is unequivocally a scientific question." Richard Dawkins
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