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Re: The extinction of small dinosaurs (long again)
Original Message by John Bois
Saturday, 14 December 2002 16:57
> > The good reason is that a) there is no evidence of competition;
> > b) we have a catastrophe, so we _have to expect_
> > a catastrophic mass extinction _a priori_.
> This is what I mean by rejecting _known_ causes.
Of _damn big mass_ extinctions? I repeat: I do think the above is a "good
reason". We have that big bowl in Yucatán. We do not have any K-T fossils
that suggest competition.
> We know competition/predation are in constant operation;
Correct. But despite this, competition is rare. It's common wisdom in ecology
(just had an exam today) that it's hard to ever see competition in action,
what you see is "the ghost of competition past", for example, closely related
species with _almost_ the same ecological niches. Over here there are 3
species of tits (Paridae) that live on broad-leaved trees -- one close to the
trunk, another on the thinnest twigs, and the third in the middle; and there
are another 3 species for conifers. Such phenomena exist because the gain by
competition -- more resources -- is less than the effort -- energy and time
that could otherwise be invested in, say, reproduction; species that do not
compete have selective advantages over ones that compete.
Predation all the way to extinction is similarly rare. Where it occurs
cases of introductions of full-blown predators to islands. When a new
predator evolves somewhere instead of immigrating, I can't imagine how it
could have such an effect there, because its prey evolves in its presence and
will therefore keep up with it should an evolutionary arms race be triggered.
> we also know they are
> difficult or impossible to detect in the fossil record;
Competition of the enormous scale you suggest (all the planet's birds vs. all
similar-sized pterosaurs; all the planet's Neornithes vs. all Enantiornithes)
should take some time, though, and might therefore be recoverable. The
present knowledge of the fossil record is probably not enough for this,
> this, for you, is
> apparently enough reason to say they do not play a role in any
> particular extinction.
In this particular mass extinction. And probably not in any global mass
extinction, but of course this must be tested by investigating each of those.
> If I said dinosaurs had red blood, you should
> probably say "there is no evidence", and then reject the claim.
Nah. The phylogenetic bracket for red blood in non-neornithean dinos is
wonderful. Plus, hemocyanin (a blue blood pigment, with copper, which occurs
in some spiders) is IIRC said to have less capacity than hemoglobin and might
not have been enough for huge bodies, not to mention endothermic ones. :-)
> > > I don't see the distinction: if a new behavior evolves that results in
> > > a competitive advantage, that species' range will increase at the
> > > expense of others'.
> > If their ranges don't already overlap.
"Range" here understood as geographical distribution.
> > Border wars between species are AFAIK
> > very rare.
Same. If that's not what you meant, see above on "the ghost of competition
> Clearly, today's borders are the result of yesterday's border wars.
I can't think of 2 species with exactly the same ecological niche that live
in directly adjacent areas, which, again, may not be what you meant.
> > > Only the most radical of proponents of punctuated
> > > equilibrium would suggest that _all_ speciation involves catastrophy.
> > Actually AFAIK nobody does, because punctuated equilibrium is not about
> > catastrophes, it's about evolutionary stasis and allopatric speciation.
> Eldredge embraces the notion of catastrophic forces clearing the way for
> allopatric speciation. I actually agree with this--
I think most of these don't even count as disasters.
> The way was certainly cleared for
> mammals after the K/T. This was a punctuation.
A very big punctuation indeed.
> > > If it is true that speciation may be slow or fast--
Both punctuated equilibrium and gradual divergence occur. The former is
normal, the latter e. g. happens in the open ocean.
Michael J. Benton: Speciation in the fossil record, TREE 16(7) (Special
issue: Speciation), 405 -- 411 (July 2001)
> > > depending on the
> > > individual circumstances, my feeling is that as species develop new,
> > > more complex, strategies and behaviors,
> > Why "more complex"?
> More moving parts...more complex neural circuitry for processing more
> complex stimuli and responding with more complex physiological/behavioral
Or instead with simpler skills, like bigger jaw muscles. Sometimes less
moving parts, such as in the akinetic skulls of lungfish and basal tetrapods
vs. the kinetic ones of other Sarcopterygii. It happens both ways.
> > > The great changes in
> > > utilization of ecospace through geological time
> > What do you mean?
> Different species dominate different niches over time. Once upon a time
> the only vertebrates on land were amphibs--->then reptiles---> and so
Ah, the "mega-dynasties" (as Bakker called them), coupled with a gradistic
classification and its traps. I see it differently, of two clades
first one takes a certain group of niches, then comes a mass extinction, and
either the same or the other gets into those niches. A few examples:
· Semiaquatic croc analogs
Permian: Temnospondyli occupy the full diversity, from (and beyond)
alligator-snouted to gavial-snouted forms, and this in all sizes. Never mind
Lanthanosuchidae, anapsids with a big temporal opening, which apparently stay
rather small and are confined to Russia.
Early Triassic: Temnospondyli still occupy the full diversity, though
different groups than before, because the mass extinction has caused quite a
decimation. Proterosuchidae may not have been as aquatic as was assumed for
long; anyway they're only around 1.5 m long (otters don't really compete with
crocs, do they?), and all have narrow snouts with the usual serrated teeth,
different from conical croc or temnospondyl teeth.
Oops, I overlooked the Permian proterosuchid *Archosaurus*. Well,
look like it was globally important. In any case it was small.
Middle Triassic: No more Proterosuchidae. AFAIK less Temnospondyli than
before, but what do I know about temnospondyls. Anyway now there's
Proterochampsidae, a croc-like clade with slightly gavial-shaped snouts, but
only in South America... there aren't so many terrestrial MTr sites in the
Late Triassic: No more Proterochampsidae. Instead Phytosauria all over the
world. Difficult to guess what all their niches were, there snouts are all
more or less gavial-like, their teeth usually aren't, neither are some of the
known gut contents... Big, broad-snouted temnospondyls like *Mastodonsaurus*
survive. The most gavial-snouted and gavial-toothed phytosaur is
*Mystriosuchus* which lives in the sea, not (only?) in freshwater.
Early Jurassic: Almost all Temnospondyli and all Phytosauria gone. Now there
are real crocs like *Calsoyasuchus*.
· Biggest terrestrial top carnivores
Lower Permian: Sphenacodontidae.
Upper Permian: Dinocephalia and Gorgonopsida. (Any temporal difference?
Dinocephalia first?) Cynodontia confined to the rather small *Procynosuchus*.
Lower Triassic: Erythrosuchidae. *Cynognathus* is smaller.
Middle Triassic: Prestosuchidae. No more Erythrosuchidae, no more big
carnivorous cynodonts all the way to the Eocene.
Late Triassic: Prestosuchidae. Other "rauisuchians" (Poposauridae:
*Postosuchus*) and Ornithosuchidae play secondary roles... but wait, there
was a little mass extinction at the Carnian-Norian boundary, and I don't know
when Prestosuchidae died out or the majority of "rauisuchians" appeared.
Looks like Prestosuchidae bought it at the end of the Carnian, and
Postosuchidae came afterwards... right? -- Then there is *Aliwalia*.
Early Jurassic: Theropods only. Protosuchia (possibly terrestrial) stay small.