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Re: Pterosaur size
-------- Original-Nachricht --------
Datum: Tue, 12 Dec 2006 16:58:10 -0800 (PST)
Von: don ohmes <email@example.com>
> > Then, is the first part of your scenario falsifiable? I'm not sure
> > it is.
> > If the biggest member of a clade is extinct, you'll say a dwindling
> > has occurred since; if it's extant, you'll say the dwindling is yet
> > to come. Right? This sounds like you would only accept cases where
> > a maximum size was reached at some point in the past _and has been
> > precisely kept ever since_ -- an extremely unlikely scenario, even
> > if only for stochastic reasons. Maybe Neosauropoda counts (the
> > Morrison supergiants + *Argentinosaurus* + *Puertasaurus*).
> Don says => It is not a hypothesis.
Of course it is. :-) It's an overarching interpretation of a rather large
number of observations. It is thus potentially useful for science. To find out
whether it is, I must know the conditions for when it should be considered
> Insects appeared in the fossil record, those insects that found size
> to be an advantage got big, then they dwindled in size.
No, they died out. The Permian mega-dragonflies are not among the ancestors of
> Amphibians appeared in the fossil record, got big, then dwindled in
I don't know if it's out of the question that the so-called *Andrias matthewi*
(the huge Miocene cryptobranchid) is the ancestor of today's *Cryptobranchus
alleghaniensis*. If so, there we have the dwindling for this one lineage.
(Remember that it's a matter of dispute if temnospondyls or lepospondyls are
> Reptiles, dinosaurs, birds, then mammals all have done the same.
What, if anything, is a "reptile" then?
> Really, it has more to do with functional equivalences, wherein systems
> are grouped by process. Egg layers, flying vertebrates, quadrupedal
> herbivores, all follow the same pattern.
> Can you think of any process where this pattern does not hold?
Many of those groupings have a size limit. Think of rodent-like seed eaters: if
they get too big, they need to become ordinary herbivores; too small, and they
couldn't sustain endothermy.
I don't think the pattern exists. I think it's an attempt to see a pattern
where none exists. Trying to figure out if actual descendants are bigger or
smaller than actual ancestors is a lot of work -- part of that work, for
dinosaurs, was my M. Sc. thesis.
Let's try if I find a counterexample... big ectothermic herbivores: the biggest
turtles, the biggest pareiasaurs, the biggest captorhinids, and the biggest
caseids were all about the same size, IIRC, and I'm not aware of any dwindling
except perhaps among the pareiasaurs.
> Question-- what is the length of time from first appearance to the
> all-time size champion in the groups listed above? In the genetically
> related subgroups of the groups listed above (eg, dino herbivores,
> ptero flyers, bird flyers, etc.)?
Well, is it "bird flyers", or is it "large bird flyers" (vultures, albatrosses,
pseudo-toothed birds)? Which should count?
> As to extants ("future dwindlees" : D), only bats come to
> mind. Note the age (~60 mys? that number still good?). Note they are
> a sub-group. I think they will get bigger, barring catastrophe.
Why should they? What advantage would they get from it?
> Want to wager they will eclipse Quetz?
Certainly not, unless all birds die out and the bats survive it.
> Don says => Sigh. Aquatic air breathers. Lots of different kinds,
> very long time, blue whale biggest. Teleosts. Sharks. Clams. Any
> clear patterns?
As clear as for terrestrial vertebrates, I'd say. You might like the fact that
the biggest ichthyosaur (and probably the biggest marine vertebrate before
*Basilosaurus*) was Triassic, though -- but then it is far from the ancestry of
the later ichthyosaurs.
> > > It is an entirely incorrect way of
> > > determining how much atmospheric N2 is subducting over time.
> > How should that happen?
> Don says => Qualitatively: seawater is at equilibrium w/ the
> atmosphere. Where goes seawater, there goes (some) atmosphere.
> Seawater penetrates _at least_ 12-14 km into ocean crust. Circulation
> intra-crust and subduction occurs. Exotic organisms that eat something,
> but nobody knows what, and p/t/ph conditions that are suitable for
> abiotic N2 fixing are down there. Continental seds with contribution
> from the atmosphere also exist. No one questions that some N2 is
> Quantitatively: How much, and what fate are the questions I am asking.
Well, what goes down must come up. The water comes up again -- there are
volcanoes that spew a lot of water vapor; I guess so does the nitrogen. After
all I'm not aware of a volcanic rock that is a nitrate or nitrite, and nitrides
don't seem to occur in nature at all.
> Don says => N2 moves along thermal gradients (hot=> cold),
Why should it?
> The thermal gradient from the earth core to the surface
> reverses just below the surface.
> Where, logically, an average wayward N2 molecule stops, making no
> contribution to atmospheric mass.
Well, there are no nitrogen bubbles at that depth, nor any other nitrogen
stores I'm aware of...
> Also, conductivity is proportional to density, given similar
> composition. Generally, as atmospheric mass increases, latitudinal
> temp gradients decrease.
I see. Now someone should quantify that.
> > > 3). The tacit and universal assumption of steady-state atmospheric
> > > N2 mass therefore colors all evaluations and models of paleo-
> > > climate, particularly those periods of the distant past wherein CO2
> > > levels were much higher.
> > Then you have to explain why those models work pretty well.
> Don says => I do? They do? That is big news. What do you mean by
They are pretty successful at predicting the climate we find to have existed by
other evidence. Like, no ice during much of the Cretaceous, and so on.
> > I defend this by saying that, having cleared such hurdles as evolving
> > feathers and wings, the time logically needed for a volant to
> > optimize wingloading is relatively short.
> But then, *Limnofregata* is IIRC Danian, and the Danian only lasted for
> around 3.8 Ma.
> Don says => ?
Maybe that counts as "relatively short" or less.
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