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Pterosaur diversity (was: Re: Waimanu)

Fair questions to ask might be: why has the new taxon moved into all of the other niches...and what is the delay in taking over the large-size niche? Are the large sized species immune to the forces that eliminated the globally
distributed smaller species?

Good questions, to be sure. I should mention at this juncture that I don't subscribe very heavily to classic niche theory (not hugely important, but a difference in opinion worth mentioning up front).

A good starting hypothesis might be that there
was something in the biology of the entire taxon that predisposed it to
global extinction. To propose a biological mechanism is not teleological.
Indeed, we apply such knowledge of predisposition-to-extinction today: e.g.,
island species are predisposed to at least local extinction due to no
defenses against continental predators!

I completely agree; half my Masters thesis was on biological traits associated with extinction. And, certainly, the specific flight dynamics and large size of late Cretaceous pterosaurs probably made them sensitive to a bolide-induced global event. However, given that they were probably also robust to so many other extinction pressures, I feel that it is inappropriate to say that they were absolutely more likely to go extinct than other clades (they were in hindsight because we know the bolide was coming). They were more likely than average to be killed by a massive global event, but those are rare.

Incidentally, island species are probably more predisposed to extinction because they have small ranges.

I know you recognize that most community structure is the result of
competition/predation past...a war-like battle to be sure!

In that sense, yes. Allow me to clarify my statement: community structure is not a matter of, for example "Team Birds vs Team Pterosaurs". Pterosaurs probably competed most heavily with other pterosaurs, and it stands to reason (given the late Jurassic trend) that large pterosaurs were doing better than small pterosaurs. Birds could be a factor, sure, but there are plenty of reasons to get directed selection for large body size. The flight dynamics of pterosaurs would have encouraged large size, for example. Populations may have become large because larger size was advantageous in and of itself, rather than because birds caused a global extinction of small pterosaurs.


I absolutely agree with this _except_ in the case where an entire
clade--pterosaurs--were selected in one direction, for one trait: bigness.
This suggests (to me) a selection _against_ other traits. I mean, to argue
for niche abandonment rather than niche replacement is romantic: species
don't simply give up niche space.

Sure they do. This is one reason I don't like the "war for niche space" model. That model essentially assumes only disruptive selection, and never directional selection. If 'bigness' was advantageous for pterosaurs (say because it improves soaring ability, which it generally does, especially for marine soarers), then selection _for_ bigness would essentially include selection _against_ smallness. If selection at the individual level produces a directional trend, there may be a decrease in population-level (and species-level) character diversity.

I would have thought that a prime engine
of morphological diversity was the tendency to divide up similar size class
niche among more species.

This is a common thought; I find actual support for it to be weak. Species produce "niches" by being in them, this creates the apparent trend of niche division when cladogenesis is rapid. As I mentioned, I don't really think "niche space" is real in the way the term is generally used.

For sure, the number of actual niches has
_increased_ over evolutionary time.


Because we have more critters.

I'm not saying there is _always_ a biological answer--just usually. And so,
the questions, at least, are valuable. 

Absolutely.

Today's marine soarers care for non-flying baby for several months. This
requires a predator-free island with close-by resources. I mean, parental
investment is very high in these species--this is "demanding" by definition!

True, but pterosaur life histories and bird life histories appear to differ a great deal. I think it is premature to make provisioning arguments at this time for all marine soarers of all clades. It is certainly something to keep in mind, though.

But there are several coastal species...and who knows what was around then.
I realize this is guesswork...but, with the likely pre K/T split, the
question of predatory birds influencing global community structure is now
askable.

I don't think it is a question we can ask just yet; we would need some fossil representatives of the Cretaceous stem falconiforms to know what predatory adaptations were actually present. A hypothetical ancestor is not sufficient for this problem.

I find this hard to believe. Are you talking from a standing start? Half a
second? For an animal with a ten meter wing span. Something like this is
shown on Walking with Dinosaurs when Q quickly takes off. My students are
quick to cry "fake"at this representation of a quick take-off. Looking to
be informed.

I'll let Jim handle this, as he is the one that made the comment and has worked out the launch parameters. I'll just say that 1) yes he means a standing start and 2) be careful not to assume that performance must be limited by size. Launch rate in large soaring birds is constrained by size because of the particular way in which they launch. A quadrapedal pterosaur would have a very different launch system (one that allows them to use the forelimbs in launching; a very important point).

It turns out that burst performance, in particular, does not drop off much with increased size (see Marden, 1994) in flying animals. Sustained performance does, but this matters little for an organism that spends most of the time in non-flapping flight phases. Note that Tobalske and Dial (2000) disagree slightly with Marden's explanation for this trend. I have, however, looked at the results of both studies pretty intensively, and I am pretty certain that their findings are actually largely congruent, if you're willing to do some algebra.

Finally, on the issue of whether or not large soaring pterosaurs were likely
to be replaced by birds with or without a global catastrophe: I am of the
impression that (absent man, at least) albatrosses are limited more by
predation/competition for predator-free nest sites close to food than by
food availabitlity _per se_.

Might be true; hard to say how they would do without human influence. Nest sites do not have to be very near food, though, because the foraging range of albatrosses is very wide (almost insane, really).

As such, the survival of a particular soarer
is likely dependent upon either its ability to drive a competitor off an
island or its ability to nest at more inaccessible sites.

May be true. However, a giant pterodactyloid may have some advantages in the "driving off competitors" column.

I would be
interested on your view about pterosaur latitudinal distribution and fish
abundance in tropical vs. arctic regions.

Given their estimated foraging range, and the fact that modern soarers occur from warm waters all the way to Antarctic coastlines, I'd hazard a guess that the latitudinal distribution would be very wide. On average, the range size of large pterosaurs should have been larger than those of most extant birds. Pseudodontorns may have had similar-sized ranges. Osteodontornis seems to have been nearly worldwide in its distribution.

Cheers,

--Mike Habib