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Re: Declining pterosaur diversity



----- Original Message -----
From: "John Bois" <jbois@umd5.umd.edu>
Sent: Sunday, November 28, 2004 4:47 PM

Nobody but you has even tried for the last ten years or more.

Thank you...but let me decline the compliment. Penny, D. and M.J. Phillips 2004 The Rise of birds and mammals: are microevolutionary processes sufficient for macroevolution? _Trends in Ecology and Evolution_ Vol.19 No.10 pp.518-522.

Oh. Let me rephrase... "no pterosaur expert has tried"... :-)

...in which is discussed five models for the biotic turnover at and/or
before the K/T boundary.  The authors propose inverting the traditional
question of "When did the last pterosaurs and dinosaurs become extinct?",
to "When did mammals and birds start replacing small dinosaurs and
pterosaurs?" (This is a question I have often asked on this list.)

Fine, fine. Let's try to answer that question.
- An SVP meeting abstract from this year says that there probably never was an adult pterosaur with less than 40 cm wingspan. So no pterosaurian hummingbirds, sparrows, blackbirds, thrushes, titmice, and lots, lots more have probably ever existed.
- The biggest known Mesozoic mammals are from the middle-late Early Cretaceous. You have tried to build arguments on the amount of large-sized mammal species in later times, but the fossil record is still so bad that not only we don't have such numbers, we still risk making gross mistakes in inferring the ecology of "the tooth, the whole tooth, and nothing but the tooth". Remember the Stagodontidae, such as *Didelphodon vorax* from the Hell Creek Fm, which I and seemingly everyone else have proclaimed to be comparable to the Tasmanian devil in both size and ecology? Looks like we were wrong. Another SVP meeting abstract from this year makes a good argument for interpreting stagodontids as otter analogs, based on the discovery of... isolated tail vertebrae. I don't know of a dinosaur that could be interpreted as an otter analog, so it doesn't look like dinosaurs occupied that niche before mammals did.


Fueling this is an observed increase in diversity of birds and mammals.
They also supply some new data.  OK...the data suffer from the usual
problems (preservational bias, etc.) but they indicate a reduction in the
diversity of small dinosaurs from three time periods: E-M Cret.,
Campanian, Maastrichtian.

Discoveries of dinosaur clades that were thought to have lived only elsewhere still happens in the Hell Creek Fm. I will not hazard a guess on what small dinosaurs lived in Africa or Australia or even southern Europe in the LK, and I certainly won't hazard a guess on what mammals could theoretically compete with a compsognathid... short of a fully formed fox (note that I'm talking about a relatively cursorial animal here).


Lastly, the pushing back of placental evolution (you will recognize the
authors from previous papers re molecular estimates of divergence) insists
that these questions be asked.

I remember Penny from a paper from 1997... it made sweeping arguments based on a catastrophically misrooted molecular tree of Neornithes (don't ask me how it passed peer-review... in Science). But enough of this ad hominem "argument". Instead let's have a look on whether placental evolution is really pushed back by molecular data.


I'll start with deliberately picking the one paper that finds the shortest Mesozoic history for Placentalia. (Considering how lengthy the paper explains its methods, this subjective choice may not be all that bad.) It is

Peter J. Waddell, Hirohisa Kishino & Rissa Ota: A Phylogenetic foundation for Comparative Mammalian Genomics, Genome Informatics 12: 141 -- 154 (December 2001)

These authors derived four different sets of divergence dates from their tree by using one of the four calibration points per set (which is a priori a bad thing). When they used the "tarsier/human split" at 55 Ma ago, they got only the following divergences in the Mesozoic:

--80--68--Paenungulata
  |   `--Afroinsectiphilia
  `--76--Xenarthra
       `--70--Supraprimates
           `--Laurasiatheria

(The word-monster Afroinsectiphilia includes Tubulidentata, Macroscelidea and Afrosoricida; Supraprimates is the same as Euarchontoglires of other authors.)

If (!) that calibration is correct, it tells us a few interesting things:
- The deepest divergences of placentals sort of look like they happened in anticipation of the K-Pg boundary.
- On a more objective note, not much diversification can have taken place during that short time. For example, if we want to find a more immediate sistergroup of (Xenarthra + Boreo[eu]theria) than what Afrotheria is, we need lots of good luck -- it would have had to appear within no more than four million years. When Xenarthra and Boreo(eu)theria diverged, they together were _more_ closely related to Afrotheria than chimpanzees are to us now.


Now let's ask whether the calibration is correct:
- Being based on fossils of presumed representatives of both lines, the date of 55 Ma is automatically too young. There's no way to quantify this, however. It might be too young by only a hundred thousand years or two if I'm optimistic -- in other words, it might be inside the error margin of the date itself.
- Oops, wait. It might be too _old_ -- if omomyids don't belong in the tarsier clade. Does someone happen to know how old the earliest anthropoids are? IIRC their age is similar, though...
- Regardless, the used model of molecular evolution does, unsurprisingly, not take the effects into account that factors like metabolism and population size have on the speed of molecular evolution. Small endothermic animals have, as a rule of thumb, higher metabolic rates and therefore higher mutation rates = faster molecular evolution. It is certainly reasonable to assume that K placentals were small -- or at least that only small ones survived the K-Pg boundary. Small animals also have, as a rule of thumb, larger populations and therefore slower rates of molecular evolution than larger animals with the same metabolic rates... it may or may not be reasonable to assume that small K placentals had smaller populations than small Cenozoic placentals, but it's certainly reasonable that the mass extinction event drastically reduced all population sizes! In sum, I can imagine that even those last 4 divergence dates will one day be crowded into the Cenozoic.
- One of the other calibration points gives ages that are just one to two Ma older. It is the rabbit/pika split at 42 Ma ago, which is rather badly constrained by fossils, so one could say that both calibration points give identical ages and thus corroborate each other -- respectively suffer from the same amount of systematical errors.


The idea that often gets aired on this
list--that there is infinite ecospace, or rather, an infinite number of
niches created by division into smaller and smaller units when
new species come along--must be held to question.

Oh, ecological niches don't always become smaller and smaller. Even if we totally ignore competition and predation, extinctions -- including mass extinctions -- still happen due to abiotic factors.


So, to suggest some species become
extinct upon the arrival of new species--even without evidence of this
extinction--is not a radical claim.  And I am certainly not the only one
making it!

That extinction can happen at the literal arrival of species that immigrate from elsewhere is indeed not a radical claim. The biggest problem I see, however, is that pterosaurs may well have retained a global distribution all the way to the end, and certainly had one in the EK -- which means an "elsewhere" may not have existed to start with.
That extinction can happen at the figurative "arrival" of species that evolve in situ, however, _is_ a radical claim. (I'm not sure if it's your claim.) When an ecological niche cannot be created, speciation cannot happen, except perhaps (!) for extreme cases like when species only differ in the time of the year when they reproduce.


By the way, the placental/marsupial split is now at around 125 million
years ago according to SVP presentation by Luo,Z., Wible, and Yuan.

"Placental-marsupial" split is misleading, because both clades are crown-groups; the actually involved sistergroups (with stem-based definitions) are Eutheria and Metatheria. That their split has about the age that was expected from longer-known fossils tells us nothing about when the crown-groups appeared, except that that didn't happen much earlier than 125 Ma ago. Indeed, fossils of Cretaceous placentals are still not known with certainty, and fossils of Cretaceous marsupials are not known at all.


BTW, some molecular divergence time estimates put the Metatheria-Eutheria split at a hundred and SEVENTY-five Ma ago, at the base of the Middle Jurassic. How's that? Fifty million years off. That's the difference between Messel and Rancho La Brea.
I think some other studies used that crazy date to _calibrate_ their trees. But perhaps I'm just getting paranoid. In other cases, however, it has repeatedly happened that people used others' molecular date estimates, stripped of their margins of error and without the slightest glance at morphology, to calibrate their own molecular dates.