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Re: Declining pterosaur diversity
----- Original Message -----
From: "John Bois" <firstname.lastname@example.org>
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.,
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
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:
(The word-monster Afroinsectiphilia includes Tubulidentata, Macroscelidea
and Afrosoricida; Supraprimates is the same as Euarchontoglires of other
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
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
- 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
- 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
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.