[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Declining pterosaur diversity
John Bois (email@example.com) wrote:
<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.
...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.)>
This is rather an over generalization. The authors proposed FIVE models,
of which only the fifth conforms to John Bois' theories. The authors
discussed this model:
Finally, the fifth model (Figure 1e) is similar to Figure 1d except
some current crown groups arose during the Late Cretaceous. Although it
is an interesting taxonomic question as to whether a fossil is a member
of a crown group, it is the ecological or evolutionary question that is
more important here. An animal near the end of the Cretaceous that is
about to be caught and eaten by a carnivore is not concerned with
whether the predator will be considered, 65Cmillion years later, as a
member of the Carnivora crown group. Alroy  has established a
database of North American mammalian fossils, and the timing of events
in his data could argue against Figure 1e with respect to mammals in
North America; however, the model depicted in Figure 1e needs to be
evaluated both in other regions and for birds."
Why does half the paragraph have to be about crown groups and wether or
not we can refer fossils to them? The authors' discussion leads one to a
possible refutation, rather than material support, in the form of Alroy
Alroy, J. 1998. Cope?s rule and the dynamics of bodymass evolution in
North American fossil mammals. _Science_ 280: 731?734.
<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.,
Indeed, the authors state:
"The main limitation of the results in Figure 3 could come from
differences between fossil sites in their potential for fossilization.
Such an ?ascertainment bias? can arise in addition to normal stochastic
effects (including local fossilization conditions). Larger dinosaurs
more likely to leave fossils, and to be discovered, studied and
published. Nevertheless, the trend in Figure 3 requires explanation and
is presented as a stimulus to further research."
So what is indicated is the reduction in DISCOVERY or COLLECTION of
small dinosaurs from these time periods. In sheer number of fossils, large
tyrannosaurs and ceratopsians tend to fill the specimen counts because of
the very bias in preservation for size in the Campanian, Nemegt _or_
Dinosaur Provincial Park. Small taxa, like the oviraptorosaurs and
dromaeosaurs, which were likely to have been as abundant, if not more so,
just as much as wolves outnumber bears in Canada, do not preserve as well,
and there is not a complete small theropod from Dinosaur Provincial Park,
even though there are complete 4m+ carnivores and herbivores. The same
sediments don't correspond to that many other predators, such as possible
conflicts with crocs, since their fossils, too, are rare.
<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.>
Based on recovered fossils, for the most part, given the recovery of
basal taxa *Sinodelphys* (Metatheria, not Marsupialia, which is the crown
group and *Sinodelphys* is not a crown marsupial) and *Eomaia* (Eutheria,
and not a placental for the same reasons given for *Sinodelphys*), the
metatherian/euherian split was near or at just less recently than 124mya,
the dating for the Yixian Formation (more recently than the slightly older
Lujiatun beds of the same formation) simply because it cannot be
constrained or extended further. Note: recent work suggests human
mitochondria actually _accelerated_ their evolution since the *Homo*/*Pan*
split, yet more work showing that mitochondria are not the stable
time-keepers we take them for. Does this mean it's true? No, but it does
allow us to start THINKING, and not BELIEVING, in possible courses, even
if Penny and Phillips would like to think that inverting the questions
solves more problems than it creates.
For instance, the quote:
"?when did mammals and birds start replacing small dinosaurs and small
pterosaurs?? This replaces the question ?when did the last dinosaur, or
the last pterosaur, perish??"
These are in fact two different questions, and both can be answered with
two entirely different scenarios, or even the same one. Replacement
effects can occur in an ecological vacuum as easily as it can in a direct
competitive scenario. The authors present no data to show who was
replacing whom, only that dinosaurs were apparently declining (which could
not be demonstrated based on the given data, see above), as there are no
2m or 50-100kg mammals in the Paleocene immediately after the K/T event
(note: the authors use Tertiary, I think out of familiarity, and the term
"K/T event" or "K/T boundary" are immediately more recognizable than "K-Pg
boundary" and so forth ... just quibbling, David). Yet the authors support
the declining diversity they see as evidence for this replacement regime.
We need more evidence for this, not ghost competitors to explain a problem
with the extinction scenario.
Jaime A. Headden
Little steps are often the hardest to take. We are too used to making leaps
in the face of adversity, that a simple skip is so hard to do. We should all
learn to walk soft, walk small, see the world around us rather than zoom by it.
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
Do you Yahoo!?
Read only the mail you want - Yahoo! Mail SpamGuard.