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Looking for patterns in extinctions.
As the recent "no bolides" discussions have shown, looking for
patterns in K/T extinctions can be a fruitless and exhaustingly
complex search. I believe there is a pattern, a very simple
pattern. But before I reveal all, I would like to criticize
1. The bolide hit and kicked up photosynthesis-blocking dust.
This caused the "75% of all species" extinction event and
selectively targeted everything over 25kg. The "75%" claim has
been effectively countered. Keller et al _are_ persuasive when
they say that marine extinctions were gradual, i.e., many of the
species were in decline and were "rare", comprising a mere ~20%
of relative abundance of the planktic foraminifera at the K/T.
The non-K/T extinctions of the rudists, the pre-K/T waning of the
ammonites and belemnites, all of these things call the surgical
bolide strike into serious question. This seemingly
instantaneous event is stretched to a larger time scale. At some
point even the term "mass extinction" becomes inapplicable.
Whether or not this clustering of extinctions is correlated with
anything is arguable. Couldn't I make the argument that the
extinctions are random, like a blind man shooting at the side of
a barn, if he shoots enough bullets, will have some clusters.
But the greatest extinction of them all, the dinosaurs, lures us
with its own catastrophic grandeur into notions of connectedness.
And we confuse correlation with causation. So when we decouple
dinosaur extinctions from marine extinctions, the evidence for
common causation disappears. Indeed, there is no evidence of
_any_ kind indicating the cause of dinosaur extinctions.
But there are patterns from which we may infer causes. Raup
(I think) claims the revealing pattern is that everything over
25kg. was killed. From this, he believes, we can infer that big
animals were selectively targeted by loss of food (caused by the
anti-photosynthesis cloud). But many things _smaller_ than 25kg.
died too (including dinosaurs). In addition, when one considers
that the overwhelming number of creatures over 25kg. _were_ the
dinosaurs, the claim becomes tautological. The point is that
there is no pattern here, only the _fact_ of dinosaur extinction.
2. Then there is the school which throws its collective hands in
the air and says: "We'll never know the cause of the extinctions
because they are too numerous and complex." Indeed, many changes
were going on over the Cretaceous and especially at the end of
the Cretaceous. But these changes - climate, vulcanism, perhaps
multiple bolide strikes (Hallam, Science vol 238, talks about
multiple iridium spikes but doubts bolides were their source)
merging continents, regressing oceans - these changes were going
on for _all_ species. Why did the dinosaurs succumb taxon-wide
to these pressures when other taxons didn't? Perhaps this school
is right. Maybe we'll never know what incredibly complex series
of interconnected factors did them in. But I believe that none
of the above selectively targets the dinosaurs. They were,
according to Gregory S. Paul in Dinosaur Eggs and Babies, r
selected. Listen to this: "The survival of the birds after the
Cretaceous is important because they may hold a clue to the ways
dinosaurs might have been able to resist extinction. Most birds
lay fewer eggs than dinosaurs and are apparently more sensitive
to environmental change than were dinosaurs. It is therefore
difficult to understand how birds could have survived (rapid
environmental change) but not the dinosaurs. If just a few
dinosaurs survived...their rapid reproduction and genetic
flexibility should have allowed them to reestablish and adapt to
a dramatically altered world. These surviving dinosaurs would
have formed the basis of a new dinosaur radiation. That this did
not happen is an unsolved puzzle."
The time period invoked for these changes in Bakker and
others is, in my view, plenty long enough for these
evolutionarily malleable creatures to find new ways to cope.
After all, Darwin's finches evolved 14 species in less than 5
million years. What rate of environmental change over what
stretch of time is fast enough to account for the putative cause
of the loss of dinosaur diversity? No, there is no causation in
this line of evidence for the dinosaurs demise.
3. Well, anyway, here is a pattern which explains survivors as
well as losers. Things that survived the K/T were stealthy egg
layers (birds, snakes, turtles, and lizards), non-egg layers
(most mammals), and crocodiles (niche sovereigns). Things that
didn't make it were non-stealthy egg-layers, i.e., the dinosaurs.
Laying eggs out in the open, in light of the ever-increasing
egg-predation ambience of the Cretaceous, was a defunct strategy.
For the dinosaurs, genetically malleable as they were, this was a
fixed liability. They could neither hide it, nor move it, nor
protect it effectively from the growing cadre of creatures that
would eat it and the dinosaur hatchlings. I'm talking about the
mammals and the birds, both of whose diversification was perhaps
coming to a sort of functional branch-point at the end of the
Cretaceous. Birds were essentially modern in their flying
ability. Mammals had achieved that zenith in reproductive
security - placental birth. They were also perfecting critical
adaptations in sensory systems, locomotion, and dentition. These
new agents amplified the already persistent attacks from perhaps
the lizards (varanids), snakes, the cursorial crocodiles, and
their own worst enemy - other dinosaurs.
It is a theory which explains alot. But it has one problem:
no evidence! Not to worry, there is no evidence of any other
cause either. The iridium layer is merely phenomenon. Even if
it indicates an extra-terrestrial visitation, this is not a
cause, only an event! The scientific literature is full of
instances where hard physical evidence is misinterpreted and
extrapolated out of reason. The crowing of people like Gould and
Raup is premature and, for scientists, unseemly. We do _not_
know what killed the dinosaurs.
The question then becomes: Which is the most feasible
hypothesis, the one which requires the least propping up. The
non-stealthy egg theory is parsimonious in that it depends on
well known ecological principles. The pattern makes sense. It
is, in fact, the _only_ comprehendible pattern to be observed in
the extinctions around the K/T.