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This is a little counterpoint to the DINO-Fest presentation:
"What the Fossil Record of Dinosaurs Tells Us"
By Peter Dodson (University of Pennsylvania)
What does the fossil record say about dinosaur extinction?
The K/T boundary is defined by a very thin boundary clay enriched
in Ir/siderophiles typical of an extraterrestrial body. Dino fossils
worldwide occur up to this boundary layer, then the density of dino
fossils drops to nothing, zilch, nada. Approximately half of the genera
of other living organisms of that time exhibit a similar pattern in
the fossil record around this K/T boundary clay. I'm not sure how much
clearer the fossil record can say "CATASTROPHE OCCURRED HERE".
The K/T boundary layer has been located and time correlated at sites
from all over the world. Ever since the Alvarez group identified the
Ir anomaly, a steady mountain of evidence has been accumulating that
there was at least one very large bolide impact at the K/T boundary
65 million years ago. Furthermore, a large(>200 km) impact structure
has been identified at Chicxulub, Mexico that dates to exactly 65 million
years ago. This crater is probably the smoking gun to the K/T catastrophe.
Fossil remains of dinosaurs occur worldwide in deposits from the
Triassic until the end of the Cretaceous. The density of fossils
varies greatly throughout the column. I'm very skeptical regarding
claims that the dinos were "dying out" before the end of the Cretaceous.
I think its plausible that they were in tough times due to the climate
and sea level fluctuations of the late Maastrictian, but the dinos had
surely survived many episodes of similar environmental stress over the
previous 150 million years. The gradualists say that dino fossils become
thinner as you approach the K/T. Lets suppose for a minute this is true,
(even though there has not been enough sampling done to confidently
state this). Now go back through the record and notice how many times
you can find a "thinning" of dino fossils over the past 150 million
years. There are lots of them, some worse than the Maastrictian. The
dinos didn't die *then* so why is a "thinning" enough to explain a
total extinction at the K/T? Now apply this fossil-density analysis to
the 3 million year period *spanning* the K/T. Right in the middle, the
density plummets to 0 and stays there for the next 65 million years.
The gradualists see the pre-K/T "thinning" as the significant data,
while the impact theorists notice the sudden drop-off to nothing in
dino fossil density and it's association with the Ir/clay layer.
The real coup de grace of the dinos correlates all to well with the
K/T boundary(and associated bolide impact) to be merely a coincidence.
There may have been a very heavy death toll in the first week after
the impact. The sun would have been effectively blocked by a dust cloud
for months, curtailing photosynthesis. The herbivores starved, the
predators followed. The smallest and most intelligent dinos had the
best chance of survival, but eventually went extinct like their larger
cousins. An early Tertiary raptor would not come as a big surprise, but
a Tertiary Edmontosaurus would. Competetion from mammal faunas may have
been a factor for the last few surviving dinos.
Revisiting the book of paleontology with the latest astrobleme data can
lead to some interesting lines of research. Have Earth's lifeforms evolved
specific anatomy/growth cyles/hibernation/habits to enable them to survive
the bolide catastrophes? Or more accurately, have large bolide impacts
selected for organisms that by chance had catastrophe-proof features?
IMHO, our recent advances of knowledge in this area is like a geologic
and evolutionary rosetta stone, unlocking some old mysteries and opening
up some fascinating new ones.