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Re: Why non-avian dinosaurs weren't able to survive
> On 11/24/2015 12:59 AM, frank bliss wrote:
> > I’m still having trouble understanding why there aren’t massive beds
> > of dinosaurian bones at the top of the K sediments. No such thing
> > here on my ranch. I see fewer and fewer bone sites as I get closer to
> > the top
> Easy answer. Every spot in the strata where you have bones is a result of a
> statistical probability of fossilization from before and after
> a specific time. As you aproach the top, especially if it is a sudden die
> off, you lose the source of half your probably candidates for any
> exact spot... and so you get the illusion of thinning population.
>The Signor-Lipps effect
This is incorrect. In the event of a sudden die-off then you would not see any
decrease in population before ground zero. This is part of the reason why some
people believe that the decrease in dinosaur fossils in the upper 3 m of the
Hell Creek (etc) is representative of a gradual extinction over the time
represented by that 3 m (almost certainly less than 30ky).
The 3 m gap is real; albeit it is not a true "gap" as you do see some fossils
in that 3 m but they are undoubtedly much more rare than in the preceding 25 m
(and more). I have some discussion as to why this might be the case in my
dissertation. I do not believe it is a signal of gradual extinction.
>> Additionally, there was a great physical destruction. Not much
>> sediment.>Exactly: you only get fossils in a depositional environment. The
>> effects of the Chicxulub impact didn't increase rates of sedimentation
>> (well, except for tsunami deposits along the Gulf...), so you don't get any
>> more chance of fossilization than other times.
>Terrestrial deposition is very episodic and spotty: it isn't like deep sea or
>lake sedimentation. You only get overbank deposits when the river floods; you
>only get channel deposits when the channel is RIGHT at that spot, etc.
The uppermost Hell Creek (towards the K-Pg) is actually a period of high
accommodation and significant deposition. The reason why the iridium layer &
various ashes are preserved at the K-Pg (and a few metres above and below) is
because there is deposition occurring. There is little evidence for any
significant hiatus through the K-Pg in the Hell Creek.
However, there is evidence for a hiatus in southern units, in for example Utah,
New Mexico, and Texas. I am not suggesting that the hiatus in the south is due
to effects of a tsunami or suchlike, just that a hiatus probably (I would say
almost certainly) exists.
Fowler Paleontology & Geology Consulting