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Re: Archies in the Morrison?!



Jerry Harris wrote: 
:>      You read my mind!  I would _LOVE_ to find some nice lithographic 
:> limestones of Late Triassic/Early Jurassic age somewhere!  And,if they 
:> _don't_ exist anywhere, barring complete erosion, could anyone suggest a 
:> reason why they _don't_ occur in those time periods?  (Barring lack of 
:> exposure, too...)

John Schneiderman <dino@revelation.unomaha.edu> wrote:  
>If I understand my historic geology right...during the Late 
>Triassic/Early Jurassic most of the continents lacked epi-continental 
>seaways and off shore shelves were quite narrow and deep.  Also this time 
>was marked by a lot of rifting...great for evaporates but lousy for those 
>calm, shallow lagoonal deposits.  
>I suggest a better source for small vertebrates are the playa deposits 
>and fine grain mud deposits (mudflats---great for footprints).

  Yeh, forgot about the lower sea-level at that time.  Also, having a
single super-continent (Pangea) probably didn't help matters, either.
Early rifting would produce isolated inland lakes and playas, so those would be
the places to look.  The lakes (a good Recent anolog would be Lake Victoria
in Africa) would be so small and short-lived that their sediments may never
have been preserved in quantity.  They would be interspersed with the more
abundant playa deposits.  I am beginning to wonder if vert-fossiliferous
strata from the Late Triassic/Early Jurassic is as well-known as we are led
to believe.  Think about it:  The only reason the Solenhoffen limestone has
produced so much vertebrate material is because it is being quarried for
lithographic limestone for artists/printers...it is NOT because the
fossils are abundant there (they are definately not abundant).  How many
other unknown Late Triassic sites are waiting to be discovered, simply
because there is no economic incentive to quarry the rocks?.
  Chatterjee has got the right idea about concentrating on his Late Triassic
Dockum Formation in Texas; unfortunately, the stuff is mostly
current-deposited mudstone, and as has been pointed out in the Protoavis
discussion we had, the bones tend to get mish-mashed together with bones of
other animals. (Someone, can't remember who, referred to Protoavis as a
dino-lizard-croc accumulation: sort of a Frankenstein's monster of the
Triassic. 
All that is missing is the bolt sticking out of the neck).
 
  Well, I'm off to the boondocks for a few weeks.  In my absence, a friend
will collect my dino-mail for me, and hopefully, store it in a file I can
find when I get back.  I could unsubscribe, but then I would have to tell
all of you to delay reporting new discoveries until I get back.