Rare and re-bedded

Phymatoceras rude, 1.4 cm

Phymatoceras rude, 1.4 cm

This wee little 1.4 cm diameter ammonite that came off an eBay auction is a big rarity both in terms of species and in terms of where it was found : Port Mulgrave.

The species isthe ultra-rare Phymatoceras rude (hurray, another species from my “wants list” down !), and it usually comes from the striatulum subzone of the upper lias, the Peak Mudstone member at Ravenscar, where the beds were protected by the downthrow of the Peak Fault from the erosion that everywhere else at the end of the upper toarcian eroded a lot of the uppermost toarcian.
Now wait, you will say, if these beds were eroded everywhere else, then how come this ammonite can be found at Port Mulgrave ?
There are two possible theories (and I think I can trust the data given by the seller, who wasn´t even selling it as a Phymatoceras, so that excludes the third option that the seller tried to make a bit of extra money by selling a rare ammonite that comes from somewhere else…) :
  1. When you look closely at the ammonite, you see a rusty brown sandy substance in the umbilicus.
    I think this is middle jurassic sandstone, and that the ammonite was naturally eroded from the striatulum subzone and then re-bedded when the aalenian sandstone was sedimented. I have seen small badly eroded fragments of ammonites in these sandstones before, but never a recognizable ammonite.
    I think somebody even told me about a nautilus from the sandstones as well…
  2. Somebody found this ammonite at Ravenscar and lost it at Port Mulgrave. Much less interesting…
I like theory 1 best, what do you think ?
AndyS
P.S.: If you are the seller of this ammonite : I know you might think that the sales price was a little low now that you´ve read this.
Console yourself with the tought that you´ve earned yourself a copy of the book once it´s ready !
Edit 09.08.2012 :
I’ve updated this post with a new set of pictures of the Phymatoceras and during our holiday in Yorkshire these last 2 weeks I’ve made of few pictures “on location” that provide some evidence for my theory number 1 :
A large aalenian sandstone block with a re-bedded layer of toarcian pebbles, including what looks like a rounded fragment of an upper toarcian Grammoceras !
I’ve looked at many of these blocks during my last collection trip and almost every one of them showed some ammonite fragments !
Aalenian sandstone block with reworked pebble layer, the tip of the hammer pointing to a Grammoceras fragment

Aalenian sandstone block with reworked pebble layer, the tip of the hammer pointing to a Grammoceras fragment

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2 Comments

  1. Trevor

     /  December 4, 2012

    Andy…just found this blog…..absolute brahma…

    Reply

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