Transitions or A late “straight-fingered” survivor on an upper toarcian bed of belemnites ?

Dactylioceras (Orthodactylites) cf. semiannulatum, as found

Dactylioceras (Orthodactylites) cf. semiannulatum, as found

This 7 cm ammonite sits above (or below ? – I have found no indication like e.g. a fossilized level that would allow this decision) a bed of belemnites and was found
at Hawsker March 13, 2008. The matrix around the ammonite was full of brownish-black glistening fragments of what I assume are belemnite hooks.
I seem to remember that the ammonite was not visible initially, and that I split the rock to make it smaller for the sole purpose of grinding and polishing the beautiful other side of the rock displaying the belemnite sections – the entry in my little red book seems to corroborate that – I found very little else on that day and must have been desperate 😉
It shows again you sometimes need to look in unusual places to find something special…

Block with belemnites from other side

Block with belemnites from other side

 

This matrix almost shouts “falcifer” zone, more specifically this could be from a belemnite accumulation usually associated with the ovatum band of the upper falciferum subzone, although this is only an educated guess, since the matrix block was found ex situ and could also come from a slightly lower bed. It´s composition and appearance, however, matches very well with one given in the paper : DOYLE, MACDONALD, 1993: Belemnite battlefields.

Dactylioceras (Orthodactylites) cf. semiannulatum on belemnite block

Dactylioceras (Orthodactylites) cf. semiannulatum on belemnite block

 

What makes this ammonite interesting is that it has some of the characteristics of the Orthodactylites (literally translated from the latin as “straight-fingered”) subgenus of Dactylioceras that I described from the lower toarcian in an earlier post (link) which predominantly have straight, single, usually non-bifurcating ribs, including the classical preservation with “capped” ribs that have a kind of predetermined breaking point, as the outer shell stayed in the negative and took the top of the ribs with it.

 

Howarth described a similar type of ammonite as Dactylioceras (Orthodactylites) semiannulatum in his 1978 paper “The stratigraphy and ammonite fauna of the Upper lias of Northamptonshire”.   It is a late survivor of the Orthodactylites subgenus, in later beds replaced by ammonites with mostly bifurcating ribs of the Dactylioceras genus. I´m hesitant to attribute this specimen to this species, however, since the measurements – apart from the whorl width, which is much smaller – would rather point to Nodicoeloceras. Howarth´s D. semiannulatum specimen were also a bit smaller, though.

There is one other option, which is Dactylioceras consimile (BUCKMAN), of which I´ve got conflicting descriptions/pictures – I need to take a look at one of these in a museum.

 

The conundrum presenting itself of course is a pattern that applies to most of the Dactylioceratidae, especially the lesser known species :

  • You can’t always rely on morphology alone, you need to know the bed from which the ammonite originates to verify the species
    and this can be surprisingly difficult when you mostly collect from the cliff debris, i.e. ex situ
  • There are transitions between the different species, as this probably is. And mutations, pathologies…
  • There is always variation within a species that can not be fully recognized when you look at a small number of specimen.
  • Early descriptions  (BUCKMAN, WRIGHT, SIMPSON…) often relied on single specimen – see above.

 

No clear solution this time, then – I will however label this one, to point out my conviction that this ammonite is closer to D. semiannulatum than to Nodicoeloceras :

Dactylioceras (Orthodactylites) cf. semiannulatum HOWARTH.

 

As promised earlier, the next couple of posts (too much for just one !) will predominantly deal with the upper toarcian Dactylioceratidae – hopefully with less undefined identifications like this one – sorry you had to wait until after the 50th post (which is this one, hurrah !)  – thanks for bearing with me for so long !

 

AndyS

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2 Comments

  1. Mark varah

     /  October 7, 2013

    Hi Andy
    Felt like commenting a few times before on your excellent blog, mainly to encourage you to look beyond the Yorkshire Lias for certain genera!!
    I agree with where you are placing this ammonite on the ID spectrum, and the many cautions on ID from past authors!
    An ammonite like this is found in the ilminster Toarcian in the falciferum zone alongside ammonites distinct from it and closer to Nodicoeloceras.
    Regards Mark

    Reply
    • Mark,
      Thanks for your kind comments.
      I did notice that BUCKMAN´s type of D. consimile (as Peridactylites consimilis) is from the Barrington / Ilminster area as well and read up about these exposures.
      My feeling (and I mustr say I´ve never been there) is that a lot what you see on the market today from this area are finds from ploughed fields
      (you can see the lighter coloured areas on google earth) where the exact bed cannot be safely identified, the Barrington Limestone member also being a
      highly condensed facies, which does not help in this respect. I do have a few (bought) reference examples of ammonites from this area in my collection, and it
      is also sometimes interesting to see how the different preservation changes the appearance of an ammonite – maybe a topic for a future blog post ?

      All the best,

      AndyS

      Reply

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