Gagaticeras or “…ribs like rows of jet beads”

Gagaticeras cf. finitimum, 5 cm

Gagaticeras cf. finitimum, 5 cm

Gagaticeras in my mind somehow is more of a coincidental find, casually picked up on the way somewhere else – which of course does not do this fine ammonite genus any justice.Around Robin Hoods Bay it has been reasonably common in the last couple of years (though I´ve noticed a drop in the last 2-3 years), I´ve mostly found them almost eroded free or in nodules that were thrown onto the beach during storms, the beds they occur in are very often sanded over – since most of the coast is a SSSI you´re not really supposed to do any large-scale digging in them anyway.

As you start collecting Gagaticeras ammonites, at first sight they mostly look the same (especially in the field), you put them in the drawer as “Gagaticeras gagateum”.
You collect some more, clean them up as good as you can (they´re not easy to prep well because of their delicate inner whorls), put them in the drawer.

After a while, when looking into the drawer at what you´ve accumulated over the years, you begin to wonder and see little differences, a more pronounced keel here, rursiradiate ribbing there, differences in rib density etc.  A while ago after I acquired an air abrader, a re-preparation helped to work out some more details in the inner whorls.

When starting to document the Gagaticeras species for the book, I was really surprised (and pleased of course 🙂 )to see that I do actually have the four species that HOWARTH mentions in his Robin Hoods Bay / Bairstow collection paper !  I must admit though that it did take me some time to find the most characteristic specimen
for each species, there seem to be many intermediates (like the one pictured above, which has a very high rib density but almost no keel and no rib angle at the venter), which all the authors that have recently written about them (HOWARTH 2002, SCHLEGELMILCH 1976, GETTY 1973,…) have taken as a sign that intra-species variation may make it appear that there are more species than there really are – but that´s always the problem when you do not have a large enough collection to do statistical tests.

Here they are :

Gagaticeras gagateum (YOUNG & BIRD, 1828)

 

Gagaticeras gagateum, 3.5 cm, with aperture & keel view

Gagaticeras gagateum, 3.5 cm, with aperture & keel view

G. gagateum has a rib density of 20 – 24 ribs / whorl, but there is only the slightest hint of a keel on the venter, sometimes completely invisible.
Whorl section is more compressed than on the other species, i.e. the whorl is thicker than high.

Gagaticeras neglectum (SIMPSON, 1855)

 

Gagaticeras neglectum, 4 cm

Gagaticeras neglectum, 4 cm

Gagaticeras neglectum, 4 cm, keel view

Gagaticeras neglectum, 4 cm, keel view

G. neglectum has a rib density of about 20 ribs / whorl, but there is a (sometimes strong) keel on the venter. Whorl section compare to G. gagateum
is less depressed, almost round. The expression of the keel seems to be very variable.

Gagaticeras finitimum (BLAKE, 1876)

Gagaticeras finitimum, 3.7 cm, with aperture & keel view

Gagaticeras finitimum, 3.7 cm, with aperture & keel view

G. finitimum has the highest rib density (between 24 and 28 ribs per whorl) and the ribs meet at an angle at the venter.
Similarly to G. neglectum, there is a keel on the venter.

Gagaticeras exortum (SIMPSON, 1855)

Gagaticeras exortum, 3 cm, with keel with

Gagaticeras exortum, 3 cm, with keel with

G. exortum is the easiest to identify : Ribs are quite rursiradiate (leaning backwards), there is just a hint of a keel.
If my collection were considered representative, it would be the rarest species of the four. Rib density on the figured specimen is
23 ribs / whorl at 30 mm diameter, another specimen I´ve seen has 26 ribs/whorl at 50 mm diameter.

In case you´re wondering what this all has to do with jet beads ?
Well, “gagateus” is the greek name for jet and the citation in this post´s title is from YOUNG & BIRD´s original 1828 description of “Ammonites gagateus”
and simply refers to similarity of the black whorls with jet beads…

AndyS
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4 Comments

  1. christine good

     /  October 28, 2012

    in the top right hand corner of the top photo is a small turtle beak shaped object(i know its not that but only way i can describe it!) do you know what it is as i found one recently at Lyme Regis and would like to know it its something interesting. many thanks

    Reply
    • Christine,
      Turtle beak shaped objects in this specific type of rock tend to be bivalves.
      It is quite possible though that your fossil is something different, e.g. a nautilus jaw.
      If you send a picture via e-mail to andysfossils@kabelmail.de or post a PM to AndyS at the UK Fossils forum at discussfossils.com, I´ll be able to take a look and let you know what it could be.
      All the best,
      AndyS

      Reply
  2. Rick Ross

     /  January 25, 2013

    Hi Andy
    I am really enjoying your blog, from Canada, I have traded with a German/Canadian friend who used to collect in Yorkshire many years ago while still living in Germany. He traded me for several ammonites, 2 of which are Gagaticeras from Robinshood Bay. One of them has an encrusting serpulid which has caused the ammonite to deviate from its original coiling.
    I find these aberrant specimens fascinating, keep up the great blog with absolutely amazing photo’s and detailed background information.
    Rick Ross

    Reply
    • Rick,
      Many thanks for your kind comments !
      If your friends first name is Roger, then I know him through the UkFossils and Steinkern forums…
      All the best,
      Andy

      Reply

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