Side by side or Vive la différence !

Somebody recently asked me about the difference between Aegoceras(Androgynoceras) and Gagaticeras, two ammonite species frequently found around Robin Hoods Bay.
This is what I´ve come up with :

Gagaticeras (left) and Aegoceras(Androgynoceras) (right) side by side

 The ammonites shown are approximately same sized, about 3.5 cm in diameter.
  • Whorl growth more rapid on A.(Androgynoceras), fewer whorls for same size
  • While their whorl section is similar in small sizes (round) , due to more rapid growth in whorl height,  the A.(Androgynoceras) has a more rectangular whorl section at greater sizes.
  • Maximum size for Gagaticeras is about 2″ / 5 cm; A.(Androgynoceras) can grow to more than double the size
  • With most species of Gagaticeras you have at least the hint of a keel, A.(Androgynoceras) has none
  • In terms of preservation, the black shell of Gagaticeras(Gagat is the german name for jet, hence the name ?) is a giveaway, as are small associated gastropods like shown in
    the aperture of the Gagaticeras.
  • Gagaticeras occurs in nodules in softer dark silty shale, sometimes as pyritized outer whorls but very rarely as flattened 2D shells
  • A.(Androgynoceras) occurs in grey nodules with a higher limestone content, but can be found flattened in the shales as well, where nodule buildup did not occur
This is where a photo of them side by side really makes a difference : You can clearly see some of the characteristics that are described in words above (apart from the keel and aperture views.)
And this is where I hope the book will shine : In showing the differences !
Leave a comment


  1. Robert Baker

     /  June 2, 2012

    Don’t know if this is relevant, but I have been trying for some time to get hold of named examples of lower lias belemnites from Yorkshire and Lyme. Not much luck so far. If you have any to sell please contact me. Thanks.

    • Robert,
      I do not have any lower liassic belemnites, but I can pass your address on to someone who might.
      All the best,

  1. Measuring an ammonite or How fat is a “fat Dac” ? « Yorkshire Ammonites (and other fossils ) revisited

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