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 !
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