This misidentification does not happen only to sellers on eBay, in fact Zugodactylites was not known to occur in Yorkshire until the late 1960s (HOWARTH 1978) – I´m sure someone must have found it before ! HOWARTH in his “The Liassic Ammonite Zones and Subzones of the North-West European Province” in 1961 still lists it to be found in Britain exclusively in Northamptonshire.

In my collection I did actually recognize it to be something different when I saw the ammonites in the drawer side by side with other Dactylioceratids, and did see the picture in HOWARTH 1961 pl. 73.1 as the closest match I could find. HOWARTHs 1978 paper about “The stratigraphy and ammonite fauna of the Upper Lias of Northamptonshire”, which I got somewhat later, then “officially” confirmed it´s occurrence in Yorkshire.

The ammonite is much rarer than the Peronoceras it is usually associated with, using HOWARTHs numbers by a factor of more than 10. Once you´ve found one, you´ll always recognize it again : I always look for the inner whorls, if you see very sharp, pronounced ribs, with tiny spines where the whorl meets the umbilical wall,  it´s most likely to be a Zugodactylites. The preservation and “behaviour” when being prepped can also give a clue : I´ve found that with Zugodactylites the “lid” on the inner whorls “pops out” very easily and usually most perfectly. Whether this has to do with the matrix it usually is preserved in or it´s shell structure I do not know…

Other characteristica include it´s flatter whorl section, the fine spines at the outer edge of the whorl and it´s pronounced constriction at the aperture.

Edit 17.09.2012 : Added a detail pictures of spines & constriction at aperture

While looking through the Zugodactylites in one of my drawers and comparing them it suddenly dawned on me that there was another species there as well : A Zugodactylites rotundiventer.

  This one is rather large with 90 mm compared to the Northamptonshire ones that HOWARTH mentions – the early jurassic “Yorkshire” seas apparently fed the ammonites better ! As the name suggests, it has a more rounded venter and the whorl is definitely thicker when looking at specimen of same sizes.

There´s potentially another species in that drawer : A Zugodactylites thompsoni.

It is half an ammonite that I picked up at Ravenscar, nicely showing the wider than high inner whorl.

As usual: If you have a better specimen, please let me know (+ let me borrow and photograph it !)

AndyS