Peronoceras fibulatum ammonite fossil - Whitby, North Yorkshire

Ammonite fossil found near Whitby, North Yorkshire on the Jurassic coast

Fibulation (looped ribs) combined with spines is usually a particular feature of most Peronoceras species; although fibulation is usually absent in P.perarmatum. In Yorkshire there are the more compressed (slender) finer-ribbed P. turriculatum and P. fibulatum (the former having less looped ribs on inner whorls), and then the depressed (thicker) coarser-ribbed P. subarmatum and P.permaratum (the latter usually possessing no looped ribs). To add to the confusion there are also many intermediary forms.  On all these species the ribs bend forward over the top, but more notably on the 2 former more slender species.  Spines are usually much more robust on the thicker 2 latter species.

Actual ammonite size: 6.7cm 

Age (lithology): Upper Lias; Toarcian; Bifrons zone; Fibulatum subzone.

Distribution: Staithes, Port Mulgrave, Runswick, Kettleness, Sandsend, Hawsker, Ravenscar.

Occurrence: Common (but much less so P.permaratum)