Apoderoceras is, in fact, a wonderful example of sexual dimorphism within ammonites as the macroconch (putative female) shell grew to diameters in excess of 40 cm – many times larger than the diameters of the microconch (putative male) shell. Apoderoceras has been found in the Lower Jurassic of Argentina, Hungary, Italy, Portugal, and most of North-West and central Europe, including as this one is, the United Kingdom. She was found on the beaches of Charmouth in West Dorset, then prepped expertly by the lovely and talented Lizzie Hingley.
Neither Apoderoceras nor Bifericeras donovani are strictly index fossils for the Taylori sibzone, the index being Phricodoceras taylori. Note that Bifericeras is typical of the earlier Oxynotum Zone, and ‘Bifericeras’ donovani is doubtfully attributable to the genus.
The International Commission on Stratigraphy (ICS) has assigned the First Appearance Datum of genus Apoderocerasas and of Bifericeras donovani the defining biological marker for the start of the Pliensbachian Stage of the Jurassic, 190.8 ± 1.0 million years ago. As the brilliant Murray Edmunds points out, this lovely large specimen (macroconch) of Apoderoceras is likely a female. Her larger body perfected for egg production.
Apoderoceras (Family Coeloceratidae) appears ‘out of nowhere’ in the basal Pliensbachian and dominates the ammonite faunas of NW Europe. It is superficially similar to the earlier Eteoderoceras (Family Eoderoceratidae, of the Raricostatum Zone), but on close inspection can be seen to be quite different. It is therefore an ‘invader’ and its ancestry is cryptic.
The Pacific ammonite Andicoeloceras, known from Chile, appears quite closely related and may be ancestral, but the time correlation of Pacific and NW European ammonite faunas is challenging. Even if Andicoeloceras is ancestral to Apoderoceras, no other preceding ammonites attributable to Coeloceratidae are known. (Maybe there are clues in the Lias of Canada?) Apoderoceras remains present in NW Europe throughout the Taylori Subzone, showing endemic evolution.
It becomes progressively more inflated during this interval of time, the adult ribs more distant, and there is evidence that the diameter of the macroconch evolved to become larger. At the end of the Taylori Subzone, Apoderoceras disappeared as suddenly as it appeared in the region, and ammonite faunas of the remaining Jamesoni Zone are dominated by the Platypleuroceras–Uptonia lineage, generally assigned (but erroneously, IMO!) to the Family Polymorphitidae.
In the NW European Taylori Subzone, Apoderoceras is accompanied (as well as by the Eoderoceratid, B. donovani, which is only documented from the Yorkshire coast, although I know of examples from Northern Ireland) by the oxycones Radstockiceras (quite common) and Oxynoticeras (very rare), the late Schlotheimid, Phricoderoceras (uncommon: note P. taylori is a microconch, and P. lamellosum the macroconch), and the Eoderoceratid, Tetraspidoceras (very rare).
Thank you to Murray Edmunds for his advice, guidance and corrections as we explore Apoderoceras and the ammonite faunas of the Pacific and NW Europe. You are deeply awesome, my friend!
Check out Murray’s Research Gate site for more interesting tidbits!