|Apodoceras / Stonebarrow Fossils|
is a wonderful example of sexual dimorphism within ammonites as the macroconch (female) shell grew to diameters in excess of 40 cm – many times larger than the diameters of the microconch (male) shell.
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. This specimen was found on the beaches of Charmouth in West Dorset.
nor Bifericeras donovani
are strictly index fossils for the Taylori
subzone, the index being Phricodoceras taylori
. Note that Bifericeras
is typical of the earlier Oxynotum Zone, and ‘Bifericeras
is doubtfully attributable to the genus. The International Commission on Stratigraphy (ICS) has assigned the First Appearance Datum of genus Apoderoceras
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.
, 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. We may yet find 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
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 (though erroneously) to the Family Polymorphitidae.
In the NW European Taylori
is accompanied (as well as by the Eoderoceratid, B. donovani, which is only documented from the Yorkshire coast, although there are known 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
(very rare). The lovely large specimen (macroconch) of Apoderoceras
pictured here is likely a female. Her larger body perfected for egg production.