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| Apoderoceras / Stonebarrow Fossils |
This beauty is a personal fav of mine and you can see why!
These elegant ammonites were masters of sexual dimorphism. The macroconchs — females — could grow to impressive diameters exceeding 40 cm, while the microconchs, the males, remained considerably more modest in size.
One can't help but imagine the males arriving fashionably late to the Pliensbachian party, only to discover their dates had turned up looking like magnificent spiral battleships.
This lovely specimen was collected from the beaches of Charmouth in West Dorset, though Apoderoceras has been found across much of Europe, as well as Argentina and beyond. During the Early Jurassic, around 190.8 million years ago, these ammonites suddenly swept into the seas of northwestern Europe with all the subtlety of an unexpected house guest who somehow ends up running the place.
In fact, Apoderoceras appears in the fossil record at the very beginning of the Pliensbachian and quickly comes to dominate the local ammonite faunas.
The puzzling bit? No one is entirely sure where it came from. It bears a passing resemblance to the earlier Eteoderoceras, but the similarities seem to be only skin deep. Its ancestry remains wonderfully mysterious.
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| Cat's Paw Suture Walls of Apoderoceras |
But correlating ammonite timelines between the Pacific and northwestern Europe is rather like trying to synchronise two watches that have spent 190 million years at the bottom of the sea.
The clues remain tantalisingly out of reach.
As the Taylori Subzone progressed, Apoderoceras evolved locally. The shells became more inflated, the ribs more widely spaced, and the macroconchs appear to have grown ever larger.
The ladies, it seems, embraced the philosophy that bigger was better, their more spacious body chambers likely offering advantages for egg production.
Then, as abruptly as they had arrived, Apoderoceras vanished from northwestern Europe. The ammonite cast changed, and other lineages stepped into the spotlight. No dramatic farewell. No lingering encore. One day they were the dominant stars of the Jurassic seas; the next, they had exited stage left.
The Taylori Subzone was not entirely an Apoderoceras production, of course. Sharing these ancient waters were the streamlined oxycones Radstockiceras and the exceedingly rare Oxynoticeras, along with Phricoderoceras, Tetraspidoceras, and the enigmatic Bifericeras donovani. It was a remarkably diverse marine community, each species carving out its own ecological niche beneath Jurassic waves.
The beautiful macroconch pictured here was almost certainly female. Her larger shell was not simply a matter of style but of biology — exquisitely adapted for producing the next generation of these curious cephalopods.
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. This specimen was found on the beaches of Charmouth in West Dorset.
Neither Apoderoceras 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’ donovani 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.
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. 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 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 (though erroneously) 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 there are known examples from Northern Ireland) by the oxycones Radstockiceras (quite common) and Oxynoticeras (very rare), the late Schlotheimid, Phricoderoceras (uncommon)
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 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, Tetraspidoceras (very rare). The lovely large specimen (macroconch) of Apoderoceras pictured here is a female — prepped beautifully by Stonebarrow Fossils — her larger body perfected for egg production.
