In May this year, Byron messaged me a picture of an ammonite which had been found
by Ricky at Ravenscar (presumably the side towards Robin Hoods Bay, i.e. Wine Haven),
asking me for an identification.
The small ammonite has a diameter of exactly 40 mm and is preserved as a somewhat
pyritic internal mould on a small block of shelly matrix.
Significantly, this matrix also contains pieces of Pinna bivalves which is usually indicative
of the jamesoni zone, taylori subzone, of the lower lias – so no guessing necessary for
identifying the stratigraphy, which can be a big bonus when identifying ex-situ ammonites
from the Yorkshire coast.
At that point I was prepping another small specimen from this zone with an Apoderoceras
so I figured it had to be the same thing and identified it as such…
A few days later Byron put this ammonite up for sale and (you know me) I could not resist….
The ammonite was picked up from Byron´s shop in Whitby this summer and went home
with me. Usually, ready-for-drawer ammonites like this one queue up on my desk for final
identification and a label.
A few months later, until this weekend in fact, it was still sitting on my desk for
identification…It had turned out that identification was not as easy as I had hoped,
it was not an Apoderoceras, no nodes on the thin ribs, and from the small remaining
bits of shell it looks like the ribs would barely be visible under the shell…
The ammonite at 40 mm diameter is a mostly complete adult, as the crowding of the sutures
suggests, and has about half a whorl of body chamber preserved.
I had identified a possible match with Epideroceras sociale, especially with the style of
ribbing on the inner whorls, but since Epideroceras is getting much larger and develops
much thicker whorls when adult, this match fell through…
Having about half an hour to kill before lunch would be ready this Sunday, I was
browsing Howarth´s 1962 “The Yorkshire Type Ammonites and Nautiloids of YOUNG
and BIRD, PHILLIPS, and Martin SIMPSON”, looking for something entirely different
and stumbled across the pictures of Polymorphites rutilans (SIMPSON, 1843)
(now Gemmellaroceras rutilans) on table 15 – a perfect match with the crenelated keel
of my ammonite.
I had not had Gemmellaroceras on my list assuming they would only be much smaller
(like Gemmellaroceras tubellum), but reading HOWARTH´s
“The Lower Lias of Robin Hood´s Bay, Yorkshire, and the work of Leslie Bairstow” again,
found that this is indeed the larger species and can be found,
(how blind can one sometimes be…) explicitly mentioned, in Bairstow bed 530,
associated with Pinna folium…
There is another subgenus which is apparently similar, just stratigraphically slightly
older – Leptonotoceras – in which the lateral lobe of the suture is only split twice
(bifid) instead of three times (trifid) as in Gemmellaroceras – the lateral lobe is trifid
in this ammonite so we´re also clear about this.
So that´s about enough to round up this identification, and it´s also a species that can be
scratched off my “Wanted” list – done !
Now let´s see if I have the smaller species of this genus, G. tubellum, hiding in my collection
* Gaetano Giorgio Gemmellaro (1832-1904) was an italian paleaontologist and geologist,
founder of the Palermo Geological museum and researcher, among many other topics,
on sicilian pygme elephants. Alpheus Hyatt named the genus after him in 1900, presumably
to reciprocate for an honour which Gemmellaro had bestowed upon him in 1887 by naming
a permian ammonite genus (Hyattoceras) after him…