Old beliefs proven wrong or Young and Old Oxynoticeras

Oxynoticeras simpsoni, 9 / 5 / 3.5 / 2.5 cm

Oxynoticeras simpsoni, 9 / 5 / 3.5 / 2.5 cm

You might have wondered why I called the Oxynoticeras in one of the last posts Oxynoticeras simpsoni and not Oxynoticeras oxynotum. I did actually have that one labeled as O. oxynotum originally, more or less assuming that this was the more common species and not looking for the differences. I had only the large Oxynoticeras where you could not overlook it’s typical O. simpsoni characteristics labeled as such. So when I looked through my collection, I actually found that I had only one potential O. oxynotum, all the other ones were O. simpsoni, just different growth stages !

This picture shows the O. cf. oxynotum , found in 1990, a lucky split, inner whorls as they could be prepped then (with a prep needle).

Oxynoticeras cf. oxynotum, 3.5 cm, 1990 style prep

Oxynoticeras cf. oxynotum, 3.5 cm, 1990 style prep

Today I gave it a little re-prep with the air abrader, just a 5 min job.
Oxynoticeras cf. oxynotum, 3.5 cm, re-prepped today

Oxynoticeras cf. oxynotum, 3.5 cm, re-prepped today

Here is a similar sized Oxynoticeras simpsoni :
Oxynoticeras simpsoni, 3.5 cm

Oxynoticeras simpsoni, 3.5 cm

So here are the main differences between the species : O. oxynotum has a smaller umbilicus at larger sizes, the whorls do not get as thick, the whorl section is more oval with a sharp keel,
there is a bit of a soft wave-like ribbing that almost vanishes at the point where the ribs bend forward towards the keel,  while O. simpsoni has a larger umbilicus, thicker whorls, and the whorl section thins out towards the venter, making it more lanceolate. O. simpsoni has very fine ribbing that continues towards the keel and can produce a crenelated keel. In comparable beds around the world, Oxynoticeras seems to be more common without shell, e.g. pyritized. I wonder if the authors describing the different species ever saw them with shell ?
At small sizes, with the shell on, they´re almost identical.
This is a “classical” larger Oxynoticeras simpsoni, 9 cm, an inner whorl of a larger, eroded specimen, the largest I have with almost complete shell on :
Oxynoticeras simpsoni, 9 cm

Oxynoticeras simpsoni, 9 cm

Both O. oxynotum and O. simpsoni apparently get very large : HOWARTH 2002 describes specimen between 25 and 40 cm !
As usual: If you do have a large specimen like that, let me know !
I have a small Oxynoticeras specimen with a crenelated keel – SCHLEGELMILCH lists this as a characteristic of another 2-3 cm small species O. bucki, which HOWARTH 2002 later lists as a synonym of O. simpsoni – I tend to agree with that, allthough since this one has its complete shell preserved, the other characteristic of O. bucki, as SCHLEGELMILCH describes it – a very simple suture – cannot be seen.
Oxynoticeras "bucki", 2 cm

Oxynoticeras “bucki”, 2 cm

AndyS

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