The result of my between-the-years prep project :
A complete Peronoceras turriculatum, 9cm, and Peronoceras subarmatum, 6.5 cm from an Alum Shale nodule, Hawsker Bottoms

At the top of the commune subzone, the spiny members of the Dactylioceratidae family developed, probably from an intermediate form between Peronoceras turriculatum and Dactylioceras athleticum. Over the years there have been some differences of opinion under which genus (Peronoceras, Porpoceras, Catacoeloceras) the different species have to be placed,
I’m here following HOWARTH’s 1978 classification which he has re-iterated in the 2013 Treatise of Invertebrate Palaeontlogy #57, Part L, Revised, Volume 3B, Chapter 4:”Psiloceratoidea, Eodoceratoidea, Hildoceratoidea” and will describe the following species under the genus Peronoceras in this post :

Peronoceras fibulatum (SOWERBY, 1823)
Peronoceras turriculatum (SIMPSON, 1855)
Peronoceras subarmatum (YOUNG & BIRD, 1822)
Peronoceras perarmatum (YOUNG & BIRD, 1822)

What unites these species is their stratigraphical range (lower part of fibulatum subzone, Whitby beds 60-63 of the lower toarcian)), and their principal style of ribbing (fibulation – ribs pairwise looped together, forming a tubercle at the end, see graphic below), so HOWARTH placed them into one genus instead of dividing them into different genera.

Fibulation ribbing pattern

Genera outside this stratigraphical range (Porpoceras – upper part of the fibulatium subzone, part of bed 72) and genera without or only very occasional fibulation (Catacoeloceras, Nodicoeloceras) will be described in later posts…

Peronoceras turriculatum (SIMPSON,1855)

Peronoceras turriculatum, 7 cm, with constriction at mouth border

P. turriculatum has very fine ribbing until approx. 3-4 cm. Ribs are sometimes looped together, but tubercles are very small or occur only occasionally.
On the outer whorl, nearly every primary rib carries a stong tubercle. The ribs cross the venter bending foward towards the aperture, almost at an angle.

Comparison of the venter of Peronoceras turriculatum (top) and Dactylioceras cf. praepositum (bottom)

When I compared one of the ammonites from the previous post about Dactylioceras (link) – I had then called it D. cf. athleticum – with the P. turriculatum in the first picture at the top of this post, it occurred to me that there is just a small step, the addition of fibulation, to go from this ammonite to a P. turriculatum.

Peronoceras fibulatum (SOWERBY, 1823)

Peronoceras fibulatum, 6 cm

P. fibulatum has stronger ribbing on inner whorls and fibulation is rather the rule than the exception. Ribs are crossing the venter bending forwards, but in a less angled, more convex way than P. turriculatum.

Peronoceras subarmatum (YOUNG & BIRD, 1822)

Peronoceras subarmatum, 6.5 cm

P. subarmatum is a more depressed (thicker) ammonite, with strong tubercles and fibulation also on the inner whorls.

Spines of Peronoceras subarmatum

Most of the time it is difficult to preserve the full beauty of the tubercles above the internal mould, but when possible like in this specimen,
where the nodule surrounding the fossil was sufficiently weathered (I found it at Bay Ness, probably from glacial drift) to soften the otherwise hard matrix, it shows what a spiny ammonite this really was when alive…

Peronoceras perarmatum (YOUNG & BIRD, 1822)

Peronoceras perarmatum, 8 cm, slight pathology on the body chamber

P. perarmatum differs from P. subarmatum in having mostly wider spaced, single ribs on the inner whorls. It tends to have even thicker whorls and very strong tubercles.

The direct comparison in detail pictures shows the diagnostic differences in the pair  of compressed forms (P. turriculatum and P. fibulatum)
and the in the pair of more depressed forms (P. subarmatum and P. perarmatum) :

Comparison of the inner whorls between Peronoceras turriculatum (left) and Peronoceras fibulatum (right), width of view both about 5 cm.

Comparison of the inner whorls between Peronoceras perarmatum (left) and Peronoceras subarmatum (right), width of view both about 5 cm.

P. fibulatum and P. turriculatum can often be found complete, with a strong constriction on the internal mould at the mouth border  – I have not seen this on P. subarmatum or P. perarmatum (and it does not show on that one complete specimen shown above – but this one has shell on the outer whorl and is also slightly pathological, so might not be representative) but this may just be a case of not having really found a fully complete, adult specimen without shell on the last whorl – if you have one with a constriction, I´d love to see it …

Looking through the lens to photograph these specimen has also (again) all too clearly shown me the limitations in my prepwork, the better specimen (especially on the inner whorls) are the results of lucky, clean splits. One of my New Year resolutions : Don’t hurry so much, take more time to do things (fossil prep work and removing dust from specimen before photographing them – amongst other things) properly…

Have a great 2014…

AndyS