2 large D. commune adult specimen – both 10 cm – as large as they come !

If there is one ammonite genus that is typical for the Yorkshire coast, more specifically for the coast around Whitby,
it has to be Dactylioceras. Do a search on UK ebay for “Whitby ammonite” and what you get is at least 50 % Dactylioceras, in natural form or sliced and polished…

The Whitby Town coat of arms (fuimus et sumus – we have been and we are – quite fitting)  bears 3 Ammonites, most likely Dactylioceras, with snake heads attached to commemorate the legend of abbess Hilda (see also link).

This all of course is due to its abundance on this stretch of coast, when walking on a beach where the cliff exposes the upper toarcian beds of the lias,
there is almost no way you cannot find a Dactylioceras (fragment), if you keep your eyes open…

A very worn pebble with a Dactylioceras sp. on Robin Hoods Bay beach

In this blog post I´d like to introduce you to the 2 most common species of Dactylioceras : Dactylioceras commune (SOWERBY, 1815) and Dactylioceras athleticum (SIMPSON, 1855).

For the main comparison I´ve chosen two similarily sized, mid size ammonites (each about 7 cm) :

Direct comparison between D. athleticum (left) and D. commune (right), both about 7 cm.

So what differences are there ?

Whorl section : First of all, D.commune has a whorl section that`s as round as a circle, while D. athleticum´s is more oval. I’ve re-prepped these two specimen especially to show this characteristic.

Ribbing : With D. commune the ribs run straight or just slightly convex across the venter, while with D. athleticum the ribs cross in a much more convex way, sometimes almost angled. With D. athleticum, ribbing is much finer on the inner whorls, and the rate of ribs/whorl does not change much on the outer whorls. D. commune has more coarse ribbing on the outer whorls, and finer ribbing on the inner whorls.

Both D. commune and D. athleticum occur in the commune subzone of the upper toarcian, in the Main Alum shale beds, D. commune in the lower part of the subzone (commune biohorizon) down into the Hard Shales (beds 49-54, approx. 15.9 m),  D. athleticum in the upper part (athleticum biohorizon), beds 55-59, approx. 2.8 m.(at Whitby according to K.N.Page in “British Lower Jurassic Stratigraphy”).

The maximum size for these 2 species seems to be around the 9-11 cm mark, with D. athleticum usually a bit smaller, but finding one of this size is quite rare these days. Adult specimen develop a constriction at the mouth border, which is mostly invisible on the surface of the shell, only showing clearly on the internal mold when the shell is removed. As seen on the D. athleticum above, there are some very fine ribs that mark the very end of the adult shell, when preserved.

So far, so good…

More ribs, less ribs, round whorl section, oval whorl section, they all do look different from the two above ammonites that I’ve shown you as characteristical for the species, but are they different species or is it just whole lot of variation ?

These were not taken from in-situ off well-defined beds, but collected from the more or less wave rolled cliff debris on the beach, as most of us collectors do, as most of previous centuries’ collectors have done.

The answer probably is : Most of it is just natural variation within a species, with some you can take established species as a reference point and call them e.g. Dactylioceras cf. athleticum (cf : latin confer, compare to) . With some of them, if you don´t really know which bed they come from, it´s sometimes best to just call them Dactylioceras sp. – until you find another one in a defined bed.
Too little work has been done to really identify bed-by-bed variation of the Dactylioceras genus – imagine what could have been done in this regard when the big Yorkshire coast alum quarries were dug in the 18th and 19th century…

Saltwick Bay at low tide – Black Nab at the water line, the disused Alum works in the left background

We’ll continue looking at the Dactylioceratidae on the next posts…

AndyS