Measuring an ammonite or How fat is a “fat Dac” ?

Measuring an ammonite with a pair of callipers

Measuring an ammonite with a pair of callipers

Perception is a wonderful thing – it´s different for everybody. When two persons look at one and the same ammonite they may come to totally different conclusions depending on what light they see it in, what they think they are seeing and maybe also what they want to see…

As I´ve shown you earlier, this even becomes more difficult when you look at the same ammonite species photographed in black & white or in color, or ammonites from different locations preserved differently – e.g. limestone or pyrite, with shell or as an internal mould.
To alleviate this problem of non-objective descriptions of ammonites, several measurements have been developed to provide objective measurements for ammonite shells.
I do not know when this actually started, I´ve seen tables with measurements in SPATH´s Liparoceratidae in 1938, wonderful scatter diagrams of various measurements in HOWARTH´s Amaltheidae and Hildoceratidae monographs from the late 1950s and early 1990s, but I think the german author Dr. Rudolf Schlegelmilch has so far provided the most all-embracing measurements for liassic (and also middle and upper jurassic in later books) ammonites so far in his 1976 publication (and the 1992 second edition)  “Die Ammoniten des süddeutschen Lias” (The ammonites of the south german lias), which is actually very much applicable to liassic ammonites from locations outside of Germany as well. I´m planning to use the types of measurements that are used in this publication for the ammonites pictured in the planned book as well – so I think it is time to explain them a little and maybe also help non-german speaking users of the SCHLEGELMILCH book, who might be struggeling with the german explanations.
For measuring the ammonite, using a vernier, dial or digital caliper is useful, since it helps in the process of taking the measurements, but precision is only needed to full millimeters.
First a picture showing the simple measurements :
Ammonite measurements

Ammonite measurements

  • Shell diameter (d – Schlegelmilch, D – Howarth) :
    It is important to take a set measurements at as close to the same diameter as possible, since
    they may vary with diameter, i.e. an ammonite´s shell can have another whorl height or width
    in an earlier stage of life than an adult ammonite.
    When giving numbers for all of the following measurements it is thus always necessary to
    also quote the diameter at which they were taken.
  • Umbilical width (n – Schlegelmilch, U – Howarth)
  • Whorl height (h – Schlegelmilch, Wh – Howarth)
  • Whorl breadth (b – Schlegelmilch, Wb – Howarth)
  • Rib count  (Z – Schlegelmilch) – Ribs per whorl at a given diameter

For comparing ammonite shells, it has proven useful to create relative, compound measurements :

  • Relative umbilical width (N – Schlegelmilch) : Umbilical width divided by diameter (N = n/d = U/D)
    A larger umbilicus would have a large N, like the quite evolute Dactylioceras commune (like the one shown above) which
    has an N between 54 and 62 %, whereas a much more involute ammonite like an Oxynoticeras exhibits an N of only 14 % at 7 cm.
  • Relative whorl height (H in the Schlegelmilch book) : Whorl height divided by diameter (H = h/d = Wh/D)
    H is a factor in how many whorls there are in a shell, when you look back at the post describing the difference between
    Gagaticeras  and Androgynoceras, I noted that there are more whorls for the same diameter in Gagaticeras and you see it in the numbers :
    Gagaticeras has a H of between 23 and 26 %  , while Androgynoceras is higher at around 30 %
  • Relative whorl section (Q in the Schlegelmilch book) : Whorl height divided by whorl width (Q = h/b = Wh/Wb)
    A large number for Q would describe an ammonite with high, slender whorls like an Amaltheus stokesi, for which Schlegelmilch
    notes a Q of 2.3 (at all sizes), whereas a low Q would describe an ammonite with whorls broader than high, usually of a coronate shell form
    like a Catacoeloceras crassum for which Schlegelmilch quotes a Q of 0.7 (at 7.2 cm diameter ;-))
When you collect a lot of ammonites of the same species, all sorts of statistical analysis becomes possible with these measurements
(as long as you´ve got a large enough sample size…), and this allows you to not only calculate variability of the species within a population,
but also show species differences independant from the perception of an individual.

A Yorkshire holiday and Be safe…

Bay Ness, Robin Hoods Bay, in the evening light at low tide

Bay Ness, Robin Hoods Bay, in the evening light at low tide

You might have noticed the rate of my posts going down in the last couple of weeks – simple reason : I was on holiday, on the Yorkshire coast, collecting and photographing ammonites.
On the collecting side, it  was hard work but some of the more remote locations I visited proved to be relatively productive – at least better than I had feared during this smooth-sea summer time. Nothing obviously being a fossil remains lying on the beach for long these days, accumulations of the so-called “cannon ball” nodules mostly containing Eleganticeras ammonites that we experienced during our first visits in the late 1980s are a thing of the past. Nowadays you need to look more closely, be at the right spot at the right time (after smaller or larger cliff falls have occurred) and be persistent, re-visit often to be there when large tides rework falls or the accumulated cliff debris on the beach.

On a more sad and earnest note, I guess tragic accidents like the one a couple of weeks ago at Burton Bradstock, Dorset,  where a young woman was killed by a huge rockfall, can happen on the Yorkshire coast as well – I have seen the craters in the shingle produced by freshly fallen large sandstone blocks from the very top, so be safe when you walk along those ever crumbling cliffs.
And if some bloke with a beard did remind you to stop your kids hammering at the foot of the cliff, that was probably me…

On the photographing side, it was most interesting – I was given the opportunity to photograph a lot of ammonites where offhand I did not have a clue as to their species, did not know that they existed in such sizes or took my breath away due to their rarity, preservation and preparation. A big “Thankyou !” to all who helped  so far shortening the “wants lists”, you know who you are, I will certainly be able to scratch out a few species off the list, once I´ve properly identified them…
Looking back at the pictures I´ve made during the holiday, I think I can say that my light “on holiday” photography setup (which I will show you in due course) did work, even in the sale room of a well-known Whitby fossil shop…
Thanks also to all old friends who came to join me again in my collecting trips, it is so much more fun to collect with you out there than alone, and thanks also to new friends who I met first time in person, it was a great pleasure to get to know you !
You might also have noticed that I have updated the earlier “Rare and re-bedded” post with some new pictures and text – I did have the ammonite sent to our holiday location, the picture shown in the post previously was by kind permission of the seller of the ammonite – thanks again, Daniel !
And also, what you might rightfully be waiting for, here are a few pictures of “teasers” – unprepped ammonites I took with me this time and a few I left behind…
I chose these especially to show you that ammonites when freshly found very, very rarely look as I´ve shown you in some of my previous posts – a lot of prep work is necessary to make them presentable. I will add pictures in their prepped state and possibly some prep photos once they´re ready…