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.
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