- 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 ;-))
(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.