Conch Parameters

From the computer drawings of the cross sections, three of the basic conch parameters can immediately be obtained for each half volution (Fig. 3.5):

Fig. 3.4 Selection of eleven cross sections of Manticoceras specimens from Coumiac (Montagne Noire); allx 0.75. A - specimen UMDK 218; B - specimen UMDK219; C - specimen UMDK 275; D - specimen UMDK202; E - specimen UMDK236; F - specimen UMDK232; G - specimen UMDK 281; H - specimen UMDK 276; I - specimen UMDK 220; J - specimen UMDK 235; K - specimen UMDK 282.

Fig. 3.4 Selection of eleven cross sections of Manticoceras specimens from Coumiac (Montagne Noire); allx 0.75. A - specimen UMDK 218; B - specimen UMDK219; C - specimen UMDK 275; D - specimen UMDK202; E - specimen UMDK236; F - specimen UMDK232; G - specimen UMDK 281; H - specimen UMDK 276; I - specimen UMDK 220; J - specimen UMDK 235; K - specimen UMDK 282.

Fig. 3.5 The conch parameters and ratios used in the text. A - conch parameters in a cross section of a Manticoceras specimen; B - parameters for calculating the flank convergence index (FCI); C - enlarged inner whorls showing the large protoconch (x 3) (dm = conch diameter; ww = whorl width; wh = whorl height; uw = umbilical width; ah = apertural height; css = cross-section surface; WER = whorl expansion rate; WHER = whorl height expansion rate; WWER = whorl width expansion rate; WSER = whorl surface expansion rate; CW1 = conch width index; WWI = whorl width index; UW1 = umbilical width index; 1ZR = imprint zone rate; FC1 = flank convergence index).

Fig. 3.5 The conch parameters and ratios used in the text. A - conch parameters in a cross section of a Manticoceras specimen; B - parameters for calculating the flank convergence index (FCI); C - enlarged inner whorls showing the large protoconch (x 3) (dm = conch diameter; ww = whorl width; wh = whorl height; uw = umbilical width; ah = apertural height; css = cross-section surface; WER = whorl expansion rate; WHER = whorl height expansion rate; WWER = whorl width expansion rate; WSER = whorl surface expansion rate; CW1 = conch width index; WWI = whorl width index; UW1 = umbilical width index; 1ZR = imprint zone rate; FC1 = flank convergence index).

Using these basic parameters, secondary parameters can easily be computed:

• imprint zone width (iz) = wht-ah or whj-(dmj-dm2)

Expansion rates (growth rates) and conch proportions were calculated in the following way by using the three basic conch parameters:

• whorl expansion rate (WER) = (dmt/dm2)2 or [dm^dmj-ah)]2

• whorl height expansion rate (WHERt) = (wht/wh2)2; WHER2 = wht/wh3

• whorl width expansion rate (WWERt) = (ww1/ww2)2; WWER2 = ww(/ww3

• whorl surface expansion rate (WSER) = css1/css2

• umbilical width index (UWI) = uw/dm1 or (dm1-wh1-wh2)/dm1

• imprint zone rate (IZR) = wh1-ah/wh1 or (wh1-(dm1-dm2))/wh1

To describe the shapes of the whorl cross sections, a further index was calculated:

• Flank convergence index (FCI) = The ratio of the maximal whorl width and the width at 0.80 of the whorl height, measured from the umbilicus. An FCI of 1 would mean nonconverging (i.e., parallel) flanks.

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