Whorl Expansion

The whorl expansion rate is the measure for the coiling of the ammonoid conch and describes the outline of the whorl spiral. Among the characters of the manticocer-atid conch, it is one of the most plastic, which reflects ontogeny (i.e., eccentricity of the whorl coil) as well as variability within the population (Fig. 3.6A, B).

Typical for manticoceratids is the relatively low WER in juveniles. After a slight initial juvenile decrease, the WER reaches approximately 2.0 at 3-5 mm conch diameter. In this interval, the variability within the material is relatively low.

In the third or fourth whorl, the specimens from Coumiac show a rapid acceleration of whorl expansion, which usually reaches a maximum of 2.5-2.75 in the fourth or fifth whorl (20-50 mm conch diameter). Thereafter, the WER stagnates or decreases in the adult whorls. In this respect, all manticoceratids reflect the morphological development of their agoniatitid ancestors (see Klug, 2001; Korn, 2001; Korn and Klug, 2001).

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♦ specimen M275

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• additional specimens

O specimen M218

• specimen M276

Fig. 3.6 Ontogenetic trajectories (A, C, E, G) and intraspecific variability (B, D, F, H) in Manticoceras from Coumiac. The bivariate diagrams (A, C, E, G) show the ontogenetic development of the whorl expansion rate (WER = (dm/dmj2), whorl height expansion rate (WHER = (wh/wh)2), whorl width expansion rate (WWER = (ww/ww2)2), and whorl surface expansion rate (WSER = css/css); highlighted are the specimens UMDK 218, 219, 275, and 276. The box-and-whiskers diagrams (B, D, F, H) show the ontogenetic variations of intraspecific variability of the major conch ratios. Bold lines refer to the total range, boxes refer to the extension of the middle two quartiles, and white lines refer to the median value. Growth stages between 1 mm and 100mm conch diameter were subdivided into 17 logarithmic intervals.

The rate of acceleration and the maximal value of the whorl expansion rate differs between individuals within the community from Coumiac (Fig. 3.6A):

• A WER of 2.25 is reached in specimen UMDK276 at 6 mm, in UMDK275 at 8 mm, in UMDK218 at 10 mm, and in UMDK219 only at 16 mm conch diameter.

• A WER of 2.50 is reached in specimen 276 at 13 mm, in 275 at 50 mm, in 218 at 12 mm, and in 219 only at 50-60 mm conch diameter.

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