Bdesheim

Of the cross sections of manticoceratids from Büdesheim (Eifel) that were published by Clausen (1969), four characteristic individuals are highlighted in Fig. 3.11. The minute pyritized material comes from dark shales, and the question arose whether these specimens, which are less than 20 mm in diameter, portray similar ontogenetic trajectories to the normal-sized material from other localities.

The manticoceratids from Büdesheim have conchs with ontogenetically accelerated coiling rate (WER; Fig. 3.11A). The figures also show an ontogenetic reduction

A

? •

f

J.tf

• •

ÎÏ

j. •

[W

(VER

C

*

■ »

••

o

£

m

1 O

IÇWIj

I i*

•7.

I

•J

1 •

B

conch diameter /mm

10 100

conch diameter /mm

• Manticoceras 'adorfense' O Manticoceras 'cordatum' O Manticoceras 'ntumescens'

• Sphaeromanticoceras 'orbiculum'

• additional specimens

D

y

• •

1,

*

J* •

[l

Wl]

10 100 conch diameter / mm

10 100 conch diameter / mm

Fig. 3.11 Bivariate diagrams showing the ontogenetic development of the whorl expansion rate (WER; in A), flank convergence index(FCI; in B), conch width index (CWI = ww/dm; in C), and umbilical width index (UWI = uw/dm; in D) of sectioned specimens of Manticoceras from Büdesheim (Eifel). Cross sections after Clausen (1969), x 0.75.

of the conch width index (CWI; Fig. 3.11C) and a narrowing of the umbilicus (UWI; Fig. 3.11D).

Particularly Fig. 3.11C shows that the species of Sphaeromanticoceras can clearly be separated by their stouter conch, whereas the other three specimens are hardly separable. In the flank conversion index (Fig. 3.11B), they do not differ at all. In the whorl expansion rate (Fig. 3.11A) and the umbilical width index (Fig. 3.11D), only minor differences between the three can be recognized. Furthermore, intermediate forms are present and connect the morphologies of the three specimens.

In the three Manticoceras species, the whorl expansion rate experiences a remarkable juvenile increase (Fig. 3.11A). In the specimen of Manticoceras adorfense, a WER of 2.50 is reached already at 6 mm conch diameter, and in the specimens of Manticoceras cordatum and Manticoceras intumescens, the value exceeds 2.50 at 10 mm diameter. This means that the acceleration of the coiling rate is more pronounced in the Büdesheim material, which suggests an earlier maturity in these forms. Remarkably the material from Büdesheim shows the same ontogenetic allometry as the large limestone specimens. It can therefore be concluded that the minute forms represent adult populations.

Was this article helpful?

0 0

Post a comment