Orientation of the Aperture in Manticoceras

It is difficult to extract unambiguous information on the organism's mode of life from the ammonoid conch alone. Nevertheless, the conch provides some information, e.g., if the aperture was pointed directly upward or if it was oriented at an oblique angle as in Recent nautilids, if the living animal was a poor swimmer but able to reach the seafloor easily with its arms (as Recent Nautilus does) or whether it was a comparatively good swimmer. The orientation of the aperture of Manticoceras throughout its life is discussed subsequently.

Raup (1967) and Okamoto (1996) have demonstrated that the body chamber length (BCL) in ammonoids is correlated with the whorl expansion rate (WER). The nonlinear correlation shows that conchs with low WER possess long body chambers, and vice versa. This model is mainly applicable to forms which do not show remarkable ontogenetic changes of coiling. Eccentrically coiled conchs are much more difficult to compute, and only approximations can be produced so far. In order to obtain more precise results, other expansion rates (such as that of the whorl width) should be taken into account, because they also influence the volume of the soft body and consequently its length. The surface expansion rate can be regarded as a summary of all expansion rates and will be discussed below.

Trueman (1941), Raup (1967), and Saunders and Shapiro (1986) showed that particularly the length of the body chamber controls the orientation of the organism with its conch in the water column, i.e., the position of the aperture. Again, it is difficult to apply this method to conchs with ontogenetic modifications, such as the closure of the umbilicus or changes in the width of the whorls.

Manticoceras conchs are remarkable for their eccentric coiling and rather sudden ontogenetic changes of some of the whorl parameters, such as the whorl width. The accretion rate of shell material at the body chamber, which is the main factor in the accumulation of weight that has to be buoyed by the phragmocone, can only be calculated when the growth of several conch features is considered: WER: The measure for the coiling of the ammonoid conch; it is practically identical with the expansion rate of the aperture.

WHER: The whorl height expansion rate is closely related to the WER. An increase of the WHER can be caused by enlargement of the aperture height (and thus also an increase of the WER), by the closure of the umbilicus (without increase of WER), or can be driven by both.

WWER: In Manticoceras, the ontogenetic development of the whorl width is largely uncoupled from the whorl height and the aperture height. WSER: The whorl surface expansion rate depends on several other measures. A higher WSER can be caused by an increase of the aperture height (WER), the whorl height (WHER), and the whorl width (WWER), but also by reduction of the umbilical width and reduction of the imprint zone rate (IZR). All these parameters are easily calculable, in contrast to the WSER which requires some technical effort. Ontogenetic changes of these conch parameters influence the body chamber length. Additionally, the shape of the whorl cross section (circular, rectangular, or subtriangular) determines the whorl cross-section surface.

As shown diagrammatically in Fig. 3.6, the curves of the WSER, WHER, and WWER of the Manticoceras conch run almost parallel, meaning that the factor of the WHER and WWER leads to an extremely close approximation of the WSER. It can also be seen that the WER curve does not significantly deviate from the WSER curve. Despite the nonparallel development of whorl height and whorl width, no dramatic errors in the calculation of the BCL can be expected. Consequently, either WER or even better, WHER together with WWER, can be used as a good approximation for the WSER.

In the following reconstruction of a living ammonoid (Figs. 3.15, 3.16), the orientation of the aperture is based on the WER, which is regarded as the cardinal character. As Klug (2001) showed, the computed body chamber length (and consequently the orientation within the water column) changed markedly in the unusual Early Devonian ammonoid Rherisites. A similar assumption can be made for Manticoceras, although the extent of allometric growth is less striking than in Rherisites. Early

body chamber length

hatching

t at r

O

n a

1

8

i

maturity maturity hatching

10 100 diameter (in mm)

1000

10 100 diameter (in mm)

Nautili

body chamber length

Nautili

maturity

maturity

Nautilus maturity maturity hatching body chamber length

hatching body chamber length

Fig. 3.15 Estimation of the orientation of the Manticoceras conch in the water column throughout ontogeny. A - diameter/whorl expansion rate diagram; B - body chamber length/whorl expansion rate diagram (modified after Okamoto, 1996); C - body chamber length/orientation of the aperture diagram (modified after Saunders and Shapiro, 1986) with values of Manticoceras. The width of the curves in the latter two diagrams (right) schematically indicates the variability of the correlation between the parameters. This is caused by the poorly known variability of size, position, and density of the soft body and of the mandibles and cameral fluids (Klug, 2001). The numbers on the thin black line indicate the growth stages with the numbers of the whorls.

Fig. 3.16 Reconstruction of the life cycle ofManticoceras, depicting the orientations of the aperture of four representative growth stages. The reconstruction of the soft part morphology is largely speculative, except for the position of the eye and the hyponome. The presence of a hood (as in Nautilus) might be indicated by rare finds of the black layer in situ Keupp, 2000; Klug et al, 2004) which perfectly fits into the aperture. Natural diameters are: egg - 2mm, hatchling - 2mm, juvenile - 6mm, preadult - 50mm, adult - 400mm.

Fig. 3.16 Reconstruction of the life cycle ofManticoceras, depicting the orientations of the aperture of four representative growth stages. The reconstruction of the soft part morphology is largely speculative, except for the position of the eye and the hyponome. The presence of a hood (as in Nautilus) might be indicated by rare finds of the black layer in situ Keupp, 2000; Klug et al, 2004) which perfectly fits into the aperture. Natural diameters are: egg - 2mm, hatchling - 2mm, juvenile - 6mm, preadult - 50mm, adult - 400mm.

juveniles of Manticoceras with a WER of 2.0 may have body chamber lengths of approximately 270°. Later in ontogeny, when the WER increased to 2.75-3.00, the BCL is reduced to 170°-180°. Using the diagram of Saunders and Shapiro (1986), the orientation of the aperture would change from approximately 90° (i.e., horizontal position) in early juveniles to 70° in preadults and 60° in adults.

Nevertheless, the effect of the jaws (Trauth, 1935; Clausen, 1969) on the orientation of the conch remains unclear, because their material and their position within the body chamber are unknown.

+1 0

Post a comment