Ornamental Polymorphism in Placenticeras kaffrarium

Klinger and Kennedy (1989) provided the first detailed taxonomic account of the species and showed that the variation within P. kaffrarium is constrained by both genetic and non-genetic factors. The ontogeny of the species is marked by strong allometry: the early whorls are smooth, the intermediate stage is marked by the appearance of umbilical tubercles and ventral clavi, and the late phragmocone - early body whorl shows the presence of both umbilical and lateral tubercles connected by z 9t

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cn 5

H2 H1

INDEX

" SAND STONE

BRYOZOAN LIMESTONE

ma NODULAR LIMESTONE

WAVY-BEDDED LIMESTONE

Fig. 5.2 Generalized stratigraphy of the Bagh Group along the Man River valley. HI and H2 are intraformational and interformational hardgrounds respectively (modified after Gangopadhyay and Bardhan, 2000). Horizontal long arrows indicate levels of maximum occurrence of P. kaffrarium.

bifurcating ribs and strong alternating ventral clavi. Ornament weakens or disappears on the later part of the body chamber, which becomes rounded on the venter. This ontogenetic scaling of ornament is, however, not observed in all adult specimens of the population. Instead, each major ontogenetic stage may be retained up to the adult stage. Thus, some adult variants are smooth and devoid of any ornament. Some specimens retain only umbilical tubercles and ventral clavi in their adult outer whorl. In the majority of specimens in the Bagh population, the outer whorl is characterized by strong umbilical and lateral tubercles along with ventral clavi. All adult variants are of similar size, and the later part of the body chamber is marked by smoothening of the shell and rounding of the venter, especially in mac-roconchs. These three morphs, therefore, exhibit discontinuous variation with respect to ornament. In addition, there are many morphotypes, which represent intermediate stages, but do not show continuous variation. For example, some adult specimens retain the smooth stage for a longer period and develop umbilical tubercles and ventral clavi only during later ontogeny. Other variants may have umbilical tubercles in early ontogeny and subsequently develop weak lateral tubercles. Klinger and Kennedy (1989), in fact, recognized seven distinct morphotypes. The above mentioned three morphs show qualitatively different aspects of ornament and represent non-sexual intraspecific polymorphism. They correspond to typical umkwelanense, subkaffrarium, and kaffrarium variants described by Klinger and Kennedy (1989). Placenticeras kaffrarium, moreover, is strongly sexually dimorphic. It shows both size and ornamental dimorphism in Bagh. In addition, Bagh records many specimens with the peristome preserved - a feature which is rarely found in the family Placenticeratidae throughout the world (see Gangopadhyay and Bardhan, 1998). Adult microconchs show a peristomal modification, with a ventral rostrum, whereas adult macroconchs have a simple aperture (for details on adult modifications in ammonites, see Davis et al., 1996). Each morph is dimorphically paired, and both antidimorphs exhibit polymorphism. On the one hand, the umkwe-lanense polymorph shows only size dimorphism; on the other hand, the kaffrarium polymorph has well-marked size and ornamental dimorphism. The microconch is relatively evolute and coarsely ornate, and the ornament continues till to end.

There is strong covariation between the strength of ornament and degree of inflation and involution among polymorphs, and this provides a distinctive look for each variant. Umkwelanense is oxyconic involute, subkaffrarium is relatively evolute and inflated, with a subtrigonal whorl section, and kaffrarium is more evolute and highly inflated with a polygonal whorl section. It appears, therefore, that P. kaffrarium shows both sexual and non-sexual polymorphism. The variants intergrade morphometrically; this suggests that they belong to the same biological species (Fig. 5.3). Type material of P. kaffrarium from Zululand is also included in the bivariate plots and forms a homogeneous distribution with the present population in Bagh. This indicates that both the Bagh and Zululand populations belong to the same species (for a detailed morphometric analysis, see Bardhan et al., 2002). The other contemporaneous species, P. mintoi, in Bagh is closely related, but morphologically quite distinct (see Ganguly and Bardhan, 1993). Here, we provide a brief taxonomic description of each polymorphic variant.

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