Introduction

Genetically controlled discontinuous variation within a single population is termed polymorphism (Ford, 1940). According to the McGraw Hill Encyclopedia (1984: 1364), genetic polymorphism is "A form of genetic variation, specifically a discontinuous variation, occurring within plant and animal species in which distinct forms exist together in the same population. ... Distinct forms must be controlled by some switch which can produce one form or the other without intermediates such as those arising from environmental differences. This clear-cut control is provided by the recombination of genes." Genetic polymorphism may produce two or more discontinuous forms of a phenotypic feature due to functional or structural variation in a population. In the present-day organic world this term is used in reference to colonial organisms such as bryozoans and cnidarians and castes in bees, ants, and wasps. Recognition of polymorphism requires study of large populations of the organism in question. That is why it is frequently observed in the living organic world but is rarely recorded in fossils.

N. H. Landman et al. (eds.), CephalopodsPresentandPast:NewInsightsandFreshPerspectives, 97-120. © 2007 Springer.

The phenomenon of polymorphism is commonly displayed by discontinuous differences in ornament together with continuously expressed variations in size and shape (Reyment, 1988). Polymorphism, as a modality of genetic intraspecific discontinuous variability (besides sexual dimorphism), has been the subject of much attention by many workers in paleontological studies, e.g., in ostracods by Kamiya (1992) and Reyment (1988), and in graptolites by Janusson (1973). Ornamental polymorphism is also reported from fossil cephalopods. For example, Halder et al. (1998) recorded this phenomenon in the Middle Jurassic nauti-loid Paracenoceras. Within ammonites, intraspecific ornamental polymorphism has been described by Tintant (1963) in the family Kosmoceratidae, and by Tintant (1980) in Dactylioceratidae, and by Melendez and Fontana (1993) in Perisphinctidae.

In the present endeavour, we discuss ornamental polymorphism within a placen-ticeratid ammonite species P. kaffrarium from the Upper Cretaceous (Coniacian) Bagh Beds, Central India. The nature of polymorphism varies from a completely smooth shell to a morph having three rows of tubercles. Ontogeny of each morph is described in terms of heterochrony. Polymorphism has evolutionary implications. The role of ornamental polymorphism in P. kaffrarium in the evolution of the subsequent placenticeratid lineage is also discussed.

Placenticeras Meek, 1876, belongs to the family Placenticeratidae Hyatt and it ranges from the Upper Albian to Upper Campanian (Wright et al., 1996; Kennedy et al., 1996). It consists of numerous dimorphic species (Klinger and Kennedy, 1989; Ganguly and Bardhan, 1993) showing latitudinally clustered distribution patterns, but the genus itself has a pandemic distribution (Bardhan et al., 2002). The species P. kaffrarium has also been reported from the Turonian-Coniacian of Madagascar (Collignon, 1965a, b) and from the precisely dated Middle Coniacian of Zululand, South Africa (Klinger and Kennedy, 1989: 242). The P. kaffrarium population discussed here comes from the Upper Cretaceous Bagh Group of shallow marine carbonates of Central India (Fig. 5.1).

The carbonate sequence consists of two mappable formal units - the lower one, the Nodular Limestone and the upper one, the Bryozoan Limestone (Bardhan et al., 2002). Taylor and Badve (1992) informally subdivided the Bryozoan Limestone into two units - Deola Chirakhan Marl and Coralline Limestone and showed categorically that the Coralline Limestone always overlies the Nodular Limestone. Recently, Kennedy et al. (2003) showed alternations of the Nodular Limestone and Coralline Limestone in a particular section, but this cannot be regionally applicable for the entire Bagh Sequence. Specimens of Placenticeras kaffrarium are abundant in the middle subunit of the Nodular Limestone (Fig. 5.2). The Bagh sediments are exposed as discrete outcrops essentially on the northern part of the Narmada River, the course of which follows a mid-continental rift that was associated with the Karoo Rift System (Boselini, 1989). The carbonates were deposited in a narrow, intracratonic Narmada trough as the eastern arm of the Tethys transgressed (Chiplankar and Badve, 1973; Jafar, 1982). The Nodular Limestone horizons yield numerous ammonite specimens, which were previously grouped into at least 23 species belonging to four different families of varying age within the Late

Fig. 5.1 Outcrops of Bagh Group of rocks lying north of the Narmada River (inset) and geographic localities around the Man River valley showing important fossil-bearing stratigraphic sections (after Bardhan et al, 2002).

Cretaceous (Chiplankar and Ghare, 1977a, b). Recent studies (Klinger and Kennedy, 1989; Ganguly and Bardhan, 1993), however, have shown that the majority of the ammonite population actually belongs to the highly variable species Placenticeras kaffrarium. The previously assigned multitude of species names perhaps resulted from the failure to recognize sexual dimorphism and wide intraspe-cific variability especially in ornamental patterns, which led to polymorphism in the population.

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