Paleobiogeography and Paleoecology of Placenticeras kaffrarium

The genus Placenticeras has a wide geographic distribution, but species in any particular time are endemic and show clustered distribution patterns (Bardhan et al., 2002). They appear to have been constrained by latitudinal factors and, at times, were highly provincialized. Throughout their geological distribution, placenticeratids flourished best in the subtropical region on either side of the equator. For example, the earliest groups during the Albian-Cenomanian were mostly clustered in Central Asia or in Europe and America in areas having more-or-less similar latitudes (see Bardhan et al., 2002: Fig. 5.3). In Turonian-Coniacian times, the Central Asian stock migrated towards the southern margin of Tethys and occupied shallow epicontinental seas in India, Madagascar, and

Fig. 5.7 Kaffrarium type of ornament, microconch. 1, lateral view, complete adult; note peristome showing ventral rostrum. 2, ventral view; note well-developed ventral clavi. 3, lateral view, small adult; note stronger lateral tubercles. 4, apertural view; note typical polygonal whorl section. 5, lateral view, adult; note increase in strength of ornamentation during ontogeny. Scale bar = 1 cm.

Fig. 5.7 Kaffrarium type of ornament, microconch. 1, lateral view, complete adult; note peristome showing ventral rostrum. 2, ventral view; note well-developed ventral clavi. 3, lateral view, small adult; note stronger lateral tubercles. 4, apertural view; note typical polygonal whorl section. 5, lateral view, adult; note increase in strength of ornamentation during ontogeny. Scale bar = 1 cm.

Fig. 5.8 Kaffrarium type of ornament, macroconch. 1, lateral view, nearly complete adult showing strong umbilical and lateral tubercles, which are connected by coarse blunt ribs. 2, apertural view. 3, lateral view, adult; note attenuation of tubercles and scaphitoid uncoiling of the body chamber. Scale bar = 1 cm.

Fig. 5.8 Kaffrarium type of ornament, macroconch. 1, lateral view, nearly complete adult showing strong umbilical and lateral tubercles, which are connected by coarse blunt ribs. 2, apertural view. 3, lateral view, adult; note attenuation of tubercles and scaphitoid uncoiling of the body chamber. Scale bar = 1 cm.

Fig. 5.9 Kaffrarium type of ornament, macroconch. 1, lateral view, large body whorl fragment; note presence of faint rib. 2, apertural view. Scale bar = 1 cm.

South Africa. It speciated quickly to produce P. kaffrarium, the most dominant species, which virtually monopolized the Indo-Madagascan Faunal Province (Bardhan et al., 2002: Fig. 5.4).

P. kaffrarium does not show any sedimentary facies control. In Zululand, it occurs mostly in siltstone and sandstone, whereas, in Bagh, the population is found only in carbonate facies. Klinger and Kennedy (1989: 383) interpreted the paleoen-vironment of the Coniacian of Zululand as "indicative of a near shore protected environment with absence of strong currents and wave action but, nevertheless, normal salinity." In Bagh, the carbonates were deposited in a narrow intracratonic trough during the Coniacian transgression. The sedimentary facies analysis, as well as stable isotope (carbon) study, reveals that carbonate horizons bearing P. kaffrarium were deposited during the maximum flooding zone and the sedimentary facies and faunas indicate a normal marine environment. The Bagh basin was basically an inland seaway far from the open ocean. The associated faunas also include normal marine groups, such as echinoids, articulate brachiopods, and cheilostome bryo-zoans. However, all are characterized by exceptionally low diversity, almost monospecific; individuals nevertheless are highly abundant. The faunal association and low diversity pattern suggest that the environment was perhaps stressful, particularly with respect to seasonal salinity fluctuations. Tsujita and Westermann (1998) believed that Placenticeras could tolerate even a brachyhaline environment. The environment was placid from the biological and taphonomic point of view. Echinoids, brachiopods, and bivalves (mainly inoceramids) are mostly found articulated and in an exceptionally high state of preservation, without showing any injury or repair marks. In the history of the placenticeratids, the peristome was rarely found elsewhere in the fossil record and Westermann (1990) believed that they were highly predated. But, in the P. kaffrarium population of Bagh, many specimens have their apertural margins intact (Gangopadhyay and Bardhan, 1998: 195, 199). This indicates that the Bagh sea was environmentally, both physically and biologically, less stressful than other placenticeratid populations inhabiting the open shelf.

Ecologically isolated regions promote great diversity. Islands and lakes of the present day are areas of adaptive radiation (Stanley, 1979: 169-174). These regions experience much less biotic pressure, and organisms living there show a great genetic variability due to eco-insular conditions (Stanley, 1979). We believe that the release of stabilizing selection pressure due to the absence of competitors and predators, triggered the great intraspecific variability within the P. kaffrarium population, which ultimately led to polymorphism. For example, Janusson (1973) has also found the widespread presence of polymorphism in graptolite communities, which facilitated great evolutionary changes. Another example is pharyngeal teeth of cichlid fishes (Sage and Selander, 1975). The polymorphic differentiation of teeth enables the species as a whole, to get access to various foods. We shall see later that the different morphs of P kaffrarium indicate niche partitioning and not a "biological extravaganza" as Arnould-Saget (1956, in Klinger and Kennedy, 1989: 383) believed for a similar variability in a population of Knemiceras Bohm, 1898. Because other polymorphic variants (umkwelanense and subkaffrarium morphs) are the scaled-up versions of different ontogenetic stages of the kaffrarium morph, they were "pre-adapted" and had no difficulties in finding different niches.

Ammonite shell forms and sculptures are functionally significant (Kennedy and Cobban, 1976; Ward, 1981; Westermann, 1990, and references therein). Placenticeras shows wide interspecific diversity with respect to size and ornament, but the shape (broadly involute and oxyconic) and sutural patterns (complex) show phylogenetic conservatism. Patterns of sedimentary facies and paleobiogeographic distribution indicate a shallow marine habitat for Placenticeras. Functional morphological analysis suggests that placenticeratids have the weakest shells known and were one of the shallowest ammonites having lived in the proximal sublittoral zone less than

50 m depth (see Westermann, 1990; Tsujita and Westermann, 1998). Analysis of the functional morphology of P. kaffrarium (Bardhan et al., 2002) reveals that individuals each have a very thin shell, thin septa with a relatively large siphuncle, and complex septal sutures with relatively low height of the sutural elements. This implies an exceptionally low SRI (Sutural Reinforcement Index) and a shallow water mode of life (Batt, 1986, 1993). The functional significance of ammonite ornament has been variously interpreted: camouflage (Kennedy and Cobban, 1976), an effective buoyancy device (Kennedy, 1986), predator avoidance (Tsujita and Westermann, 1998), and hydrodynamic device (Chamberlain and Westermann, 1976). The presence of great intraspecific variability with respect to ornament and form suggests that the population was not under strong selection pressure (Seilacher, 1972), and the absence of septate spines indicates that buoyancy regulation was not a constraint for the P. kaffrarium population (Kennedy, 1986). It is generally believed that individuals of ammonite species showing marked ontogenetic changes are vertical migrants during their lives (Westermann, 1990; Batt, 1993). Batt (1993) has shown that ammonite morphotypes are bathymetrically controlled. The smooth, oxyconic form (umkwelanense type) inhabited the shallowest part of the basin margin (see also Tsujita and Westermann, 1998), whereas strongly sculpted, nodose shells with polygonal whorl section (kaffrarium type) lived in the relatively deeper zone. Chamberlain and Westermann (1976) suggested that ammonite ornament also has hydrodynamic significance. Strong sculpture, especially located on the outer flanks (like the kaffrarium morph) can produce drag, thereby making the animal a slow swimmer. On the other hand, smooth forms, like the umkwelanense type with compressed shells, would have experienced minimum drag and, therefore, an enhanced swimming ability.

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