function of the ammonite fluted septal margins, The

Journal of Paleontology, Jan 2002 by Lewy, Zeev

ABSTRACT-The fluted margins of ammonite septa were thought to resist the hydrostatic pressure upon the phragmocone while the ammonoid dived. However, ammonoids probably did not dive deeper than the extant nautilids, whose conchs, with the simple septa, sustain pressure correlative to depth of about 800 m. The backward and forward stretching lobes and saddles actually provide resistance to pressure perpendicular to the septum. Ammonoids lived for about three to five years, and septa were precipitated in intervals of nearly two weeks to two days, which explain the small dimensions of the scars of the adductor muscles, which were periodically detached and reattached. The weak hold between these small muscles and the buoyant conch was compensated for by the backward branching and expanding folds (forming the sutural lobes), into which the soft tissue penetrated and stiffened for a required period to firmly anchor the body to the conch throughout its whole circumference. The greater the complexity of the septa marginal fluting, the better the ammonoid could withstand the dragging force between the body and the buoyant conch, and hence the more aggressively the ammonoid predated and competed with other creatures.

INTRODUCTION

HE AMMONOIDS evolved in the Devonian, comprising evolute to involute planispirally coiled conchs quite similar to that of the contemporaneous nautiloids. However, in the ammonoid conch the siphuncle lies in a ventral position (dorsal in Upper Devonian clymeniids), and the septal margins develop more folds than in nautiloids (more sutural elements). The main evolutionary trend within the Ammonoidea is evidenced by the increase in the number of septal marginal folds and by their complexity, in contrast to the rather simple bowl-shape septum in most of the nautiloids. Since their origin in the Devonian, ammonoids highly diversified and became abundant in the Mesozoic oceans, whereas nautiloids decreased in diversity to about five genera towards the end of the era (Moore, 1964).

There is no doubt among paleontologists that the great advantage of the ammonoids over the nautilids was achieved through the fluting of the septa margins; however, the actual function of this marginal fluting is still disputed. Kennedy and Cobban (1976, p. 23; with references) evaluate the functions attributed to the septal marginal fluting. They mention the strengthening of the phragmocone against implosion under hydrostatic pressure (Buckland, 1836). The enlarged surface of the similarly folded posterior part of the body may improve physiological functions (Newell, 1949). The enlarged surface of the folded septa increases the overall weight of the conch that may contribute to buoyancy control (Reyment, 1958). The marginal folding secures body attachment to the shell during the secretion of new septa (Spath, 1919). Saunders (1995) analyzed Paleozoic ammonites suture patterns and raised doubts on Buckland's (1836) hypothesis on the mechanical support by the septal fluting. Alternatively, Saunders (1995) suggests that this marginal fluting forms septal recesses where camera] liquid is stored to improve buoyancy adjustment, However, Buckland's (1836) hypothesis was firmly advocated with mathematical calculations and hydrostatic experiments (e.g., Westermann, 1971, 1975; Hewitt and Westermann, 1986, 1987, 1997) as the functional explanation for septal marginal fluting. Nevertheless, such calculations may be wrongly interpreted. For example, trochospirally coiled gastropods are most resistant to pressure along their coiling axis, which does not mean that this mode of coiling evolved to resist such pressures. In light of Buckland's theory the observed evolutionary increasing complexity in the fluting of the septal margins (sutural complexity) implies a gradual shift of ammonoid activity into the dark, deep marine water, where ammonoid prey is scarce. This contradicts the abundance of ammonites in shelf sediments not as shoreward drifted conchs (Kennedy and Cobban, 1976, p. 36). Actually, most of the ammonoids are regarded epipelagic creatures (up to 250 m water depth), even by the main supporters of Buckland's theory (e.g., Batt, 1991; Westermann, 1990; Westermann and Tsujita, 1999).

The marginal folds of the septa strongly bend forward and backward, approaching the inner wall of the ammonite at a low angle. The thickness of a pachydiscid ammonite septum decreases from 0.9 mm in the center to less than 0.3 mm at the margin and to about 0.1 mm in the folioles and lobules (Westermann, 1975; Westermann and Tsujita, 1999). This construction and the marginally thinning of the septa [Westermann, 1975; later suggested to act "like a spring or shock absorber"; (Westermann, 1999b, p. 120)] actually reduces the resistance to external hydrostatic pressure (e.g., Daniel et al., 1997). Therefore, Saunders et al. (1999) suggested an opposing hydrostatic control on ammonoid bathymetric range, whereby more complexly sutured ammonites had shallower depth limits.