Reply to Checa and to Hewitt and Westermann

Journal of Paleontology, Jul 2003 by Lewy, Z

THE COMMENTS of A. G. Checa and R. A. Hewitt and G. E. G. Westermann on my article (Lewy, 2002a) about the functional morphology of the ammonite fluted septal margins do not mainly relate to this paper. Also they completely misinterpret its presentation. Both commentaries erroneously blame Lewy (2002a) for supporting the old idea of muscle attachment onto the complexly folded ammonite septum without adding new data (Checa, 2003), or without knowing that it was rejected a long time ago ("ignorance of their history"; Hewitt and Westermann, 2003). Lewy (2002a) clearly pointed out in the abstract, text, and conclusion that the backward expanding, septal marginal folds provided temporary anchorage sites (not attachment surface) for the soft tissue of the ammonoid, which lacked sufficiently large adductor muscles. After misunderstanding the rationale of the paper, each of these critics discussed minor issues and tried to undermine my scientific credibility.

Checa (2003) re-describes his model for the formation of the septal marginal folding by the physical interaction of two differently viscous fluids (cameral fluid and ammonoid soft tissue) on both sides of a membrane. This viscous-fingering model together with the tie-point model of Seilacher (1975) are claimed to be "currently the only ones aiming to explain the complex shape of the septal template by means of simple biophysical processes." Lewy (2002a) clarified that the tie-point model (Seilacher, 1975) requires the posterior margins of the ammonoid mantle to attach to the inner wall in a certain pattern, gradually and slightly changing with ontogeny. The addition of more such tie-points will form a continuous attachment band (Blind, 1975, 1980) in the shape of the septal marginal folding (suture line). Thereby the configuration of the septum is formed and the viscous-fingering model is completely superfluous.

Checa (2003) does not understand the evidence Lewy (2002a) presented to oppose the biophysical model of the viscous-fingering for shaping septa. The figured two specimens of Libycoceras show a shift in the position of successive septa (represented by the internal molds), whereby the tips of the lobules fold inward over the previous umbilical saddles. In case of the tie-point model the shifted location of these points over the umbilical saddles should have changed the suture pattern.

Hewitt and Westermann (2003) emend their citations from my papers and omit the significant words so as to make them sound absurd. They do not refer to their favored alleged hypothesis on the resistance of the septal marginal folding to external hydrostatic pressure, which is the main issue of my criticized paper. They prefer to discuss the observed close relationships between ammonoids and octopods (Lewy, 1996, 2000), which they name "nude-ammonites-became-octopods" instead of "octopods are nude ammonoids" (Lewy, 1996). They discuss my observations on the restrictions of life functions inflicted on ammonoids in modified terminal body chambers, which in many cases may have caused their ultimate death. They cite "therefore, they must have been made shortly before the ammonoid died in order to fulfill certain purposes of great significance" (Lewy, 1996, p. 627). They skip much of the text of my conclusion to the paragraph: "these modified, terminal body chambers probably served as egg cases" (ibid., p. 628) without referring to the significant purpose of these modifications, which is breeding and ensuring the descendents. In this manner Hewitt and Westermann (2003) summarize in four sentences some of my ideas in an incorrect way by omitting the main facts. Therefore I have to repeat my observations and conclusion herein.

Some ammonites change considerably the shape of their terminal body chamber. Most heteromorphs form a U-shaped chamber, whereas many normally coiled ammonites inflate or enlarge the terminal body chamber and constrict the aperture. The floating orientation of these modified conchs is with the aperture pointing upward, and in some cases against the preceding whorls. Authors have tried to explain how these ammonoids could feed without thinking whether they could live under such settings. These observations, together with the discovery of fossil egg masses in macroconch body chambers (Dreyfuss, 1933; Lehmann, 1966, 1981; Muller, 1969, 1978), triggered the interpretation of the modified terminal body chambers as floating egg cases comparable to that of the extant octopod Argonauta. Hewitt and Westermann (2003) do not refer to these latter references about tiny spherical eggs in ammonite internal molds. Alternatively, they discuss in detail a concentration of egg-like spheres (Michael, 1894), which turned out to be a stomach content. Despite this misleading argumentation by Hewitt and Westermann (2003) against my egg-case interpretation, they mention the similarity of the tiny, spherical fossil eggs of ammonoids and the extant Argonauta species (Boletzky, 1992).

Based on the co-occurrence of ammonites with and without modified terminal body chambers, the restricted living conditions in the modified chambers, and the similarity of the tiny spherical eggs in ammonites and extant argonautids, Lewy (1996) concluded that the ammonoids carried out the two modes of breeding as executed by the extant incirrate octopods: 1) large, yolk-rich eggs attached as bundles to a substrate; and 2) numerous, tiny yolk-poor eggs laid by the female in a floating egg case, in which she is situated as well (argonautids), or held in a modified arm of the bolitaenid octopod (Boletzky, 1986, p. 223). The first, stationary mode of breeding concentrates broods in a few attachment sites and therefore requires to protect the eggs from predators and continuously aerate and clean them from encrusting plants and organisms. This intensive brood care by the parents prevents these parent octopods from eating, resulting in their death after several months of starvation during which the eggs develop into the young. In the second, pelagic mode of breeding the brood cases are dispersed over the ocean, and the encased eggs are aerated and cleaned while each case is drifted by the currents.