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1 Régis Chirat. CNRS UMR PEPS Paléoenvironnements et Paléobiosphère, U.F.R. des Sciences de la Terre, Université Claude Bernard Lyon I, 27-43 Boulevard du 11 Novembre 1918, F-69622 Villeurbanne Cedex, France. chirat{at}univ-lyon1.fr
No egg of any fossil nautiloid has yet been discovered. However, anomalies of embryonic shell growth, described for the first time in several Mesozoic Nautilida, provide important clues on morphology, structure, and size of their egg capsules; on the physical characteristics where egg laying occurred; and on the hatching processes. Roughness inside the inner egg capsule—caused by hard and uneven egg-laying substrate, locally and temporarily slowing down or stopping the apertural shell growth—could cause temporary deformations of growth lines. Such roughness, caused by stone, is described inside an egg capsule of Nautilus, which was fixed obliquely relative to the egg-laying substrate. This reduced the space between the inner and outer capsules, which locally fused together. The lateral-umbilical grooves, furrows, and deformations of growth lines were probably caused by the inner egg capsule during the prehatching stage. In fossil Nautilida, as in Nautilus, the size of this capsule was relatively small compared with the shell diameter at hatching. During the last stages of embryonic development, the shell extended backwards outside the egg capsule before hatching. This prehatching stage, during which the egg capsule continued to press against the shell, can be marked by a prehatching constriction. In fossil species, as in Nautilus, the inner capsule constituted a kind of "straitjacket" during the last stages of embryonic development. The expansion in whorl width at hatching, in normal as well as in abnormal shells, marks release of this straitjacket. Important deformations of the whorl section probably result from an abnormal form and size of the egg capsules mainly caused by the manipulations by the female during the egg laying on a hard and hollow substrate, increasing the straitjacket effect. An alternative explanation could be that the chorion did not expand adequately. From relatively early embryonic stages (approximately 180° adapical of the nepionic constriction) to hatching, both flaps of the hyponome could be turned backward under the shell, jammed between the inner wall of the egg capsule and the mantle margin, resulting in the formation of paired ventral parallel grooves. Many normal features of the embryonic development of nautiloids can be clarified through the study of the anomalies of embryonic shell growth.
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