DECAY AND FOSSILIZATION OF NONMINERALIZED TISSUE IN COLEOID CEPHALOPODS

Decay experiments were carried out on three Recent species of coleoid cephalopods (the squids Alloteuthis' subulata and Loligo forbesi, and the sepiolid Sepiola atlantica) over a period of 1 day to 50 weeks. The morphological sequence of degradation and the fate of the more decay resistant organs (beaks, radula, suckers, gladius, statoliths, eye lenses) were recorded. Crystalline magnesium phosphate precipitated, but tissue ultrastructure was not preserved. Sex and stage of maturity may influence rate of degradation. Differences in buoyancy mechanism, physiological changes during reproduction, and post-mortem decay processes affect the highly variable preservation potential of modern coleoids. Of the six genera (Belemnotheutis, Mastigophora, Loligosepia, Geopeltis, Plesioteuthis and Trachyteuthis) of exceptionally preserved Jurassic fossil coleoids examined for evidence of ultrastructural preservation, Mastigophora exhibits a continuous series of tissues from the outer tunic, through the mantle and gladius, to the muscular sheath of the digestive gland. In Belemnotheutis and Mastigophora the radial and circular muscle, the outer collagenous tunic and the supporting meshwork of intramuscular fibres are all preserved. Longitudinal fibres are evident in the arms and in the mantle of some specimens. The texture of the calcium phosphate replacing the soft-tissue varies even within a specimen. Muscles may be represented by the fibres, or only the sarcolemma. The microspheres of calcium phosphate are 1-2 mu m in diameter in the former (perhaps representing the microbes themselves), but only 0.1 mu m in the latter (where precipitation is induced by microbial processes). Microspheres in the tunic are 0.5-0.25 mu m in diameter. Muscle, tunic, intramuscular fibres and ink are preserved in calcium phosphate. Gladius material is finely banded, with varying proportions of diagenetic calcium phosphate and calcium carbonate in each of the layers in Geopeltis from Charmouth. The mantle morphology found in Mastigophora and Belemnotheutis corresponds with that found in living coleoid cephalopods and indicates that this structure had evolved by the Early Jurassic. This calls into question the systematic position of Belemnotheutis as a member of the Belemnitida. It is clear that phosphatization of ultrastructural detail is not confined to a small number of unusual localities. There is considerable potential for histological investigations of the soft-tissues of a range of extinct organisms.