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. Th e morphological sequence of degradation and the fate of the more decay resistant organs (beaks, radula, suckers, gladius, statoliths, eye le nses) were recorded. Crystalline magnesium phosphate precipitated, but tissue ultrastructure was not preserved. Sex and stage of maturity ma y influence rate of degradation. Differences in buoyancy mechanism, ph ysiological changes during reproduction, and post-mortem decay process es affect the highly variable preservation potential of modern coleoid s.Of the six genera (Belemnotheutis, Mastigophora, Loligosepia, Geopel tis, Plesioteuthis and Trachyteuthis) of exceptionally preserved Juras sic fossil coleoids examined for evidence of ultrastructural preservat ion, Mastigophora exhibits a continuous series of tissues from the out er tunic, through the mantle and gladius, to the muscular sheath of th e digestive gland. In Belemnotheutis and Mastigophora the radial and c ircular muscle, the outer collagenous tunic and the supporting meshwor k of intramuscular fibres are all preserved. Longitudinal fibres are e vident 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 sarcol emma. The microspheres of calcium phosphate are 1-2 mu m in diameter i n the former (perhaps representing the microbes themselves), but only 0.1 mu m in the latter (where precipitation is induced by microbial pr ocesses). Microspheres in the tunic are 0.5-0.25 mu m in diameter. Mus cle, tunic, intramuscular fibres and ink are preserved in calcium phos phate. Gladius material is finely banded, with varying proportions of diagenetic calcium phosphate and calcium carbonate in each of the laye rs in Geopeltis from Charmouth. The mantle morphology found in Mastigo phora and Belemnotheutis corresponds with that found in living coleoid cephalopods and indicates that this structure had evolved by the Earl y Jurassic. This calls into question the systematic position of Belemn otheutis as a member of the Belemnitida. It is clear that phosphatizat ion of ultrastructural detail is not confined to a small number of unu sual localities. There is considerable potential for histological inve stigations of the soft-tissues of a range of extinct organisms.