ULTRASTRUCTURAL AND CHEMICAL COMPARISON BETWEEN GLADII IN LIVING COLEOIDS AND APTIAN COLEOIDS FROM CENTRAL RUSSIA

Abstract

The paper reports ultrastructural and chemical studies on gladii of living and fossil coleoids in order to elucidate the original composition of the fossil gladii. Gladii of the Aptian Nesisoteuthis simbirskensis Doguzhaeva and of the living coleoids Berryteuthis magister Berry and Loligo sp. were studied with scanning electron microscopy (SEM) and energy dispersive spectrometry (Link). In all three taxa the gladius is laminated, being composed of alterating solid and less solid laminae. In living coleoids each lamina consists of fibrous sheaths. In the fossil N. simbirskensis the laminae are composed of globular aggregates of tiny granules. The globules are ca. 0. 1-0.4 pm in diameter and arranged in chains. Link analyses on the gladii of living coleoids show the elements: S, P, Si, Fe, Cl, Al, K and Ca. The first two elements, S and P, are dominant, whereas Ca has the lowest concentration. The fossil gladius and ink were analysed with the same instrument in the holotype of N. simbirskensis and in an imperfectly preserved, unnamed Aptian coleoid (Hecker & Hecker, 1955, p. 37, note 2). In the two specimens the gladius and ink both show Ca as the dominant element followed by P and S. This indicates that they are composed of calcium phosphate. In N. simbirskensis the split surfaces between the part and counterpart of the gladius are covered by a thin layers of barite. The fossil ink in the two Aptian specimens, like the dried ink in the living B. magister and Loligo sp, has a globular ultrastructure. Originally organic, but post-mortally phosphatized, fish scales, which co-occur with N. simbirskensis, also have a globular ultrastructure but the globules are smaller than those in the gladius. Also the horny<inf/> siphonal tube in ammonites found in the same beds has a globular ultrastructure, whereas the ammonite shell wall and septa show a well-preserved nacreous structure. The role of bacteria in the post-mortern phosphatization which gives rise to the globular ultrastructure of the fossil gladius, ink and organic debris in N. simbirskensis is discussed. 'Me presence of morphologically similar globules is shown in the soft tissues of Recent Loligo after a year of drying, and in the muscle tissue of Recent Nautilus after 20 years in alcohol. In Loligo the globular aggregates represent colonies of bacteria; in Nautilus they seem to be formed by coagulation of proteins in the muscle fibres. The laminated gladius in N. simbirskensis is here interpreted as originally composed of an organic, probably chitinous, material that became phospbatised during fossilization. This agrees with the results previously obtained in Jurassic "fossil squids" (Doguzhaeva & Mutvei, 2003). It is still unclear whether the gladius in extinct squid-like coleoids lost its mineral composition in pre-Jurassic times or whether it was composed of organic material from the beginning. In belemnoids the mainly organic pro-ostracum seems to have developed as an innovative structure (Doguzhaeva & al., 2002, 2005a, b).