A B C D E F G H I J K L M N O P Q R S T U V W X Y Z #0-9

Drushchits V.V., Doguzhayeva L.A. (1978) The structure of the skeletal elements of Early Cretaceous ammonites and their systematic importance // Malacological Review, Vol. 11. Pp. 134-137

The introduction of the scanning electron microscope into the practice of paleontological research has made it poSSiole to study the texture, microstructure and ultrastructure of the skeletal elements of cephalopod molluslcs and to clarify their significance in systematics. To pursue this study the shells of 20 genera of early Cretaceous ammonites belonging to 3 orders (Phylloceratida, Lytoceratida and Ammonitida) were studied. The protoconch of all the ammonites studied was either globular or roller-like in form, with a diameter ranging from 0.25 to 0.8 mm. Its wall consists of 1 prismatic sublayer in the apical portion, and 2 prismatic sublayers in the region where the proseptum is attached to the ventral wall. The wall of the 1st whorl consists of 1 prismatic layer. The primary varix, which is a thickening of the shell wall, is located at the end of the 1 st whorl; its position is defined by "the angle of the primary varix," the size of which ranges from 225 to 340 degrees. It is formed at the expense of the nacreous layer which appears immediately before it or somewhat earlier, remaining very thin at some distance from the primary varix. The primary varix is usually of an ir belinia sp. In regard to the trematodes, metacercariae of didymozoids (type Torticaecum; 12.5%), Hirudinella ventricosa (1.05%) and a single mature didymozoid trematoda (0.09%) were found. An analysis of the species composition of helrninths from ommastrephids has revealed 2 helminthcoenoses, coastal and oceanic. The main coastal forms are Anisakis sp. 1 1 , Phyllobothrium sp. 12, Dinobothrium sp. and Scolex pleuronectis. The main oceanic forms are Tentacularia coryphaenae, Nybelinia yamagutii, Phyllobothrium sp. 11, Anisakis sp. 12, Porrocaecum sp. and metacercariae of the Didymozoidae. Such distinct differences in the species composition of the above helrninthcoenoses refer not only to ommastrephids, but are a reflection of coastal and oceanic biocoenoses existing in the ocean. A great specific variety of the helrninths, and rather high indices of extensity and intensity of infection in the oceanic ommastrephids, are stipulated by their high ecological valence, as compared to the coastal representatives of this family. The intraspecies and local variability in squid infection depends on the presence or absence of masses of parasites of ommastrephids, definitive hosts of helrninths, and lastly on the historically formed trophic relations of the squid in a given region. All helrninths found in ommastrephids possess a wide specificify, and use for the same developmental stages many species of teleost fishes as intermediate hosts. The ancestors of ommastrephids "conquered" recent ecological niches on account of this or that group of teleost fishes, and acquired their helrninths. Thus, the helrninth fauna of ommastrephids is secondary in character and consists of typical "fish" forms .