Astraspis

Astraspis desiderata

• North America (Colorado, Arizona, Oklahoma, Wyoming and Quebec)

Astraspis ( = " star- covered tank" ) is a genus of extinct, fishy, armored vertebrates and known from fossils from marine sediments of the Ordovician. Fossils of the group were discovered in the 19th century. They were the first vertebrate fossils from the Ordovician until the discovery of Arandaspida in the 1970s.

Features

In Astraspis head and oval in cross-section front hull by dorsal and Ventralschild and a number were in between of sheltered Branchialschilde. Both shields consisted of numerous mushroom-like, star-shaped or polygonal tiles ( tesserae ) of aspidin that were occupied in the center with large tubercles of dentine, which in turn contributed a cap of translucent enamel. The bone shields are relatively thick and consisted of three layers, a lower disc layer, a middle traversed by vessels spongösen layer and the superficial tubercles, each possessed of a limited dentinartiger substance pulp. The central tubercle is the place, ran out of ossification of the juvenile specimens. They grew by the addition of additional smaller tubercles. The Dorsalschild possessed strong longitudinal ridges. Between the Branchialschilden were eight ( Benton ) or ten gill openings. The Carapace show signs of local resolutions and recharge. This could indicate injury and subsequent healing processes. The rear hull was protected by rhombic scales and ended in a rounded caudal fin. Paired fins, dorsal and anal fin missing. The eyes were sideways. A lateral line system is present. It is located in pits in the bone shields. The internal anatomy of animals is unknown.

System

Astraspis is assigned to the Pteraspidomorphi and was initially placed within the Heterostraci within this group because of the acellular structure of the large dorsal and Ventralschilder. However, since these had only an external gill opening per head, Astraspis but up to ten, they are now placed in the taxon Astraspida ( Astraspidiformes in Nelson). Two species have been described, Astraspis desiderata and Astraspis splendens. Some authors even the fragmentary remains known only by a few species Eriptychius from the Harding sandstone on the edge of the Rocky Mountains ( Middle Ordovician ) is ranked among the Astraspida (otherwise Eriptychiida ).