Arandaspida

Sacabambaspis from Bolivia, the best researched Arandaspide.

• South America ( Bolivia and Argentina)
• Australia
• Oman

The Arandaspida, also called Arandaspidida, are a group of extinct, fishy, armored vertebrates and known from fossils from sediments of the Ordovician. They occurred exclusively in the sea.

Features

The Arandaspida were small to medium sized fish with body lengths of 10 to 40 cm. They had a fish-like, elongated shape with a tail fin. The head and the front part of the body was covered by a rather flat dorsal and convex ventral bone plate, between the gill openings were with special gill plates. Dorsal and ventral bone plate were composed of smaller bone plate ( " tesserae " ), who were joined at their base. On its outer surface, they showed teardrop-shaped or shaped like oak leaves tubercles. They probably had no Dentinauflage. Between dorsal and ventral bone plate located approximately 20 smaller, diamond-shaped plates between which were the numerous (10 to more than 15 ), small gill openings were. All bone plates were very thin (<1 mm), and consists of three layers of a thin basal layer, a sponge- like middle layer and a porous outer layer. The eyes that were above the horizontal, surrounded by numerous, elongated, small shed mouth opening in a very far forward bulge of Dorsalschildes were protected by a strong bone ring. Between the eyes were two small, closely together standing openings that may have been nares or contained a paired pineal gland. Like most recent fish possessed the Arandaspida a lateral line, which lay in narrow pits between the tubercles on dorsal and Ventralschild.

The internal anatomy of animals is unknown, the rear body may have been laterally flattened and covered with long, vertically standing shed. The only incomplete handed tail fin was spade-shaped and diphycerk policy. Paired fins, dorsal and anal fin missing.

System

The Arandaspida are the first vertebrates, whose membership is not doubted to the Vertebrata ( in contrast to the Conodonta ). Within the vertebrates they belong because of their large dorsal and Ventralplatten to Pteraspidomorphi. There are four genera known, of which Sacabambaspis is best explored. All are from the middle Ordovician, fragmented bones of Arandaspida but were also recovered from the lower and upper Ordovician.

Genera and localities:

• Arandaspis (Australia)
• Porophoraspis (Australia)
• Sacabambaspis (Argentina, Bolivia, and Oman)
• Andinaspis (Bolivia )