Osteostraci "bony shields". The most advanced agnathans

Australia: The Land Where Time Began

Osteostraci - (ostracoderms) "bony shields" The most advanced agnathans

These fish have not been found in Australia, being restricted to the Euramerican landmass. They have been found in Old Red Sandstone deposits in Britain, Europe, western Russia, Spitsbergen and North America from the Late Silurian to Late Devonian. They had round openings for the eyes in the bony shield, and between these openings, a small pineal opening. There was also a small key-shaped opening in the shield for the nasal organs. There are sensory fields on the sides of the shield and on top of it. There were well-developed spiny processes, cornua, that projected towards the rear, in many species. In juveniles the shields was comprised of many tesserated smaller plates that fused into a single shield at maturity. They had well-developed pectoral fins. Internally the fins were attached to simple ossified shoulder girdle bones, scapulocoracoids, a simple paddle-shaped cartilage support articulated to the girdle Ateleaspis from Scotland had 2 dorsal fins and it is believed many other species may have had 1 or 2 dorsal fins.

The paired fins disappeared secondarily in some osteostracans such as Termataspis; in these species the shield had the shape of an olive.

The underside of the shield is mostly open beneath the cavity of the mouth and gills, but was covered with many small platelets when alive. There were up to 10 gill slits. Brain casts have been found in a number of osteostracan fossils, indicating that their inner ear was composed of 2 semicircular canals, 3 being present in higher vertebrates. The plan of the cranial nerves and blood supply to the head was similar to that of lamprey larvae (Janvier, 1995). A great diversity of forms resulted from a major radiation of the osteostracans, ranging from simple head shields with a semicircular shape, as occurred in Cephalaspis. Mchairaspis had prominent dorsal spines, while in others, such as Thyestes, Nectaspis and Dartmuthia, had elongated shields that covered most of the body. The scales on the tail were thick as arranged in rectangular units capped by a series of smaller scales on the back, with series of smaller scales on the ventral side.

The lifestyles of osteostracans have been reconstructed (Afanassieva, 1992), suggesting that there were 2 kinds, mostly bottom dwellers and strong swimmers that were not confined to the ocean floor. The benthic forms had heavy shields and lacked paired fins, making them less mobile. It is suggested that for forms such as Termataspis their only means of movement was by using their short tails to move them by flits. According to the suggestion that they remained on the sea floor sifting through the mud for nutrients, the weight of their shields aiding in keeping them on the mud.

The osteostracans that were strong swimmers had long tails and well-developed pectoral fins, as in Cephalaspis and related forms. As in forms such as Parameteoraspis, some had wide shields that had bizarre shapes as a defense. They are believed to have been powerful swimmers that could rapidly move away from danger. It has been suggested that in all osteostracans the pore-canal system could possibly been an organ that secreted mucous to reduce drag from the water, allowing them to swim faster or move along the bottom. Thick scales have been found on all well-preserved fossil osteostracans. These scales were often arranged in rectangular units that were vertically arranged and capped by a series of smaller ridge scales along the back, meeting another series on the ventral side.

Recent work has greatly elucidated the phylogeny of the osteostracans (Sansom, Robert, 2009).

They became extinct early in the Late Devonian.

Sources & Further reading

John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995
Long, John A., 2011, The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 2011

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