ICHTHYOLITES OF THE OLD RED SANDSTONE

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OSTEOSTRACI

 Catalogue of Fossil Fishes AS Woodward 1891 Cephalaspis murchisoni - Ledbury

The Osteostraci or Cephalaspids as they were once known are the most ‘famous’ of all fossil agnathans. The first forms are seen in the Wenlockian of Scotland (Ateleaspis) and the last (Escuminaspis) in the Frasian of Miguasha, Canada. They are thought to have lived in shallow marine/deltaic environments, but there is some suggestion that rivers may have played a part in their life-cycle. It is thought they were mostly suspension-feeders and may have been made extinct by the massive expansion of the antiarch placoderms which occupied the same niche in the mid-Devonian

They are identified by a large bony headshield containing the eyes, nasal apparatus, pineal opening and the characteristic cephalic sensory fields at the sides of the shield. Most Osteostraci have a horseshoe-shaped headshield with sharp posterolateral cornual processes, to the inner margins of which the pectoral fins are attached.

Osteostraci Classification

Osteostraci

        Ateleaspis - Early Silurian (Wenlockian), Generalised form

        Hirella

        Hemicyclaspis

        Cornuata

                Cephalaspidida

                Zenaspidida

                Thyestiida

It has been suggested that most of the osteostracan diversication occurred before the Wenlockian because this period sees both Ateleaspis the most generalised primitive form and derived cornuates such as Tremataspis.

They are widely distributed across the Old Red Sandstone of Europe and North America. The UK has some spectacular sites.

Osteostraci in my collection (click on thumbnails to see larger images)

Ateleaspis tessellata traquair           Silurian (Wenlockian)    Lesmahagow/Hagshaw inlier, Scotland

Ateleaspis is earliest vertebrate to have paired appendages. As with all Osteostraci, they also have a network of sensory fields made up of loose platelets of bone  (not differentiated in ateleaspis) located along the mid-line and the lateral edges of the headshield. These sensory areas are  connected to a complex of nerve channels and ducts which eventually connect with the labyrinth of the brain. It is thought that these area function as follows; when the animal moves up in the water the platelets are pushed down on the channels below them sending a signal to the portion of the brain associated with orientation and movement 

In Ateleaspis, the sensory fields  extend into the fins which are an extension of the headshield. Thus it has been suggested that the fins and fields developed together as means of stabilizing swimming along with the sense organs to provide feed-back control for it. 

           An extraordinary complete dorsal fish with the head and body preserved in a nodule and with the tail preserved in  fish-bed. A true masterpiece of fossil preparation.

        An almost complete animal ventrally preserved in a nodule.

   Pos/Neg of an Ateleaspis head preserved in fishbed

       Pos/Neg of 3D Ateleaspis found in nodule

              Pos/neg of 3D preserved oralobranchial chamber

Hirella gracilis    Upper Silurian    Ringerike, Norway

Hirella is a basal osteostracan only marginally more complex than Ateleaspis. It has narrow paired fins and an oralobranchial chamber closed ventrally by large dermal platelets

   From Heintz 1939

       Mass-mortality plate

       Ventral presentation     

    Tail

Zenaspids 

The Zenaspids are usually large osteostracans with tuberculated ornamentation and a large hypophyseal foramen. They are found in the Lower Devonian of Britain and Podolia  They have a massive headshield with posteriorly enlarged sensory fields and thick, narrow cornual processes.

Cephalaspis sp. x 2  Welsh borders (part of the Wonderblock...see also acanthodian)

For the last 150 years, the Lower Devonian  outcrops along the banks of the Dniester river in Podolia have been yielding wonderful fish remains. Despite the first descriptions of cephalaspids from the region being in 1874, little work has been done on the 19 species known, although recent papers by Janvier and Voichyshyn go a long way to addressing this lack of information.

The Osteostracans of Saarema island in Estonia, while being very early in the fossil record are a highly specialised group of small thyestiida. While most of this group show the classic cornuate morphology (Thyestes) others such as Witaaspis and Tremataspis are non-cornuate.