Palaeos:		Pteraspidomorphi
The Vertebrates		Overview

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Pteraspidomorphi: Overview

Abbreviated Dendrogram

Vertebrata
|--Conodonta
`--+--Thelodonti
   `--Pteraspidomorphi
      |--Arandaspida
      `--Heterostracomorphi
         |--Astraspidae
         `--Heterostraci
            |--Cyathaspidiformes
            |  |--Cyathaspidida 
            |  `--Amphiaspidida
            `--Pteraspidiformes
               |--Pteraspididae
               `--Psammosteida
                  |--Drepanaspis
                  `--Psammosteidae

Overview
Pteraspidomorphi
Heterostraci
Heterostraci (2)
Amphiaspidida
Pteraspidiformes
Psammosteida (1)
Psammosteida (2)
Dendrogram
References

Overview

Pteraspis. Pragian (Early Devonian) of southeast Euramerica. Length 20 cm. Order Pteraspidiformes

The Pteraspidomorphi are sometimes referred to as Heterostraci, although that term is more properly applied to the specialised Silurian and Devonian forms. They are an extinct clade of jawless fish, and were the most abundant and diverse vertebrates of the Silurian. They are distinguished by having an exoskeleton or bony shield composed of several plates, usually a dorsal (upper or back) shield, a ventral (lower or belly) shield, branchial (gill) plates, and a number of smaller plates around the areas of the mouth (oral plates) and eyes. These plates consist of a mostly acellular form of bone, called aspidine, which is believed to be the ancestral condition for the dermal or skin/exoskeleton bone of all bony vertebrates. The body is covered by many scales, each of which has ornament matching the type seen on the larger dermal bones. In all of the more derived forms, there is only one, common gill opening on each side.

Ordovician Pteraspidomorphs: 1. Astraspida; 2. Arandaspida

The Pteraspidomorphi include the earliest known vertebrates. The oldest certain remains date to the Early or Middle Ordovician, although the disputed and fragmentary Anatolepis probably goes back to the Furongian. In any case, by the middle and late Ordovician, there were several different lineages evolving in isolation different parts of the world, as this map indicates.

These Ordovician forms, the Arandaspida, Astraspida and Eriptychiida, were formerly grouped with the Silurio-Devonian Heterostraci, but are rather more primitive. These early types share some unique features with heterostracans, such as the presence of large median dorsal and ventral plates, but do not possess common external branchial openings.

By the start of the Silurian period these lineages had died out, possibly as another result of the mass extinction at the end of the Ordovician. They were replaced by the Heterostraci. This latter group underwent an evolutionary radiation, dividing into a number of lineages and reaching their peak during Late Silurian and Early Devonian times, when a variety of different types evolved and flourished, from mud-eating bottom-dwellers to free-swimming filter-feeders. All had the characteristic head shield, which could grow throughout the life of the animal.

By the middle Devonian the Pteraspidomorphi went into decline, with only a single family of giant (by agnath standards) flattened bottom-dwellers, the Psammosteidans, continuing almost until the end of the Devonian, the last and also the largest of the armoured jawless fish. MAK000112.

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