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