The Pteraspidomorphi are sometimes referred to as Heterostraci, although that term is more
properly applied to the specialised Silurian and
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.
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,
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.