A side benefit for paleontologists is that vertebrates
with hard skeletons had a much better chance of being preserved.
There are fragments of bone-like tissues from as early as the Late Cambrian
with the oldest fossils that are truly recognizable as fishes come from
the Middle Ordovician from North America, South America and Australia.
At the time, South America and
Australia were part of a supercontinent called Gondwana.
North America was part of another supercontinent called Laurentia and
the two were separated by deep oceans.
These two super continents, and others that were also present,
were partially covered by shallow equatorial seas and
the continents themselves were barren and rocky.
Land plants didn't evolve until later in the Silurian Period.
In these shallow equatorial seas, a large diverse and
widespread group of armored, jawless fishes evolved:
the Pteraspidomorphi.
The first of our three groups of ostracoderms.
The Pteraspidomorphi are divided into three major groups:
the Astraspida, the Arandaspida and the Heterostraci.
The oldest and most primitive pteraspidomorphs were the Astraspida and
the Arandaspida.
You'll notice that all three of these taxon names
contain 'aspid', which means shield.
This is because these early fishes and many of the Pteraspidomorphi,
possessed large plates of dermal bone at the anterior end of their bodies.
This dermal armor was very common in early vertebrates, but
it was lost in their descendants.
The Arandaspida are represented by two well-known genera:
<i>Sacabampaspis</i>, from South America and <i>Arandaspis</i> from Australia.
<i>Arandaspis</i> has large, simple, dorsal and ventral head shields.
Their bodies were fusiform, which means they were shaped sort of like a spindle,
fat in the middle and tapering at both ends.