During the reading material, we know that 450 million years ago there are a group kind of marines called Agnostids had became extinct. Not there are some fossil of Agnostids remain in many areas of the world.  We only found 3 theories about how agnostids may have lived.
The first one is that agnostids may have been free-swimming predators in order to hunted smaller animals. And from listening, I've known that agnostids have very small eyes, some almost blind.
Secondly, agnostids may be seafloor dwellers, they may live on the seafloor before.