At first, people had only found disembodied plates of this weird fish, then they uncovered a complete skeleton preserved in very good detail.
Phyllolepis was a freshwater fish of Europe and North America.
Even though it was jawless, it was a predator, so it had to suck in its prey. It started the chase by a quick movement of its long, tapering tail, which would make it a very fast hunter. Another thing that it could use to be a very good predator would be ambush. It sat at the bottom of the body of water, then when prey passed, it swam up and sucked its prey in.
One of the strange things is that it either was blind, or had the poorest eyesight of any fish, such as all other phyllolepids. Its name means "leaf scale." Exactly how it sensed its prey is known. It had grooves that could sense any movement in the water, just like Cephalaspis did.
There was another phyllolepid that looks remarkably like it. It is called Austrophyllolepis. It's from Australia, and its name means "Australia leaf scale."
Phyllolepis, like all phyllolepids, was flat. Being flat is perfect for sitting on the bottom of a body of water without being seen by its prey.
In the Devonian Period, placoderms had the widest variety than any other time. That's probably why giant placoderms such as Dunkleosteus and Titanichthys ever evolved. Suddenly, at the end of the Devonian, there was a mass extinction that wiped out all the placoderms, because at that time the placoderms were the top predators and the top organisms of anywhere in the seas. The seas were really the only place on earth where life was really going on very well.