Cephalaspis.

Cephalaspis (seff-uh-LAS-pis) was a bizarre-looking fish that lived in the early Devonian period in fresh water streams and estuaries. It had a horseshoe-shaped headshield, which it could have used for protection or for digging up prey. It probably dug up worms and other burrowing creatures to eat. There were also sensory organs on this headshield. 


Cephalaspis could grow to about one foot long, about the size of a trout. The headshield probably would have slowed Cephalaspis down, because carrying around a heavy shield would be hard to do, even in water, especially for a small fish. Cephalaspis certainly wasn't very fast, so it probably would have relied on the tough head shield for defense. 

©Copyright 2008 by Mike Viney

Cephalaspis had two fins right behind the headshield, and also a long, powerful tail with what looks to me like muscle bands, similar to those on a lancelet. They were probably bottom dwellers, hiding among rocks and debris like modern catfish, sturgeon, and stingrays. 

Most fossils of armored fish only preserve the bony headshield, but in the case of Cephalaspis many fossils also reveal the tail. 


Cephaslaspis was a jawless fish, so it probably had to eat very small prey. But with its bony headshield, it still could have been a formidable hunter of these tiny creatures. It could easily dig up these tiny burrowers with its head and then suck them in quickly, like a modern angel shark. 


Cephalaspis probably had a similar lifestyle to Bothriolepis, a bizarre placoderm from the late Devonian.

References:

http://en.wikipedia.org/wiki/Cephalaspis

http://dinosaurs.about.com/od/tetrapodsandamphibians/p/cephalaspis.htm

http://www.prehistoric-wildlife.com/species/c/cephalaspis.html

http://palaeontology.palass-pubs.org/pdf/Vol%201/Pages%2099-105.pdf

Schinderhannes bartelsi.

Schinderhannes bartelsi was an anomalocarid from the early Devonian. It was 10 cm long. It propelled itself through the water with the two paddles on the head, and steered itself with the smaller paddles near the spine at the posterior end.

Schinderhannes bartelsi had eleven armored segments which had gills on the bottom. Schinderhannes bartelsi is known from one specimen from Germany.

Schinderhannes bartelsi probably hunted trilobites and ate them just like its relative Anomalocaris from the Cambrian and Ordovician periods.

Doryaspis.

Doryaspis was a weird jawless placoderm Heterostracan with a mouth on top of its saw, and two fin-like appendages that could have worked as pectoral fins. But they were not fins, they were part of the head armor. 

Like all placoderms, except for the giant placoderms like Dunkleosteus and Titanichthys, it had to stay near the bottom because of its heavy head armor.

This image shows a different Doryaspis. But all the species of Doryaspsis are pretty much the same. All the species of Doryaspis have two pectoral spines and one spine right under the mouth in front of their head. Which is why Doryaspsis's mouth probably had to point up.