1) Aquatic vertebrate 
• (n.) One of the Placodermi.
any member of an extinct group (Placodermi) of primitive jawed fishes known only from fossil remains. Placoderms existed throughout the Devonian ... [5 related articles]
Placoderms meaning plated skin) were armored fish that evolved during the Silurian Period, about 420 million years ago. They diviersified and came to dominate the seas by the Devonian. They went extinct bout 355 million years ago. They had hinged bony armor on their head and thorax. They had no teeth, but did have bony ridges that acted like teeth....
Plac'o·derm noun [ Greek ..., ..., tablet + ... skin.] (Paleon.) One of the Placodermi. 
[n] - fishlike vertebrate with bony plates on head and upper body
noun fishlike vertebrate with bony plates on head and upper body; dominant in seas and rivers during the Devonian; considered the earliest vertebrate with jaws