Top left is Loganellia scotica ("Logan's creature from Scotland"), a thelodont. Logan was a scaley beastie with a very interesting feature: the branchial bars (primitive gill-like organs) were lined with denticle whorls, much like those of modern sharks. These denticles grew inside the throat, and may have functioned like teeth - meaning that Loganellia might be the earliest organism to need a dentist.
Top right is Jamoytius kerwoodi ("J.A. Moy-Thomas' creature from Kerwood"). Jammy has been the centre of controversy since its discovery: was it a basal chordate, an anaspid, a petromyzontiform, a larval ostracoderm, or a stem gnathostom? Current thinking is the latter, though debate rages on like only palaeontological phylogenics can. What is known about Jamm is that it was weird: it sported a single nostril, had no teeth, and 10+ branchial openings which functioned like gills. As of writing, Jamoytius is also a significant record holder: it is one of the oldest vertebrates yet discovered.
Swimming at the bottom is Lanarkia horrida ("Lanark creature horror"), another thelodont. Lanny is noted for its particularly pronounced spiny scales, again similar to modern sharks, albeit much more prominent than in its kindred taxon Loganellia.
I'm imagining them singing a doo-wop trio, which is rather difficult to achieve underwater, I'm sure you'll agree.
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