Man, this is much more difficult than I thought it was going to be!
So, Carboniferous Scotland, and let's take a look at Crassigyrinus scoticus ("Thick Tadpole of Scotland"), another tetrapod - but one with
something of a twist. While most of Crassi's contemporaries were making
their steps onto land, this beastie was going in the opposite direction -
its limbs atrophying, its tail fin becoming more powerful and pronounced, its general morphology starting to evoke that of an eel or tadpole.
In the time Crassi was discovered (a lower jaw previously ascribed to
Macromerium before a more complete skeleton in the 1980s prompted
reappraisal), such an apparent backwards step would be considered
de-evolution, and rather perplexing to palaeontologists. Yet Crassi's
developments show it was greatly suited for its environment, and likely a
highly effective predator in the shallow river and inlets it prowled.
Its comparatively large eyes would've been very helpful in murky waters,
perhaps even at night; its skull was reinforced to absorb the shock of
struggling prey; it even had a double row of teeth - the interior row
with nasty fangs - to hold its meal in its enormous jaws. At 1.5m long,
it could have been one of the top predators of its domain.
Crassi has had a bit of a moment in popular culture spotlight: it
appeared in an episode of Prehistoric Park, where beloved animal
botherer Nigel Marven wrangled with one on a foray to the Carboniferous
period (presumably before his untimely death at the hands of a rampaging
"Oh, so yer aw goan ontae laund no, are yez?
Watter naw guid enoch fur yez? Whit, ye hink yer better than yer
ancestors, dae ye? Weel AHM gangin' back tae th' sea. See if I care!"