Gerrothorax pulcherrimus, a temnospondyl amphibian from the Middle and Late Triassic (~235-201 mya) of Germany, Greenland, and Sweden. Growing to about 1m long (3′3″) it had an incredibly flattened body, upwards-facing eyes, and kept its external larval gills even as an adult – adaptations for a life spent lurking at the bottom of rivers and lakes, waiting for prey to swim close to its huge mouth.
Since laying flat would have made it difficult for Gerrothorax to move its lower jaw, it had to develop an entirely different way of opening its mouth. Instead, it lifted its whole head upwards in a motion that’s been compared to a toilet seat.
I suppose you could say it had a potty mouth, then.