Complex sentence: The two dancing terrapins first appeared on the cover of the 1977 album "Terrapin Station", which was drawn by Kelley and Mouse, but based on a drawing by Heinrich Kley.
Split into simpler sentences: The two dancing terrapins first appeared on the cover of the album "Terrapin Station" (1977). They were drawn by Kelley and Mouse, based on a drawing by Heinrich Kley.