Complex sentence: Landseer had asked for a dead lion to be brought to his studio from London Zoo, but took so long to complete sketches that it had begun to decompose so much that some parts were improvised.
Split into simpler sentences: Landseer had asked for a lion which had died at the London Zoo to be brought to his studio. Afterwards he took so long to complete sketches that the lion's corpse had begun to decompose so much that some parts were improvised.