From “Squid Ink from Jurassic Period Identical to Modern Cuttlefish Ink” (ScienceDaily, May 21, 2012), we learn, that two ink sacs from a cephalopod of 160 million years ago contain melanin “essentially identical” to what we might find in cuttlefish today:
The finding — in an extremely rare case of being able to study organic material that is hundreds of millions of years old — suggests that the ink-screen escape mechanism of cephalopods — cuttlefish, squid and octopuses — has not evolved since the Jurassic period, and that melanin could be preserved intact in the fossils of a range of organisms.
One wonders what would happen if such finds were more common. Apparently melanin resists degradation much more than other organic pigments.