Firstly, series which were completely independent of each other when they were created – often by different writers, and sometimes even in different comics – were later linked to each other. This could be done either by a crossover story in which two or more characters from the respective series meet each other, or by a caption or piece of dialogue explicitly referring to events which occurred or characters who appeared in an earlier story in another series. Two examples of the former kind are the "Judge Dredd" stories "Top Dog" and "Judgement Day", which both featured Johnny Alpha from the series "Strontium Dog" in a major role. Alpha lived over half a century in Dredd's future, and the crossover was accomplished by having Alpha time-travel back to Dredd's era. An example of the second kind is the "Judge Dredd" story "The Cursed Earth," which featured a dinosaur called Satanus who was described as the clone of the offspring of the female dinosaur which had appeared in most episodes of the earlier series "Flesh." "Flesh" was a previously unconnected series about humans time travelling back to the Cretaceous era to harvest dinosaurs for their meat.