When time travel was invented, it of course became a very important industry,
being used for many purposes. One of the first things to be done was to
bring back extinct and endangered species. They were all very popular attractions
at zoos and parks for a generation. But for the next generation, who had
never known a world without them, they held no special interest. They were
still popular; just no more so than any creature that had always been abundant.
Of course, there was one exception to this. The dinosaurs' popularity never waned. In fact, the many Prehistory Parks damn near put Disney out of the amusement park business. Because of their immense popularity, more dinosaurs were always being imported from the past. That is, until there were none left in the past. That was when everyone realized that dinosaurs had not become extinct at all, they had merely been moved from one time to another.
There would have been no problem in this, except that not all the dinosaurs had been imported by the proper authorities. There had always been people who went back in time to engage in illegal poaching. Some killed dinosaurs for sport, while others were professionals who captured dinosaurs to sell on the black market to private collectors. And as many of these were stuffed and mounted as kept alive.
When the past failed to provide new specimens, the poachers turned their efforts towards the Prehistory Parks. No security was enough to keep them from stealing the dinosaurs (surprising but true).
This went on for months. And these days it was nearly impossible to keep live dinosaurs and not be found out by the World Bureau of Investigations. No one wanted to chance the mandatory life sentence for such a crime, so they usually had their showpieces killed. Stuffed dinosaurs were easier to keep undetected, even if the penalty for that was far greater. The dinosaurs were becoming extinct. For real this time.
Then, an unexplainable thing happened. Every live dinosaur suddenly died, overnight. The only apparent reason was old age, even though none of them had been nearly as old as their bodies now seemed to be. A few people thought of an answer, and hoped they were right. But they said nothing. And so, the mystery went unsolved...
"...until a hundred years ago, when we started bringing the dinosaurs in from the Old Parks to the safety of our own far more secure wildlife reserves. Years later, when they died of old age, we returned their bodies to the Old Parks, on the night from which they were taken. Since then, their descendants have remained in parks like this one, undisturbed by the outside world. We hope you've enjoyed the tour. Please come again."