The theory is that the attempt of a group of researchers to scan the later
Jurassic period, around 152 million years ago, botched. The botch sent the cities around a dozen major universities back in time.
Luckily, late TL9 cities with cheap fusion and vertical agriculture are less fragile than most earlier cities. But the next generation was still brutal. Now, fifty years after the transference, the cities have become stable. Now there is an urge to explore this alien Earth.
Basically, reread
Xenozoic Tales and rewatch
Land of the Lost(1974), then blend to taste. Pulpy weirdness to taste.