Final answer:
The zenith of hypsilophodontid diversity was during the Late Cretaceous period, the final period of the Mesozoic Era, when dinosaurs reached their peak in diversity and distribution.
Step-by-step explanation:
The zenith of hypsilophodontid diversity was during the Late Cretaceous. Hypsilophodontids, a group of small to medium-sized, fast-running dinosaurs, thrived during this period alongside other diverse dinosaur species. While the Jurassic period was a significant time of growth and spread of dinosaurs, the Cretaceous period, which ranges from about 145 to 66 million years ago, is when dinosaurs, including hypsilophodontids, reached their peak in size, diversity, and distribution.
The Cretaceous was the third and final period of the Mesozoic Era and ended with a significant mass extinction event, the Cretaceous-Paleogene extinction, that resulted in the demise of the non-avian dinosaurs. This period was pivotal for many species and witnessed the evolution of early birds from dinosaur ancestors, and a continued diversification of mammals.
The zenith of hypsilophodontid diversity was during the Late Jurassic and Early Cretaceous period. The Mesozoic Era, also known as the Age of Reptiles, saw the rise and domination of dinosaurs, with the Jurassic period being a golden age for dinosaurs. However, at the end of the Cretaceous period, there was a mass extinction event that led to the extinction of dinosaurs, except for their descendants, birds.