Final answer:
Ammonites were most prominent during the Mesozoic era, also known as the 'Age of Reptiles,' which is marked by the dominance of dinosaurs and the flourishing of various molluscs including ammonites until their extinction at the end of this era.
Step-by-step explanation:
Ammonites were most prominent during the Mesozoic era. This era is sometimes referred to as the 'Age of Reptiles' because dinosaurs dominated throughout this period. Ammonites, a group of molluscs, flourished and diversified after the decline of the trilobites from the Paleozoic era. The Mesozoic is divided into three periods: the Triassic, Jurassic, and Cretaceous. Ammonites thrived until their extinction at the end of the Cretaceous, around 66 million years ago, which was also the end of the Mesozoic era and when most large-bodied animals of that era disappeared in the Cretaceous-Tertiary extinction.