Observe the given evolutionary sequence leading to mammals: Early reptiles Synapsids Pelycosaurs [X] Mammals Identify the missing ancestor [X] and the geological era in which mammals first appeared on Earth.

Step-by-step solution

According to the evolutionary history of vertebrates, the synapsid lineage gave rise to mammals. The correct sequence is: Early reptiles Synapsids Pelycosaurs Therapsids Mammals. Therefore, [X] is Therapsids. While mammals dominated the Earth during the Cenozoic era, they first originated much earlier during the Triassic period, which is a part of the Mesozoic era. Thecodonts belong to the sauropsid lineage, which led to dinosaurs, crocodiles, and birds, not mammals. Answer: [X] = Therapsids ; Era = Mesozoic