Final answer:
The formation of the Atlantic Ocean during the Triassic Period came first, around 245 to 200 million years ago, followed by the dinosaur extinction at the end of the Cretaceous Period about 65 million years ago.
Step-by-step explanation:
Which Event Came First: Formation of Atlantic Ocean or Dinosaur Extinction?
The Atlantic Ocean began to form at the end of the Triassic Period, characterized by massive volcanic activity that contributed to the break-up of Pangea. This event occurred roughly between 245 and 200 million years ago. The mass extinction of dinosaurs, however, occurred later at the end of the Cretaceous Period approximately 65 million years ago. This dinosaur extinction was marked by a large asteroid impact, which caused a drastic change in climate and life on Earth, leading to the disappearance of dinosaurs and the rise of mammals.