Which phase of the Paleozoic Era includes the Mississippian and Pennsylvanian periods?

The Upper Paleozoic Era encompasses the latter part of the Paleozoic Era, which includes the Mississippian and Pennsylvanian periods. This era is characterized by significant geological and biological changes, including the development of vast coal swamp forests and the diversification of amphibians and early reptiles. The Mississippian Period spans from around 359 to 323 million years ago, while the Pennsylvanian Period follows, lasting from about 323 to 299 million years ago. These periods are known for their rich deposits of coal, formed from the dense vegetation that thrived in warm, humid environments.

In contrast, the Lower Paleozoic Era includes earlier periods such as the Cambrian and Ordovician, which are known for the emergence of diverse marine life but do not include the specific time frames mentioned in the question. The Mesozoic Era, which follows the Paleozoic, is characterized by the age of reptiles and the presence of dinosaurs, while the Cenozoic Era focuses on the development of mammals and birds. Thus, the Upper Paleozoic Era is the only correct choice for this question.