Final answer:
The warmest climates since the Mesozoic occurred in the Early Eocene, characterized by high global temperatures and evidence from isotope records and subtropical fossils. The Tertiary Period's generally warm climate allowed mammalian evolution, contrasting with the cooler Pleistocene's ice ages.
Step-by-step explanation:
The warmest climates since the Mesozoic occurred in the Early Eocene. The Early Eocene is marked by some of the highest global temperatures of the Cenozoic era. During this time, environments such as subtropical climates extended to higher latitudes, and we have evidence of this warmth from multiple data sets, including isotopic records and the presence of fossils that indicate such climates, like the palm trees from the Green River Formation in Wyoming.
The Tertiary Period was generally characterized by a warm and humid climate that enabled mammals to thrive and evolve. In contrast, the Pleistocene experienced a cooler climate that led to ice ages. Despite temporary warm intervals, the average overall temperature was much lower than in the Early Eocene.
Therefore, the addition of various geological and fossil records all support the conclusion that the Early Eocene contained the warmest climates since the Mesozoic era. These warm and humid conditions facilitated the expansion of diverse ecosystems, including the tropical forests symbolized by palm trees, and laid the foundations for many modern species and environments.