Final answer:
Mass extinctions at the end of the Mesozoic and Paleozoic eras were due to an asteroid impact and massive volcanic activity, respectively, leading to drastic environmental changes and temperature shifts.
Step-by-step explanation:
By the end of the Mesozoic Era, a major mass extinction event took place, commonly known as the Cretaceous-Paleogene (K-Pg) extinction event. Around 65 million years ago, this catastrophic event was triggered by the impact of a large asteroid, resulting in darkness and a drop in global temperatures that devastated plant life and, subsequently, the herbivores and carnivores that depended on plants for food. The end of the Mesozoic marked the transition from the so-called 'Age of Reptiles' to a period of mammalian diversification in the Cenozoic Era.
Similarly, at the close of the Paleozoic Era, the Permian extinction occurred, known as 'the great dying'. This event saw a massive decline in diversity, affecting both terrestrial and marine life, coinciding with significant climatic changes and possibly massive volcanic eruptions. Unlike the mass extinction at the end of the Mesozoic, the end of the Paleozoic was not primarily attributed to an asteroid impact but rather to extreme environmental changes and an increase in global temperatures.
To summarize, large asteroid impacts and massive volcanic activity accompanied by global temperature changes were principal causes of the mass extinctions at the end of both the Mesozoic and Paleozoic eras.