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The answer is Cenozoic

Mammals are classified as organisms consisting of mammary glands and give rise to their young ones. The origin of mammals occurred in the:

  • Cenozoic era

The Cenozoic era began 66 million years ago, which extends till the present. The Cenozoic era is also known as the Age of Mammals, as it led to the thriving of smaller species such as rodents. The large animals like predators were mostly endangered. The era is characterized by the presence of mammals, birds, gymnosperms, and angiosperms.

Reasons for incorrect answers:

  • The Mesozoic era existed 252-66 million years ago. It is characterized by the dominance of dinosaurs and the Triassic and Jurassic periods.
  • The Palaeozoic era began 541 million years ago. It is characterized by the extreme thriving and diversification of marine organisms.
  • The Precambrian era is one of the oldest geologic ages. It is marked by different layers of sedimentary rocks. It consists of fossils of many plants and animals buried in that age.

Therefore, the Cenozoic era is the geologic age in which mammals diversified and thrived.

To know more about Cenozoic era, refer to the following link:

https://brainly.com/question/1367601?referrer=searchResults

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