Introduction

The appearance of mammals is a pivotal chapter in the story of life on Earth. These warm-blooded, fur-bearing vertebrates would eventually give rise to an incredible diversity of forms, including humans. But when did the first true mammals emerge? Understanding the geological period that marks their origin sheds light on the evolutionary journey that shaped modern ecosystems.

The First Mammals: A Triassic Origin

• Fossil evidence and evolutionary studies reveal that the first true mammals appeared during the late Triassic period, about 225 million years ago1358.

• These early mammals were small, shrew-like creatures that coexisted with dinosaurs, often living in the shadows of these dominant reptiles135.

• The Triassic period, part of the Mesozoic era, was a time of significant evolutionary experimentation, with mammals evolving from synapsid ancestors known as therapsids48.

Why Not Other Periods?

• Tertiary: This period is much later, beginning after the extinction of dinosaurs. While mammals diversified and became dominant during the Tertiary, their origin predates it by over 150 million years35.

• Cretaceous: Mammals were already present by the Cretaceous, but this period is best known for the dominance of dinosaurs and the appearance of flowering plants35.

• Permian: The Permian period saw the rise of mammal-like reptiles (synapsids), but true mammals had not yet evolved45.

• Triassic: The correct answer, as this is when the first mammals appeared and began their evolutionary journey alongside early dinosaurs13578.

Evolutionary Significance

• The emergence of mammals during the Triassic set the stage for their later diversification, especially after the extinction of dinosaurs at the end of the Cretaceous.

• Early mammals were mostly nocturnal and small, adaptations that helped them survive in a world dominated by reptiles15.

• Over millions of years, mammals evolved a range of adaptations, including endothermy (warm-bloodedness), hair, and live birth, which contributed to their evolutionary success.

The Correct Answer

Given the options:

1. Tertiary

2. Cretaceous

3. Permian

4. Triassic

The correct answer is:

(4) Triassic

Conclusion

Mammals first appeared during the Triassic period, marking a crucial evolutionary milestone. Their humble beginnings in the late Triassic paved the way for the incredible diversity and ecological dominance that mammals would achieve in the eras that followed.