Early Mammals (Mesozoic Era):
* Morganucodon: A small, shrew-like creature from the Late Triassic, often considered a prime candidate for the earliest mammal. It possessed some key mammalian features like a dentary bone in the lower jaw, but also retained some reptilian characteristics.
* Hesperosaurus: This Late Jurassic mammal, with its mix of mammal and reptile traits, was likely an early ancestor of modern mammals.
Specific Groups:
* Placentals: The earliest fossil evidence for placental mammals comes from the Late Cretaceous, around 100 million years ago. These early placentals were diverse and likely gave rise to many of the mammal groups we see today.
* Marsupials: The earliest marsupial fossils date back to the Early Cretaceous, around 140 million years ago. They were likely small, insectivorous creatures that eventually evolved into the diverse array of marsupials we see today in Australia and the Americas.
* Monotremes: The earliest monotreme fossils are found in the Late Cretaceous, around 110 million years ago. These egg-laying mammals are the most ancient lineage of mammals and are unique for their combination of primitive and specialized features.
Important Note: The evolution of mammals is a complex and ongoing field of study. New discoveries and interpretations can change our understanding of which groups emerged first and their relationships to each other.
Overall: It's more accurate to speak of the *earliest known members* of each group, rather than definitively claiming the "first." The fossil record provides a glimpse into a long and fascinating evolutionary history, but it's constantly being refined with new discoveries.