What made placental mammals more successful than marsupial mammals?
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The Permian-Triassic extinction event, which occurred approximately 250 million years ago, led to the extinction of the dominant group of carnivorous animals known as Therapsids. These Therapsids played a significant role in giving rise to the defining characteristics of present-day mammals. Recent studies suggest that the common ancestor of Placental Mammals (Eutheria) and Marsupials (Metatheria), known as the first Therians, emerged during the late Triassic epoch. While initially insectivorous, this group diversified and adapted during the Mid Jurassic epoch.
It’s important to note that during this period, Earth’s landmasses were merged together as the supercontinent Pangaea.
The earliest known Metatherian fossil dates back to around 125 million years ago. Scientific consensus points to Asia as the location where the divergence of Metatheria from Eutheria occurred, approximately 125 million years ago.
From Asia, a group of Marsupials migrated to North America, which was still connected to Europe at that time. Subsequently, they further migrated to South America, which was joined with North America, giving rise to the diverse populations of marsupials found in present-day South America. Another group of Marsupials migrated southward to Oceania, including present-day Australia. However, the initial population that reached Laurasia (Europe and North America) became extinct.
The exact reasons for the extinction of the original population are unknown, although Metatherians were heavily affected by the K-Pg extinction event. It is also worth noting that there is no direct correlation between Metatherian and Eutherian diversity. Early Metatherians occupied open arboreal territories.
Due to the separation of Australia from Pangaea, the continent became home to a large population of Metatherians, while Eutherians did not reach this isolated landmass. Both Metatherians and Eutherians have their own successful evolutionary strategies, but the isolated nature of Australia allowed for the development of a diverse range of marsupials at their own pace.