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The dinosaurs of the Jurassic era (206–144 million years ago) were the tyrants of their world. But, concurrent with those dinosaurs, a mammal-like reptile gained arboreal proficiency, permitting exploitation of the insect fauna that was co-evolving and diversifying with the flowering plants. This key step would eventually lead to the evolution of the monotremes, marsupials and placental mammals.
Marsupials were early descendents of that last common ancestor and were established between 180 and 130 million years ago. The marsupial radiation separated from the placental mammals, and can provide us clues to the details of retinal evolution.
Around 180 million years ago, Pangea, the last supercontinent, began separating into Laurasia and Gondwanaland. That separation coincided with the last common ancestor to the mammalian clades. This continental drift would eventually isolate marsupials to Gondwanaland, especially to Australia, Antarctica and South America. Once isolated on South America and Australia, perhaps with minimal competition from placental mammals, radiation continued, resulting in different orders of marsupials. Seven such orders are extant, with three represented in the New World. With the eventual connection of South America to North America through …