Placental Mammals Debut
With the exception of rodents, whales, seals, and sea lions every major group of placental mammals first appeared during the age of dinosaurs. A number very early in the history of the order. Where once it was thought that diversification didn't really start until after the dinosaurs had gone extinct, it is now understood that the placental mammals (hereafter, just 'mammals) began to diversify soon after their appearance in the early Jurassic, and began the great radiation in the Middle Cretaceous.
For most of this time mammals were mostly small insectivores. But there were a few that had other diets, and some did get to a (for a primitive mammal) substantial size. In the Middle Cretaceous larger mammals began to appear, omnivores for the most part, but with some herbivores and carnivores in the mix. It is around this time that the dinosaurs began to die off. In the period between the Middle Cretaceous and the Cretaceous/Tertiary (K/T) boundary entire genera and families of dinosaurs became extinct. Mammals began to diversify, taking advantage of the new environments opening up for them.
[sidebar]
The Troodon
This animal first appeared in the Late Cretaceous. Not the first nocturnal dinosaur to evolve, it was the first one to evolve outside either of the polar regions. It was nocturnal because it hunted mammals, a nocturnal bunch at the time. It became possible for the troodon to exist because by the Late Cretaceous because mammals were now plentiful and active in the open, whereas before they had rare and tended to remain hidden. As you can see, where dinosaurs were going extinct in general, there were new species evolving to take advantage of the changing conditions.[/sidebar]
Conditions were changing. Pangea -the super continent who's formation had been instrumental in the Permian Extinction- was now breaking up. It had started in the Late Jurassic with the split between Laurasia and Gondwana. Now Australia/Antarctica, Madagascar, and what are now the Seychelles were starting to go their own way. Up north the nascent Atlantic Ocean was starting to appear between North America and Eurasia. The dinosaurs had evolved under much different conditions. Back when they first arose Earth's climate was hot and humid, with a substantially higher percentage of oxygen than today. But the start of the Cretaceous the climate was cooler and drier, with a lower oxygen concentration. Evolved for more 'salubrious' conditions a number of dinosaur groups went extinct, to be replaced by new dinosaurs better able to survive the changes. It is only in South America/Africa that the sauropods and stegosaurs manage to survive, along with the (comparatively) primitive theropods. In the north they were replaced by the hadrosaurs, ceratopsians, and tyrannosaurids, with dromaesaurids taking the place of earlier small predatory dinosaurs. It is at this time that the largely reptilian dinosaurs (but more advanced reptiles than any around today) became more avian in biology and behavior. Indeed, had such as the microraptors survived to modern times, we'd likely know them as birds.
For mammals things were starting to look up. More environments to occupy, more territories. Some became diurnal, a number larger in size. More took up eating foods other than bugs. But, inheriting the Earth was not guaranteed. From all the evidence the large northern dinosaurs were likely doomed. But not the southern, who lived in a more agreeable clime. The more advanced small dinosaurs had the best chance to adapt to the new conditions, and may have survived to our time. But events put a stop to that.
[sidebar]
The Deccan Traps
Some 70 million years ago in what is now the Deccan region of India there began what is the largest and most extensive series of volcanic eruptions since the Siberian Traps instrumental in the end of the Permian age. It is around this time that the dinosaurs, already in decline, started a precipitious collapse. Even the small, avian-like dinosaurs began to disappear in great numbers. It is also around this time many species of shellfish started to go extinct. By the K/T boundary the coastal seas and estuaries had seen entire orders go extinct, and the great shell fish beds of the Cretaceous were wastelands. By 65 million years ago dinosaurs were rare indeed, and the mammals began to 'take off'.[/sidebar]
By the time of the impact the dinosaurs were pretty much on the way out. Without the meteor strike maybe some small, predatory dinosaurs may have survived, but not the larger dinosaurs. In ironic contrast it's more likely the (relatively) primitive sauropods of South America could've made it through, thanks to the local climate. But the meteor did strike, and entire orders of life that had once paraded across a trembling land died out.
In the immediate aftermath such as the crocodiles and birds diversified in amazing ways, but their 'domination' would be short lived. Soon enough it would be the day of the mammal, but that is a story for a later time.
With the exception of rodents, whales, seals, and sea lions every major group of placental mammals first appeared during the age of dinosaurs. A number very early in the history of the order. Where once it was thought that diversification didn't really start until after the dinosaurs had gone extinct, it is now understood that the placental mammals (hereafter, just 'mammals) began to diversify soon after their appearance in the early Jurassic, and began the great radiation in the Middle Cretaceous.
For most of this time mammals were mostly small insectivores. But there were a few that had other diets, and some did get to a (for a primitive mammal) substantial size. In the Middle Cretaceous larger mammals began to appear, omnivores for the most part, but with some herbivores and carnivores in the mix. It is around this time that the dinosaurs began to die off. In the period between the Middle Cretaceous and the Cretaceous/Tertiary (K/T) boundary entire genera and families of dinosaurs became extinct. Mammals began to diversify, taking advantage of the new environments opening up for them.
[sidebar]
The Troodon
This animal first appeared in the Late Cretaceous. Not the first nocturnal dinosaur to evolve, it was the first one to evolve outside either of the polar regions. It was nocturnal because it hunted mammals, a nocturnal bunch at the time. It became possible for the troodon to exist because by the Late Cretaceous because mammals were now plentiful and active in the open, whereas before they had rare and tended to remain hidden. As you can see, where dinosaurs were going extinct in general, there were new species evolving to take advantage of the changing conditions.[/sidebar]
Conditions were changing. Pangea -the super continent who's formation had been instrumental in the Permian Extinction- was now breaking up. It had started in the Late Jurassic with the split between Laurasia and Gondwana. Now Australia/Antarctica, Madagascar, and what are now the Seychelles were starting to go their own way. Up north the nascent Atlantic Ocean was starting to appear between North America and Eurasia. The dinosaurs had evolved under much different conditions. Back when they first arose Earth's climate was hot and humid, with a substantially higher percentage of oxygen than today. But the start of the Cretaceous the climate was cooler and drier, with a lower oxygen concentration. Evolved for more 'salubrious' conditions a number of dinosaur groups went extinct, to be replaced by new dinosaurs better able to survive the changes. It is only in South America/Africa that the sauropods and stegosaurs manage to survive, along with the (comparatively) primitive theropods. In the north they were replaced by the hadrosaurs, ceratopsians, and tyrannosaurids, with dromaesaurids taking the place of earlier small predatory dinosaurs. It is at this time that the largely reptilian dinosaurs (but more advanced reptiles than any around today) became more avian in biology and behavior. Indeed, had such as the microraptors survived to modern times, we'd likely know them as birds.
For mammals things were starting to look up. More environments to occupy, more territories. Some became diurnal, a number larger in size. More took up eating foods other than bugs. But, inheriting the Earth was not guaranteed. From all the evidence the large northern dinosaurs were likely doomed. But not the southern, who lived in a more agreeable clime. The more advanced small dinosaurs had the best chance to adapt to the new conditions, and may have survived to our time. But events put a stop to that.
[sidebar]
The Deccan Traps
Some 70 million years ago in what is now the Deccan region of India there began what is the largest and most extensive series of volcanic eruptions since the Siberian Traps instrumental in the end of the Permian age. It is around this time that the dinosaurs, already in decline, started a precipitious collapse. Even the small, avian-like dinosaurs began to disappear in great numbers. It is also around this time many species of shellfish started to go extinct. By the K/T boundary the coastal seas and estuaries had seen entire orders go extinct, and the great shell fish beds of the Cretaceous were wastelands. By 65 million years ago dinosaurs were rare indeed, and the mammals began to 'take off'.[/sidebar]
By the time of the impact the dinosaurs were pretty much on the way out. Without the meteor strike maybe some small, predatory dinosaurs may have survived, but not the larger dinosaurs. In ironic contrast it's more likely the (relatively) primitive sauropods of South America could've made it through, thanks to the local climate. But the meteor did strike, and entire orders of life that had once paraded across a trembling land died out.
In the immediate aftermath such as the crocodiles and birds diversified in amazing ways, but their 'domination' would be short lived. Soon enough it would be the day of the mammal, but that is a story for a later time.
