Climate change as a selection pressure: Cretaceous-Palaeogene extinction
Cretaceous life and climate pre-extinction -
Approximately 65 million years ago the second greatest mass extinction in Earth history had unfolded. The Cretaceous-Palaeocene extinction was a mass extinction of huge proportions which brought an end to 50% of the world’s species along with the dinosaurs in Earth’s most famous mass extinction. It marks the end of the Mesozoic era in which the supercontinent Pangaea began separate into its individual landmasses.
The climate change during this period was significant in relation to the mass extinction that occurred. During the Cretaceous period Earth experienced a relatively warm climate shown by tropical sea surface temperatures which averaged at about 37 °C and for a short time reached up to 42 °C. The warm climate trend was due extensive amounts of carbon dioxide derived from intense volcanic activity and, an increase in magma quantities which further raised sea levels meaning large areas of continental crust were covered with shallow seas. These hot conditions began to change after the end of the Mesozoic era as the Palaeogene period displayed a cooling and drying trend. A cause of this was due to the formation of the Antarctic Circular Component, an oceanic current flowing clockwise from west to east around Antarctica thus cooling the oceanic water temperatures.
The life present before the extinction event consisted of many terrestrial organisms some of which include Ceratopsians (herbivore dinosaurs with horns present on their heads) such as Triceratops, Pachycephalosaurs (smaller dome-headed, herbivore dinosaurs), and Hadrosaurs (‘duck-billed’ dinosaurs) such as Edmontosaurus. The plant life that dominated the Cretaceous period consisted of ferns, seed ferns, Bennettitaleans, conifers, and cycads, but angiosperms (flowering plants) also became part of the flora at this time, about 135 million years ago. But these species would not remain for much longer as mass extinction fell upon the Earth.
Causes of extinction -
There are several theories about the causes of extinction most focusing on climate change due to volcanism and shifting continents. It was thought that tectonically induced climate change disrupted food chains enough to create wide spread extinction amongst the different organisms. The changing climate from hot and humid conditions to cooler and dryer ones left dominant species, such as dinosaurs, unable to compete with mammals for the necessities of food and water meaning their extinction was inevitable. However, this isn't the only factor likely responsible for this mass extinction. Most palaeontologists believe that an asteroid impact is the main reason for this extinction which is supported by evidence found at the Cretaceous-Palaeogene boundary. It was here that large concentrations of iridium were found thus leading researchers to believe that an asteroid struck Earth at the Cretaceous-Palaeogene boundary (as iridium is abundant in comets and asteroids). This asteroid impact would generate a dust cloud which blocked sunlight for many years and also insert sulphuric acid aerosols into the stratosphere meaning the sunlight reaching Earth was reduced by 10-20%. Therefore the process of photosynthesis was inhibited, which would also affect any organisms that depended on such plants (e.g. herbivores).
Life and climate post extinction -
After the extinction it was evident that mammals were rapidly diversifying and transformed from few forms to undergo evolution into most of the species of present day. Some of these mammals evolved in such a way that led them to dominate the land. Plesiadapiformes (an order of primate-like mammals common in the Palaeogene thus eventually leading to humans), carnivorans (any member of the mammalian order Carnivora, which includes dogs, cats, sea lions) and primitive herbivores (such as Condylarths and early Uintatheres) inhabited the forests. Some plant species also survived and evolved into the Palaeogene period, most of which included flowering plants and conifers with ferns remaining abundant for a brief period after the Cretaceous-Palaeogene extinction.
Approximately 65 million years ago the second greatest mass extinction in Earth history had unfolded. The Cretaceous-Palaeocene extinction was a mass extinction of huge proportions which brought an end to 50% of the world’s species along with the dinosaurs in Earth’s most famous mass extinction. It marks the end of the Mesozoic era in which the supercontinent Pangaea began separate into its individual landmasses.
The climate change during this period was significant in relation to the mass extinction that occurred. During the Cretaceous period Earth experienced a relatively warm climate shown by tropical sea surface temperatures which averaged at about 37 °C and for a short time reached up to 42 °C. The warm climate trend was due extensive amounts of carbon dioxide derived from intense volcanic activity and, an increase in magma quantities which further raised sea levels meaning large areas of continental crust were covered with shallow seas. These hot conditions began to change after the end of the Mesozoic era as the Palaeogene period displayed a cooling and drying trend. A cause of this was due to the formation of the Antarctic Circular Component, an oceanic current flowing clockwise from west to east around Antarctica thus cooling the oceanic water temperatures.
The life present before the extinction event consisted of many terrestrial organisms some of which include Ceratopsians (herbivore dinosaurs with horns present on their heads) such as Triceratops, Pachycephalosaurs (smaller dome-headed, herbivore dinosaurs), and Hadrosaurs (‘duck-billed’ dinosaurs) such as Edmontosaurus. The plant life that dominated the Cretaceous period consisted of ferns, seed ferns, Bennettitaleans, conifers, and cycads, but angiosperms (flowering plants) also became part of the flora at this time, about 135 million years ago. But these species would not remain for much longer as mass extinction fell upon the Earth.
Causes of extinction -
There are several theories about the causes of extinction most focusing on climate change due to volcanism and shifting continents. It was thought that tectonically induced climate change disrupted food chains enough to create wide spread extinction amongst the different organisms. The changing climate from hot and humid conditions to cooler and dryer ones left dominant species, such as dinosaurs, unable to compete with mammals for the necessities of food and water meaning their extinction was inevitable. However, this isn't the only factor likely responsible for this mass extinction. Most palaeontologists believe that an asteroid impact is the main reason for this extinction which is supported by evidence found at the Cretaceous-Palaeogene boundary. It was here that large concentrations of iridium were found thus leading researchers to believe that an asteroid struck Earth at the Cretaceous-Palaeogene boundary (as iridium is abundant in comets and asteroids). This asteroid impact would generate a dust cloud which blocked sunlight for many years and also insert sulphuric acid aerosols into the stratosphere meaning the sunlight reaching Earth was reduced by 10-20%. Therefore the process of photosynthesis was inhibited, which would also affect any organisms that depended on such plants (e.g. herbivores).
Life and climate post extinction -
After the extinction it was evident that mammals were rapidly diversifying and transformed from few forms to undergo evolution into most of the species of present day. Some of these mammals evolved in such a way that led them to dominate the land. Plesiadapiformes (an order of primate-like mammals common in the Palaeogene thus eventually leading to humans), carnivorans (any member of the mammalian order Carnivora, which includes dogs, cats, sea lions) and primitive herbivores (such as Condylarths and early Uintatheres) inhabited the forests. Some plant species also survived and evolved into the Palaeogene period, most of which included flowering plants and conifers with ferns remaining abundant for a brief period after the Cretaceous-Palaeogene extinction.