This page explains the current hypotheses about the way an impact could have selectively killed the Mesozoic animals, as well as a description of the disappeared species.
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The scheme of the extinction
Debates last for a long time about the sudden or gradual nature of the extinction. The data for terrestrial vertebrates are quite poor, and the statistics have been very, very controversial, but the study of marine sediments (see disappeared species) seems to speak in favour of suddenness.
How did the asteroid impact kill the dinosaurs (and the lots of other species which disappeared at the same time, see disappeared species) ? The violence of the impact itself (possibly with a great blaze) is not likely to have killed some species world-wide and left some others unharmed. Acid rains have been suggested, but they mainly affect aquatic species, lots of which survived. A long-term cooling of the climate is subject to the same restrictions than those for the marine regression hypothesis.
The most harmful consequence of the impact was probably the dense dust cloud due to the explosion. It prevented sunlight from reaching the earth. The same dust cloud settled on the ground a few months later, leaving the iridium-bearing layer. Arguably, this cold and dark period could have been followed by a much warmer period due to some greenhouse effect.
The selectivity of the extinction is (roughly) explained by studying the different food chains. The obscurity halted photosynthesis, and plants were the first to be affected, together with the phytoplankton in the seas. Plant-eating animals were decimated, and flesh-eating animals went together with them. On the contrary, all the invertebrates living on corpses and aquatic organic waste flourished, and with them, the small, insectivorous vertebrates: fishes, lizards, mammals etc. The animals in the plant-based food chain, like the big dinosaurs, disappeared, whereas the insect-based food chain was not affected, and the kind of small dinosaurs that birds were survived.
The size of the animals may have played a role too. Big animals are generally less numerous, so more vulnerable and then statistically more likely to disappear, the more as they have greater feeding needs.
Thus, the consequences of the impact itself seem sufficient to explain the extinctions.
The disappeared species
The species extinction rate at the K-T limit has been evaluated from 50% to 80%. In any case, the extinction caused a global reorganisation of the ecosystem, mammals taking the place of dinosaurs as the dominant land vertebrates. In the air, however, the success of the group of dinosaurs known as 'birds' is impressive: there are today about 6000 species of mammals, and some 8000 species of flying dinosaurs.
On land, all the non-avian dinosaurs disappeared (the stegosaurs became extinct in the middle Cretaceous, before the K-T event). There have been some records of dinosaur fossils in the Tertiary, all of which appeared to be fossilisation artefacts. Pterosaurs disappeared as well. Other land vertebrates were severely affected but did survive, especially marsupials and multituberculates. A few placental mammals and turtles became extinct.
In the seas, plesiosaurs and marine lizards (mosasaurs) became extinct (ichthyosaurs had already disappeared). Fishes were affected, especially the sharks. Among the invertebrates, belemnites and ammonites, which had been very successful throughout the Cretaceous, disappeared (it had been thought that ammonites had declined before the K-T, but this has recently be shown to be false). Some other less well-known groups of invertebrates became extinct just after having known a radiation.
The most drastic extinction regards the plankton. Coccolithophores (unicellular algae with a shell made of minuscule calcareous scales) and planktonic foramenifera (unicellular organisms with a calcareous shell) became almost extinct. This results in a sudden and important fall in the carbonate concentration of the sediment, which is simultaneous to the iridium-bearing layer, an argument in favour of a catastrophic rather than gradual extinction. Diatoms and radiolarians (with siliceous skeletons) were severely affected, though they were very diverse at the very end of the Cretaceous. On the other hand, the benthic (living on the sea-bottom) species resisted well to the extinction. The dinoflagellates (with chitinous shell) throve as well, probably due to their ability to migrate and hibernate.
The terrestrial plants were affected too, though less severely due to the resistance of their spores or rhizomes. Ferns, as usual after a catastrophe, appear to have been the first plants to recover (one more hint in favour of a catastrophic, not slow, change), as great amounts of spores are found just above the iridium. Strangely, tropical plants were less affected than the north plants, supposedly better adapted to the cold.
In the oceans, the small organisms rapidly got back their previous diversity (except for the totally extinct ammonites and belemnites), which is compatible with a short and intense crisis. The plants quickly recovered as well. The main consequence of the K-T extinction remains the arising of the mammalian age, and we are the first concerned.
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