The two main hypotheses that compete with the impact theory are the marine regression hypothesis (with a cooling of the climate) and the volcanic hypothesis. There are still some more hypotheses, some funny.
The marine regression hypothesis
This is the main alternative to the meteoritic hypothesis. It does not exclude the existence of an asteroid impact, but the impact is considered as a deathblow to an already declining ecosystem.
The Cretaceous marine regression is well established by several independent geological data. In the seas, the regression would have dried up a great part of the continental shelf, killing lots of the species living there. On land, the consequence of the regression would have been a more continental climate, with more severe winters. The harsh climate would have been fatal to dinosaurs. Moreover, the coastal plains, where lots of dinosaurs lived, would have been reduced by the regression. Lots of the in-land seas, common at that time, disappeared.
Yet the precise scheme of the extinction is not compatible with this hypothesis. First, it is often claimed that the fossil records show a decrease in dinosaur diversity at the end of the Cretaceous. The main marine regression took place a few million years before the end of the Cretaceous, at a time when dinosaurs were actually flourishing. The (terrestrial) fossil record at the very end of the Cretaceous is too poor to tell whether the extinction was sudden or slow.
Second, in the sea, a marine regression affects more the benthic fauna living on the disappearing continental shelf, than the planktonic, floating fauna. Yet the contrary is observed.
Third, the cooling must have been very severe to kill dinosaurs, even in the tropics (where, for instance, lots of species sheltered during the Quaternary glaciations). The more that some dinosaurs were adapted to live inside the polar circles (see polar dinosaurs). Now some other species survived the K-T crisis, which were more sensible to a cooling than dinosaurs: crocodiles, turtles, frogs... Actually, dinosaurs survived other coolings and marine regressions in their 150-million-year-long history, without being particularly affected.
Thus, the marine regression seems not to precisely fit the K-T extinction scheme.
It has been suggested that the iridium excess might come from an intense volcanic activity, since it is more frequent is the earth' s mantle. There is precisely an important volcanic event at the time of the K-T boundary, which left the Deccan Trapps in India. The consequences of a great volcanic explosion would have been roughly the same as those of a meteorite impact: a big dust cloud.
However, the iridium-bearing layer has been found there between two lava flows.
Second, the Deccan volcanism is not of the explosive type which is required to explain the world-wide expansion of the layer of iridium.
Third, the volcanic hypothesis cannot explain the presence of the magnetites, nor the irregular crystallisation structure of the glass droplets, and shocked quartz was never found in volcanic rock (some claim that shocked quartz could come from volcanoes, but this has been much debated). There had been claims that the iridium/ruthenium/rhodium rates in the K-T layer were much closer to those of meteorites than of lava, but they are in fact roughly identical.
Fourth, the huge amount of iridium found seems difficult to reach by volcanic explosions.
Thus, the Deccan volcanism cannot stand in for the meteoritic impact.
Yet the two most important volcanic events that left a trace on the earth are the Siberian Trapps and the Deccan Trapps, and their dates correspond to those of the two greatest mass extinctions, the Permo-Triassic and the K-T ones (though the volcanic events are more spread out in time than the extinctions). The claims that these volcanic events were caused by asteroid impacts seem not satisfactory, and this correlation remains unexplained.
There have been lots of other hypotheses to explain the dinosaurs' extinction. Some are not really scientific (extermination by extraterrestrial creatures, changes in the laws of gravitation...). Others have commonly been put forward by palaeontologists. In general, they suffer from their inability to explain the general extinction of animals other than dinosaurs (for example the marine invertebrates) that took place at the K-T limit. Their limitation to the dinosaurs is nearly sufficient to dismiss them.
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Vegetation changes
The plant-eating dinosaurs would have been unable to eat the newly appeared angiosperms ('flower plants'), and would have disappeared. In fact, the emergence of the angiosperms took place well before the end of the Cretaceous, and the fossil record shows a perfect coevolution of dinosaurs and plants, with the arrival of successful new herbivorous dinosaur families such as hadrosaurs and ceratopsians, equipped with impressive masticatory features.
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Mammal competition
The mammals were not in direct competition with dinosaurs, because they did not at all occupy the same ecological niches. However, they supposedly ate the dinosaur eggs. Yet mammals and dinosaurs appeared roughly at the same time, at the end of the Triassic, and one cannot explain why the mammals would have been dominated during 150 million years and then suddenly would begin to eat dinosaur eggs with enough appetite to kill them all. Furthermore, several dinosaurs are now known to have taken care of their eggs.
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Epidemics
It is difficult to imagine an epidemic killing the very varied species of dinosaurs, the plankton, and not the lizards or birds.
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Senescence of the group
Dinosaurs have been said to have disappeared because they were too big (though lots of them were small), because they developed bizarre excrescences (though these can be explained if one accepts to consider that dinosaurs could be adapted), or simply because of a badly-definite 'group senescence' (though no biological explanation of what caused it or even of what it is is known). Yet in the late Cretaceous, dinosaurs were more diverse than ever.
- Poisonous plants or mushrooms, collective suicide, extermination by extraterrestrial creatures, constipation, too thick eggshells, blindness due to supernova radiations, too weak sexual activity, increase of the universal gravitation, death from AIDS...
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