The Cretaceous–Tertiary extinction event, commonly known as the K-T extinction, was the mass-extinction of a large percentage of plant and animal species 65 million years ago, including all non-avian dinosaurs.

It is generally believed that the Chicxulub asteroid impact triggered the K-T extinction due to some combination of the following effects of the impact: global firestorms, infrared radiation, an intense greenhouse effect due to CO2 released by fires, and a reduction in sunlight due to dust released into the atmosphere.

I contend that the primary cause of the mass extinction of animals in the K-T extinction was oxygen deprivation caused by a reduction in atmospheric oxygen due to effects of the Chicxulub asteroid impact.

The support for this theory can be found by looking at which species survived the event, and which did not.

Marine Animals:

Most species of coral close to the surface died out, while those living in oxygen-poor areas below the photic zone survived. 1 Species of Echinoderms (the phylum that includes sea urchins) that lived in shallow waters suffered the highest extinction rates while a those that lived in deeper waters had higher survival rates. 2 

Many species of cold-blooded marine vertebrates like fish and crocodyliforms, though no large members of the crocodilian clade, survived. 3 No species of mosasaurs or plesiosaurs, which were warm-blooded 4, survived.

Terrestrial Animals:

All non-avian dinosaurs died out, while avian dinosaurs (birds) survived. Small burrowing mammals survived. 5

Terrestial Plants:

A high percentage of plant species found in North America went extinct 6 while those further from the asteroid impact site, in New Zealand and Antarctica, were less affected. 7

For the animal species, the major factors correlating with survival seem to be:

• Small size

• Cold-bloodedness

• Burrowing adaptations

• Flight adaptations

• Deep-sea adaptations

The biological advantage that all of the above are associated with is lower oxygen requirements. Smaller sized animals have lower environmental oxygen requirements than large one. 8 Cold-blooded animals have slower metabolisms and thus lower oxygen requirements than warm-blooded animals.

Burrowing animals are adapted to survive in low oxygen underground environments. 9 Birds are adapted to survive in low oxygen high altitude environments. 10 Deep-sea marine life is adapted to survive in low oxygen aquatic environments. 11

I contend that the decline in atmospheric O2 of the early Tertiary period which followed the Chicxulub asteroid impact, perhaps due to oxygen combustion in widespread fires or oxygen being used in the oxidation of the massive amounts of sulfur released by the impact 12, wiped out all species not able to survive in this new low oxygen environment.

Marine animal life adapted to low oxygen deep-sea environments would have been able to ascend to shallow higher oxygen depths.

Burrowing animals could have emerged from their burrows into the more oxygen rich outer environment.

Birds could have stayed at sea level where oxygen levels are higher than high altitudes.

Cold-blooded animals, particularly small ones, could have reduced their activity to slow their metabolism and require less oxygen.

These options didn’t exist for animals adapted to live in oxygen rich surface environments and, due to warm-blooded-ness and larger-size, had higher atmospheric oxygen requirements.

The effect of the K-T extinction on plant species supports this theory. If high levels of infrared radiation or reduced photosynthesis due to particles reducing the amount of sunlight reaching the surface of the earth were the cause of the mass-extinction of animals, then plants would have seen a globally uniform reduction in species numbers, as animals did, since they would have been equally affected by these effects.

Instead, plant extinctions occurred at their highest rate near the impact, where the firestorms would have been, and at their lowest rates in the southern hemisphere far from the impact.

The Chicxulub asteroid impact would have devastated animal and plant life in the Western hemisphere due to the blast of hot ejecta that it emitted from the impact site.

The ejecta blast created by the Chicxulub impact caused immediate destruction of animal and plant life near the impact site and reduced atmospheric oxygen levels, and the result was a globally uniform extinction of animal species poorly adapted to survive a low oxygen environment, and a more localized mass-extinction of plants that were killed by firestorms caused by the impact.