What Really Killed the Dinosaurs: Earth's Past Major Life Extinction Events

Life on Earth will not last forever. Several near life extinction events have happened on Earth in the past. Here they are listed in the order in which they have occurred.

To put everything in context, the Earth is about 4.5 billion years old. Single celled living organisms first appeared between 3.5 and 4 billion years ago. Complex life composed of multicellular organisms first appeared as much as 1.6 billion years ago. Plants first colonized land 515 million years ago. The first member of the genus homo, to which modern humans belong, appeared about 2.8 million years ago.

Ordovician Extinction
About 440 million years ago on Earth, over a nine million year period, three pulses of high volcanic activity produced very high CO₂ levels and ocean acidification. These pulses alternated with snowball Earth conditions followed by rapid thawing and floods. Loss of life is estimated to have been 85% of all species.

Devonian Extinction
About 359 million years ago, one or more supernova explosions directed enough radiation onto Earth to eliminate the ozone layer and cause prolonged exposure to ultraviolet radiation, causing an extinction event. This ended the Devonian period and ushering in the Carboniferous. The supernova are estimated to have been 65 million light-years away. A supernova within 25 million light-years of Earth can extinguish all life. Loss of life is estimated to have been 70% of all species.

Permian Extinction "The Great Dying"
About 252 million years ago, a large meteor hit what is now Antarctica nearest Australia. The crater, newly found under Antarctic ice, is twice as large as the one that landed 66 million years ago. Most life, which was then ocean dwelling, died. This was the end of the Permian period and the start of the Triassic. Loss of life is estimated at 90-96% of all species.

Triassic-Jurassic Extinction
About 200 million years ago, a series of large scale volcanic eruptions happened over an 18 million year period. This caused massive climate change and altered ocean acidity levels. Loss of life is estimated at 50% of all species.

Cretaceous Extinction
66 million years ago, a large asteroid hit the Earth and famously wiped out the non-flying dinosaurs, the pterosaurs, the plesiosaurs, and other animal groups. The angle at which it hit the Earth, plus the location, a shallow seabed next to the Yucatan peninsula full of sulfur-containing gypsum, made it even more deadly. The impact spewed vast quantities of vaporized seabed into the upper atmosphere. If the asteroid had been larger, it could have ended most multicellular life. If it had struck in the middle of the Pacific ocean, there would have been no lasting significance.

Most plant and animal species did not die immediately. The sulfur compounds and other fine particles in the upper atmosphere reduced sunlight to only 10% of normal during the first year after the impact. Worldwide temperatures plunged to severe winter cold for years.

Ichthyosaur skeleton by Vizetelly, Pixabay

Most living plants did not receive enough sunlight or warmth and died in the first year. Fifty percent of all plant species went extinct. The ones that survived came back from their seed and roots. Of those that survived, many of the flowering plant species had an unusual genetic adaptation. They were polyploidy, that is, having two or more whole copies of their genome. This made them more adaptable, and able survive a greater range of environmental conditions. Some lichen and fungi species also thrived, living off massive amounts of dead plant material.

As living plants expired in the cold and gloom, the plant-eating dinosaurs died with them. The remaining carnivorous dinosaurs followed soon after.

Many bird species survived the immediate impact, but fossil evidence indicates that brain size mattered. Only the larger brained bird species were able to locate enough food in the first few years to avoid starvation. Our current bird species evolved from them.

Of mammalian species, the marsupials were dominate during the dinosaur era, getting in some cases to be as large as a modern badger, and strong enough to take down some dinosaurs more than twice their size. The placental mammals were smaller, resembling rodents, living in underground burrows, and hunting at night for insects and other tiny morsels. After the impact, many marsupial species died off. The placental mammals, having little competition once sunlight returned, exploded in size and variety, taking over most of the world.

Ten years after impact, enough fine particles and sulfur compounds had drifted out of the atmosphere that sunlight and temperatures were nearly back to normal. The impact ended the Cretaceous period and started the Paleogene. Estimated loss of life: 75% of all species.