The reptiles we classify under the superorder Dinosauria, and commonly call dinosaurs, consisted of a vast array of different species. While species of dinosaurs roamed and dominated the terrestrial (land) environments of the Earth for approximately 160 million years, up to 65 millions years ago, not a single one of those species is known to have existed throughout that period. Individual species evolved through speciation, existed for a time, then went extinct.
Natural selection occurs when times are hard. Those best suited to the particular hardship(s) are most likely to survive to breeding age and propagate, the process best known as the “survival of the fittest”. When two or more populations of a species are divided geographically, perhaps by a rising mountain chain or a sinking land-bridge, the environments and hardships faced by the separated populations is likely to differ. This can result in their evolutionary path taking different directions and producing different results, a process called speciation.
Species tend to evolve increased complexity far more often than not. This can lead to specialization, a particular species filling a niche in the local environment far better than any other. This is highly beneficial to the individuals of the species as long as the environment doesn’t change. When the environment does change, specialized organisms are often unable to adapt to the quick changes or evolve appropriately for those that occur slowly over multiple generations. So they die out, as 99 percent of all species in the world’s history have. Those that haven’t are predominantly the most recent results of speciation, including us.
We consider a species to be extinct when no more individuals of that species live; whether they have descendant species created by speciation or died out completely. This is an ongoing natural fact of life that has been occurring since life first developed on the planet. It is described as the background level of extinction. Distinct from that are extinction events. Paleontologists have determined that there have been five major extinction events in the past, and biologists have determined the 6th major extinction event is happening now. Despite that, the most well known one is still the major extinction event that occurred around 65 million years ago at the end of the Cretaceous Period of the Mesozoic Era; ending the dominance of the reptiles and clearing the way for the rise of the mammals. All of the reptilian species under the taxonomic super order classification Dinosauria became extinct during this event.
Despite, or perhaps because of, the complexities of our lives, people like to try to keep some things simple. What caused the dinosaurs to go extinct is one of these, we seem to want a single simple answer. That answer might be what has come to be called a dinosaur-killer asteroid, large-scale volcanic eruptions causing a “volcanic winter”, climate change, disease, fungal infections, reversal of the Earth’s magnetic field, increased sunspot activity, a large solar flare, intense gamma radiation arriving from a distant supernova, extraterrestrials clearing the way for a colony that didn’t eventuate or an Act of God, but we seem to want it to be just one of them. We should know by now that life is rarely simple. The extinction of the dinosaurs is most probably due to a combination of these more likely hypotheses:
The dinosaur-killer asteroid
Besides the sun, planets and moons, the solar system contains a vast number of objects ranging in size from specks of dust to asteroids and planetoids many kilometers in diameter. All of them are moving, orbiting the sun. Collisions and close encounters with the gravitational fields of larger objects can cause a change of direction, modifying their orbital paths. This might throw them outward but usually sends them inward. As the craters on the moon make clear, this can result in impacts of considerable magnitude. The Chicxulub structure of Yucatan in Mexico is the result of an impact by an asteroid estimated at some 10 kilometers in diameter, thought to have occurred 65 million years ago.
All life within a thousand kilometer or more radius would have been eradicated and the shock-wave from the impact would have flattened trees and broken all the bones in the bodies of larger animals to a much greater distance. But this is mainly proposed as a cause for major extinctions because of the ongoing effects. The ejecta from the impact, pulverized rock and ash, would be thrown up into the atmosphere, forming a layer that would have blocked out sunlight for years.
The reduction in sunlight reaching the Earth’s surface would lower the average global temperature significantly, and the lack of solar energy for photosynthesis would cause much of the plant life to die off, particularly larger plants. The herbivorous dinosaurs that survived the impact and the cold would die from starvation, and their carnivorous predators with them. Only smaller animals might survive this, particularly the omnivorous scavengers with heat containing outer protections. The proto-mammals with their fur and the small dinosaur species with feathers. Those early mammals evolved to dominate Earth in this new era, the Cenozoic, while those few surviving dinosaurs evolved into the avian species of today.
Extreme Volcanism
Approximately 65 million years ago the tectonic plate that the landmass of India resides upon was moving slowly north to intersect Asia. This resulted in massive volcanic activity, the Deccan Traps near Bombay in western India still have lava fields two kilometers thick after 65 million years of erosion. Major volcanic eruptions such as Krakatoa in 1883 have changed the Earth’s weather for months, super volcanoes like the Taupo Tephra of 160 AD have cooled the planet for years. The lava fields of the Deccan Traps were laid down by multiple volcanic eruptions that may have continued for hundreds, possibly thousands of years.
Such an extended volcanic event would have produced climate change beyond our imagination, even today when current climate change is such a major environmental concern. After 160 million years or so of evolution, most dinosaur species then extant would have been highly specialized, their ability to adapt long enough to evolve for such changed conditions minimal.
Pandemic Diseases and Fungal Infections
The first life, as we know it, to evolve on Earth were single-celled microbes. Even now, they make up the majority of our planet’s species. While most ignore or live in harmony with multi-cellular life, a small percentage have evolved to predate or parasitize multi-cellular plants and animals. Their short lifespans and the ease with which they mutate can result in pandemic diseases. While most such usually affect only a limited number of animal species, it is not beyond the realms of reason for a super-bug, or multiple subspecies of one, to have arisen capable of infecting large numbers of the reptiles, the most numerous prey available to them.
Fungal diseases are quite common in avian species. If we accept that avian species are the descendants of dinosaurs, as seems likely based on current scientific studies, they may have played a significant role in the dinosaurs demise.
Conclusion
Shock-waves propagate from the epicenter of an earthquake, both across the surface and through the center of the Earth. Perhaps the Yucatan impact triggered the start of the Deccan Traps volcanic eruptions on the other side of the world. Between them creating an abrupt cooling of the Earth’s temperature and climate change lasting thousands of years. Stressed, weakened and injured dinosaur survivors would have been more susceptible to parasitic microbes and fungi, passing such strengthened infections on to their healthier pack and herd mates, as well as the predators and scavengers that ate them.
When populations in an area decrease below a limit specific to their species, they go into what is called the extinction spiral, which inevitably results in local extinction if no artificial intervention occurs. Passenger pigeons still numbered in the billions in the middle of the 19th
century, yet the last one died in 1914. The minimum viable population for a species in the wild varies, but for some species it can be surprisingly high. It may have been so for most dinosaur species; it is not something we can actually determine from fossil remains.
A combination of abrupt climate change caused by an asteroid impact, sustained for generations by extreme volcanic activity and supplemented by diseases preying on weakened individuals is probably the best hypothesis for this major extinction event available, based on the limited evidence we can unearth after 65 million years.
