What Killed the Dinosaurs?

Mammals co-existed with the dinosaurs for most of the dinosaurs’ existence. However, almost all of the mammals remained small creatures—they generally could not outcompete the dinosaurs for the big-creature jobs.
Extinction is a normal process. A new species may arise and be more successful than an existing type, pushing the old type to extinction. Diseases, accidents, or other events may kill an entire population. And, extinction is forever. As populations vary owing to random factors, sometimes the population drops to zero. But when the population hits zero, the species can never come back up—you can’t just borrow a few creatures from a future generation and bring them back to fill in the gap. So all sorts of random events cause extinctions, and most of the species that have lived on Earth have become extinct. (This characteristic of extinction, that when you hit zero, you're gone, also applies to gamblers at casinos. To see why you’re likely to lose if you gamble, take a look at Module 11 Enrichment.)
About 65 million years ago, at the end of the Cretaceous Period of the Mesozoic Era and the start of the Paleogene Period of the Cenozoic Era (the K/Pg boundary, because K is used for Cretaceous and Pg for Paleogene; note that old literature used Tertiary rather than Paleogene and called this the K/T boundary), a very large extinction event killed most of the living dinosaurs except for the birds. At the same time, many other types became extinct—more than half of the species known from fossils near the end of the Cretaceous became extinct at the end of the Cretaceous. Because the survival of even a few individuals from a species can allow the species to persist, it is likely that almost all living things on the planet were killed. It was a catastrophic event, one of the most catastrophic in the history of the Earth.
The solution to this puzzle—how the dinosaurs and others were killed—was not found until fairly recently, but now we have a lot of important data. At the K/Pg boundary, sedimentary rocks around much of the world contain a thin clay layer. This layer is rich in iridium, an element rare on Earth but common in meteorites. This clay layer contains bits of rock that were melted and refrozen rapidly to form glass, such as are produced by meteorite impacts. Quartz grains in the layer contain shock features, which are caused by very high pressures applied very rapidly, but by no other known mechanisms such as volcanic eruptions. The layer is rich in soot (black carbon) from fires. The layer is thicker in and near the Americas than elsewhere. Around the Caribbean Sea, the layer includes a deposit of broken-up rock such as would be produced by a huge wave. And, on the Yucatan Peninsula is a large crater, the Chicxulub Structure, that is dated to the K/Pg boundary. The crater is partially buried by younger rocks but easily detected using geophysical techniques, drilling, etc. The crater is at least 110 miles (180 km) across, and perhaps as much as 180 miles (300 km) across.
This evidence indicates that a large meteorite, perhaps 6 miles (10 km) across, hit the Earth (the hole or crater made by an energetic projectile is usually a whole lot bigger than the projectile). Such a collision would have released more energy than all of the nuclear bombs that were on Earth when the U.S. and Soviet arsenals were at their largest.
The impact broke huge amounts of rock into small pieces, from the meteorite and from the Earth where the meteorite hit, and melted much more rock, blasting the solid pieces and melted drops of rock into the air and even above the atmosphere into space. As large pieces fell rapidly back to Earth, friction with the air generated heat in the same way that a re-entering space capsule or a “shooting star” is heated. For a little while, the air would have been like a toaster-broiler oven, lighting wildfires around much of the Earth that produced the soot in the fallout layer and that killed many things.
Following that, cold and dark descended. The impact site included sulfur-rich rocks. The heat of the impact vaporized some of those rocks, and that vapor cooled later to form sulfuric-acid clouds in the stratosphere. The small particles of these clouds didn't fall fast enough to heat up much, just as raindrops and dust particles do not heat up when they fall today. The many, many small particles, plus fine dust and soot from the fires blocked the sun and cooled the Earth. We know that such cooling occurs with modern volcanic eruptions—big ones such as Mt. Pinatubo in 1992 cool the Earth by a part of one degree for a year or two. A nuclear war might do much more, creating a nuclear winter or at least a nuclear fall. Even more of the sunlight would have been blocked after a huge meteorite impact, and the world may have frozen for a few years. And, with the sunlight blocked, photosynthesis would have stopped, which would have been very bad for plants that rely on photosynthesis and animals and fungi that rely on plants.
The sulfur particles, when they fell, would have made sulfuric acid, giving much stronger acid rain than the recent human-produced pollution. The sulfur in the stratospheric may have damaged the ozone layer, allowing dangerous UV radiation to penetrate as the dust and soot started to clear.
The impact site included a lot of carbonate rocks, and those broke down in the intense heat, releasing carbon dioxide. After the dust and sulfur cleared and the freezing ended, the world became anomalously hot for perhaps 100,000 years or so, from the enhanced greenhouse effect caused by all that carbon dioxide.

The meteorite impact was not nearly big enough to roll the Earth over, notably move the orbit, rearrange the continents, or anything similarly cataclysmic for the physical behavior of the planet beyond the few years or decades of the heat, cold and acid — the energy brought by the impactor was enough to move the Earth in its orbit roughly 1 cm, or a bit less than ½ inch, NOT dramatic. But the event was cataclysmic for life—almost all of the living things on Earth died. Who survived? Plants with long-lasting seeds, hibernators, things that live in ocean sediment or along spreading ridges, scavengers, and probably some others with appropriate characteristics. The general pattern is that the surviving animals were small, and mammals did better than dinosaurs. (Although, yes, birds are a branch of the dinosaurs, and the birds are still with us.)
After the fire and ice was over, the “jobs” (ecological niches) of many of the dinosaurs were left open. There were no big plant eaters or big meat eaters left, for example. Over tens of millions of years, the mammals, freed of the competition from the large dinosaurs, slowly evolved to take over the jobs of the dinosaurs. Some of the larger offspring of some species were successful, although most mammals remained small (mice, voles, etc.). Lining up the fossils over time, we see an evolutionary shrubbery—lots of branches, many extinctions where those branches were cut off, the persistence of small creatures, but the appearance of some large creatures, with those leading to modern lions and tigers and bears—and people. Biodiversity was hugely reduced by the meteorite, but over the next millions of years, new species appeared a little more often than existing species went extinct, so that diversity increased back to more-or-less what it was before the meteorite, just with different species doing the jobs.
If today, Coke and Pepsi and all other soft-drink companies suddenly magically disappeared, new soft-drink companies would be started fairly soon, not because of a magical tendency for soft-drink companies to appear, not because soft-drink companies must exist, but because we usually figure out how to take advantage of opportunities. In the same way, wiping out dinosaurs opened up a space for the evolution of mammals.