A few days ago I finished reading the wonderful new book by paleontologist Riley Black, The Last Days of the Dinosaurs: An Asteroid, Extinction, and the Beginning of Our World. I can't say enough positive things about it -- it tells the gripping story of the impact of the seven-kilometer-wide Chicxulub Meteorite, which hit a spot just north of the Yucatán Peninsula so hard that most of the giant rock vaporized, what was left punched twenty kilometers into the Earth's crust, and it left an impact crater 180 kilometers across.
Black gives us a vivid description of the event and its aftermath, each chapter from the point of view of one individual animal who experienced it (not necessarily lived through it, of course). The day before the impact; the impact itself; the first hour; the first day; the first year; and so on, up until a hundred thousand years after the strike, at which point the Earth's ecosystems had largely recovered -- albeit with a completely different assemblage of species than it had before.
Black's contention, which is generally accepted by researchers, is that there's little truth to the old trope of the dinosaurs being a moribund group anyhow and the asteroid just finished them off. The dinosaurs were doing just fine. While some species were headed toward extinction, that had been happening during the group's 190 million year hegemony (and has happened in every single group of life forms ever evolved). Dinosaurs as a group were still widespread and diverse -- and if it hadn't been for the impact, it's pretty likely that they would have remained in charge (as it were) for millions of years afterward.
Which means that it's probable that mammals would never have taken off the way they did. (More accurately, "the way we did.") It's also an incorrect understanding that mammals only launched after the dinosaurs were "out of the way." Mammals had been around for a very long time themselves (the first ones, the morganucodontids and multituberculates, overlapped the dinosaurs by over a hundred million years). What seems to be true, though, is that the dinosaurs occupied most of the large-apex-predator and giant-herbivore niches, so mammal groups were mostly small, and a lot of them were burrowers -- something that was an adaptation to there being a lot of carnivores around, but turned into a key to their survival during the searing infrared surge that swept across the world the day the asteroid hit.
What brings this up, besides my wanting to promote Riley Black's awesome book, is a link sent to me by a friend and loyal reader of Skeptophilia about a series of recent discoveries by paleontologist Robert DePalma at a dig site in Tanis, North Dakota. What's stunning about these finds is that DePalma believes -- and the evidence seems strong -- that they represent the remains of organisms that died on the day of the Chicxulub Impact.
In other words, we're looking at a snapshot of the event that killed every non-avian dinosaur species, and changed the face of the world permanently.
Hard as it is to imagine, in the late Cretaceous, what is now North Dakota was a tropical wetland bordering the Western Interior Seaway -- an inlet of the ocean that has since vanished from a combination of uplift, the Rocky Mountain Orogeny, and simple evaporation. Picture southern Louisiana, and you have an idea of what North Dakota looked like.
Then the meteorite struck.
Despite the fact that the distance between the impact site and the Tanis wetland is around four thousand kilometers, it only took an hour before there was a blast of heat, a rain of red-hot debris, and a series of earthquakes. The first-mentioned is probably what did the most immediate damage; large animals that were too big to shelter were probably all dead within minutes after the the infrared surge started, as were just about all the terrestrial plants. Even aquatic organisms weren't safe, though. One of the more horrifying fossil finds was a turtle -- that had a stick driven all the way through its body. The earthquakes triggered a series of seiche waves, which occur when an enclosed body of water is shaken laterally. (Picture the sloshing of water in a metal tub if you jostle it back and forth.) The seiche in the Western Interior Seaway and nearby lakes flung aquatic animals onto shore and then buried them under tons of debris -- DePalma and his team found layers of fish fossils right at the K-T Boundary Layer that were also victims of that awful day the impact occurred.
I've written about this event before, of course; I've always had a fascination with things that are big and powerful and can kill you. But what made me decide to revisit it was a new discovery at Tanis of amber that contains glass spherules. Amber, you probably know, is fossilized tree sap; it can contain other fossils, including pollen and animals that were trapped in the sap before it hardened (made famous by Jurassic Park, although it must be added that there's never been any found with intact DNA). But these glass spherules were altogether different. Silicate rocks turn to glass when they're melted and then cooled quickly; that's where the rock obsidian comes from. But an analysis of the spherules showed something fascinating. There were inclusions in the glass of tiny chips of two different kinds of rock; one type was high in calcium, while the other was largely metallic, with high content of chromium, nickel, and other heavy elements.
The first, DePalma says, are the remnants of the limestone bedrock from the spot in the Yucatán where Chicxulub hit, blasted into the air and landing four thousand kilometers away.
The other are the (thus far) only actual pieces of the meteorite itself which have ever been found.
It's absolutely astonishing that we can identify rocks and fossils that formed on a specific day 66 million years ago, and doubly so that it was a day when an event occurred that quite literally changed the course of life on Earth. As horrifying as the Chicxulub Impact was -- Riley Black calls it "the worst day the Earth ever experienced," and it seems an apt description -- in a real sense, we owe our existence to it.
Without Chicxulub, it's pretty likely it'd still be a dinosaur-dominated world -- and one in which mammals were still small, furtive furballs that never had a chance to control their own destiny.