The K-T Mass Extinction
The history of life is punctuated by mass extinctions- severe, global, and geologically rapid events that wipe out vast numbers of species. Five extinctions are ranked as more severe, more intense, than all the others. These are the Ordivician-Silurian extinction, the late Devonian extinction, the Permian-Triassic extinction, the Triassic-Jurassic extinction, and the Cretaceous-Paleogene (K-Pg) extinction. The K-Pg extinction used to be the Cretaceous-Tertiary or K-T extinction (which is unfortunate because 'K-Pg' is less catchy than 'K-T'. And 'Pg' makes you think of a Disney movie that is fun for the whole family, which it most certainly would not have been).
The Cretaceous-Paleogene mass extinction occurred 66 million years ago. Its causes were once a source of debate, but the evidence now shows that the extinction coincided with the impact of the Chicxulub asteroid. The asteroid was 6 miles in diameter and slammed into the earth at 50,000 miles per hour; the result was an explosion on the order of 100 million megatons of TNT, or a million of the most powerful H-bombs ever built. It is thought that the extinction would have launched a huge amount of debris into the air, blocking sunlight and causing photosynthesis to shut down, causing the collapse of the food chain.
The primary evidence for this hypothesis is a sedimentary layer that is distributed around the world. This layer is the Cretaceous-Tertiary boundary clay (it's visible in the photograph at right as a thin grey layer below the coal at the top). It marks the extinction interval. Below the K-Pg boundary, you get dinosaurs and other Cretaceous organisms. Above it, you don't.
In 1980, Alvarez et al. looked at iridium in the boundary layer. Iridium is a metal that's rare on earth but common in meteorites. Their clever idea: a slow, constant rain of iridium from tiny meteorites could be used as a clock to determine how long the boundary clay took to form. The less iridium, the shorter the boundary layer and the faster the extinction. Their project failed in the most spectacular way possible: they found orders of magnitude more iridium than expected. The only way they could explain it was with a giant asteroid, which they calculated as being ~ 6 miles in diameter. Further studies revealed that the boundary layer contained minute spherules, which were minute beads of rock that had been melted by impact, shot into the sky, and then solidified. The layer also got thicker and thicker in southern North America, hinting at a location somewhere near the Gulf of Mexico. Eventually, the giant, 100-mile-diameter Chicxculub crater was discovered in the Yucatan.
There is no longer any serious debate about the causes of the extinction, a few holdouts notwithstanding. The key thing is the timing- the Cretaceous species go right up to the iridium layer and vanish suddenly. The fact that the timing of extinction coincides perfectly with the asteroid impact- the biggest extinction in the past 250 million years happened at the same time as the biggest asteroid impact in the past 250 million years- simply can't be a coincidence. Other events just don't fit in terms of timing. The eruption of the Deccan Traps in India, for example, occurs around 500,000 years before the K-Pg boundary, and yet there is little evidence that dinosaurs even noticed. Triceratops and T. rex were around before the eruption, and around afterwards.
But the effects of the extinction on diversity still remain poorly understood. Over the past few years, I've been restudying late Maastrichtian and early Paleocene fossils to try to get a better picture of the extinction patterns.
The first study to emerge from this research examined the birds (Longrich et al. 2011, PNAS). The Cretaceous is dominated by archaic bird groups- the weird Enantiornithes, the toothed Hesperornithes and Ichthyornithiformes- but it was unclear whether they dwindled gradually leading up to the K-Pg boundary, or went extinct when the asteroid hit. My colleagues and I examined bird fossils (at right) from the late Maastrichtian. These are pretty scrappy; but they are also the most informative fossils we have regarding this problem, because they are virtually the only fossils we have. The key here is that we used a phylogeny to constrain the placement of these fossils. Instead of simply comparing these fossils to modern birds (the approach used previously) we were able to say something about what they were related to.
We showed that archaic birds remained a diverse component of the bird fauna until a few hundred thousand years before the K-Pg boundary; we also found no evidence for a radiation of modern birds before the asteroid impact. In the ten million years after the extinction we see the sudden appearance of many modern bird groups, however. The implication is that the asteroid wiped out archaic birds, and in so doing, created the opportunity for modern birds to radiate.
The next part of this research focused on the lizards. This project examined the diversity of lizards and snakes before and after the K-Pg event (Longrich et al. 2012, PNAS). The consensus was that the K-Pg event didn't really affect lizards much. My colleagues and I tested this idea. We showed that lizards were extremely diverse before the end of the Cretaceous, with 27 species known- 24 lizards and 3 snakes.
New Late Cretaceous lizards and snakes (From Longrich et al., 2012)
We found that only five of these survived. The other 83% became extinct. The extinction took out all the big lizards and snakes, so there is strong size selectivity. Finally, the extinction took out major branches of the lizard and snake family tree. Whole groups disappeared, including the Polyglyphanodontia, which were far and away the most diverse lizards of the time.
In the wake of the extinction, modern lizards- iguanas, monitor lizards, boa constrictors- appeared. Far from being an evolutionary non-event, the K-Pg event defined the modern lizard and snake community. Insofar as lizards are a proxy for terrestrial diversity, the implication is that you simply cannot understand the evolution of the terrestrial ecosystem without understanding the K-Pg event.
The next step is to take a broader look- to look at the diversity of dinosaurs, mammals, crocodiles, pterosaurs, mosasaurs, and fish in the leadup to the extinction, to examine extinction rates and selectivity for the entire fauna, to get a better idea of what the dynamics of extinction are like. In particular, I'm interested in the idea that the poor quality of the fossil record has obscured the severity of the mass extinction. In other words, the "75% of all species" casualty estimate that is often tossed around may actually be too conservative.
here's the bird paper: http://www.pnas.org/content/108/37/15253.abstract
and here's the lizard paper: http://www.pnas.org/content/109/52/21396.abstract