Research

Our lab uses the fossil record to understand major events in evolution- how evolution produces new kinds of organisms and new kinds of ecosystems. Rather than focusing narrowly on a particular taxon or problem, our work ranges broadly- from dinosaurs and birds to snakes and crocodilians (and the odd anomalocarid)- to look at fundamental evolutionary processes such as speciation, extinction, and adaptive radiation.

Speciation and Adaptation

At the level of the organism, we focus on everything from species-level systematics- describing new kinds of dinosaurs, or crocodiles, or lizards- to the origins of entirely new kinds of organisms, such as birds and snakes. Describing new dinosaurs and figuring out their affinities- well, partly, it's just a thrill to find something completely new, that no one has ever seen before, and to put a name on it.

Pentaceratops aquilonius, a new species of horned dinosaur I described in 2014.

But understanding the species-level relationships of dinosaurs, or lizards, or whatever, gives one a very up-close view of how evolution happens. And as Darwin argued, its ultimately these minor changes that add up over time. An evolutionary journey of a thousand miles begins with a single step- speciation. That's why Darwin called his book On the Origin of Species- he was after something much bigger, but figured if he could solve the much smaller problem- one species turning into another- we could explain much bigger problems, such as the origins of complex animals or human beings. In cases, we can even see it happen- one species of dinosaur actually turning into another.

A pair of four-winged Archaeopteryx

And fossils also tell us how these little events add up to create entirely novel kinds of organisms. How did birds evolve? From carnivorous dinosaurs similar to T. rex, it turns out. And how did flight evolve? By taking feathers- which originally evolved for insulation- and layering them to create airfoils. The transitional forms recorded in the fossil record often tell us surprising things. Archaeopteryx, for example, turns out to have flight feathers not just on the forelimbs and tail, but on the hindlimbs as well. It suggests a configuration similar to what is seen in many parachuting and gliding animals- all four limbs are thrown out to the sides to maximize stability and surface area- and suggests that birds may have originated from tree-dwelling parachuters and gliders.

More recently, a remarkable new fossil- Tetrapodophis amplectus, an early Cretaceous, four-legged snake- has helped us understand the transition from snake to lizard. There's been a longstanding debate over whether the ancestors of snakes evolved the long, skinny body to help them move in water, or underground. The new snake seems to settle the issue. It has the classic proportions of a burrowing lizard- a very long trunk and a short tail- and lacks features such as fins or dense bones for locomotion in water. But the legs are truly a surprise. Rather than being reduced to functionless vestiges- or specialized for digging- they have very long, skinny little fingers that resemble the feet of bats and pterodactyls- where the limbs are specialized for grasping. This is not what anyone- advocates of the burrowing hypothesis or the marine hypothesis- would have predicted.

It's astonishing, and certainly not what you'd expect. I think this is a major reason so many people have been reluctant to accept it as a snake (which seems a bit ass-backwards to me: if the facts don't fit your hypothesis, well I don't know... have you considered that maybe your hypothesis is wrong?). But it shows us that evolution tends to be unpredictable. The shortest evolutionary distance is rarely the path taken. The quickest way from a dinosaur to a bird is to add a pair of wings- but evolution added four wings, then eliminated the second pair. The quickest way from lizard to a snake is to elongate the body and eliminate the limbs- but after snakes stopped walking, and start slithering the limbs didn't become vestigial. They started using their limbs for grasping.

Extinction and Radiation

But the fossil record doesn't just show us how individual organisms change, it shows us how whole ecosystems can change dramatically. This goes back to a debate that originated in the 18th century. Cuvier looked at extinct organisms- Pleistocene mastodons, Maastrichtian Mosasaurus- and saw well-adapted organisms that simply vanished with no descendants, and nothing else to take their place. He couldn't see why well-adapted organisms would suddenly survive- unless they encountered conditions beyond what they were adapted for- catastrophes.

Darwin had no patience for catastrophes. Disasters of Biblical proportions seemed too fantastic, too miraculous, too, well, Biblical for a materialistic explanation of the world. He thought that the present was the key to the past- and tried to use everyday processes such as environmental changes, competition, predation, and mate choice to explain how species adapted, or became extinct.

But there's a fundamental flaw here- can you extrapolate from the modern world to periods when the physical environment and the biotic environment may have been quite different? And can you reliably extrapolate from a few centuries of scientific observation of the world to understand vast spans of time? The lifetime of a human being- or a whole civilization- may not be enough to see events that occur once every thousand, hundred thousand, or hundred million years. The geological record shows us that events of inconceivable magnitude- asteroid impacts, volcanic eruptions, ice ages- have fundamentally altered the ecosystem.

We've been working to better understand these events- extinctions and the radiations that they spawned in their aftermath. Looking at fossil birds, for example, we've been able to show that a diverse fauna of archaic birds survives into the Late Maastrichtian, just before the asteroid impact- corroborating the hypothesis that it was a mass extinction of birds that drove the radiation of the modern bird orders.

We have another record of this event- the DNA of modern birds- and its increasingly supporting this scenario in the recent molecular clocks. Modern birds appear to have originated in a sudden burst of evolution near the K-Pg boundary. The driver of this radiation is the extinction of the archaic birds- enantiornithines, hesperornithes- as well as the pterosaurs- which allowed a handful of surviving species- perhaps as few as 6 to 12 species (!) to radiate. It was like the Darwin's Finch radiation- except instead of 3 million years and a little island, it's had 66 million years and 7 continents.

I don't think that this is a peculiarity of the birds. You just cannot wipe out the dinosaurs, pterosaurs, birds, mosasaurs, ammonites, and not affect everything else as well. It's an ecosystem; its connected: any event severe enough to take out dinosaurs must have done a number on everything else in the same ecosystem.

So we decided to take a look at lizards. Conventional wisdom was that they weren't affected much at all. There are lizards and snakes in the Cretaceous, there are lizards and snakes today. But they're different lizards and snakes. What we found is that over the K-Pg boundary, over 80% of the lizard and snake species vanished- and whole groups, entire families, including the dominant lizard groups- vanished. In the aftermath, formerly marginal groups- like the iguanas- take off.

I don't think there's anything special about lizards- I think most groups of animals and plants show the same dynamic. Offhand, I'd guess that the K-Pg extinction wiped out 90-95% of all species on earth- maybe more. We've been revisiting the fossil record of mammals, mosasaurs, pterosaurs, crocodiles, dinosaurs- in each case, we're finding that late Maastrichtian diversity was much richer- and therefore the extinction was more severe- than had been appreciated before.

Some groups were less affected- in the sense that 90% extinction is better than 100% extinction- but it radically altered the ecosystem. Many of the groups that dominate today- not just birds and mammals, but groups of snakes, lizards, amphibians, insects, plants- aren't abundant or diverse before the K-Pg boundary. Butterflies and ants, grasses and orchids- they are either marginal players or nonexistent before the K-Pg boundary. It was likely the extinction of their competitors that allowed them to radiate.

Extinction drives evolution; its the vacant niches in the Paleogene that spur modern groups to radiate. We're seeing this now in studies of marine reptiles: the extinction of the mosasaurs and plesiosaurs that dominated Cretaceous seas is followed in the first 10 million years of the Cenozoic by the explosive diversification of marine crocodiles, sea turtles, and giant sea snakes up to 30 feet long.

Does extinction simply replace what was lost, or actually change ecosystem structure? Hard to say. To a degree, evolution simply replaces one group with another. It's a bit like recasting a play: if your play's Hamlet dies after a crazy weekend in Vegas, cast another actor else. To a degree, you could argue that the community structure is fairly similar: replace your Triceratops with rhinos, your Pteranodon with pelicans. And yet there are species alive today- venomous snakes, echolocating bats, grasses that form vast savannahs, bipedal apes that transmit JPEGs of cats using communications networks- that are radically different from anything in the Cretaceous. Little brown bats, rattlesnakes and prairie grasses and humans- we occupy niches that simply did not exist in the Cretaceous. It's not just that the actors are different, the play is somehow rewritten. Its as if not only has Hamlet been recast, he and Ophelia now live happily ever after.

Its not simply that the K-Pg event has been underestimated, but that the effects of mass extinction in general have been. It follows that if the biggest mass extinction in the past 100 million years could go overlooked for centuries, then smaller events may be overlooked as well. If asteroid impact can play such a profound role in evolution, then perhaps other severe, rapid global shifts in the environment drive severe, rapid, global extinctions.

Recently, I’ve been testing this idea by studying the Eocene-Oligocene event. It turns out that the marine reptiles that evolved after the K-Pg event- crocodilians, turtles, giant sea snakes- persisted up until the Priabonian then disappears near the E-O boundary along with archaeocete whales. The most likely explanation is that severe cooling and a drop in productivity caused by Antarctic glaciation drove a mass extinction. This event may in turn have driven the evolution of modern marine communities, dominated by marine mammals and diving birds.

Darwin was right about a lot of things, but he got mass extinction wrong. He thought that it was the slow, gradual action of everyday processes that drove extinction and radiation. But Cuvier was onto something. Catastrophes- despite or more likely because of their rarity- can cause profound changes. We cannot understand the modern world without understanding these events, and we cannot understand these events except by studying fossils and geology. The present isn't the key to the past. The past is the key to the present.