Nestled among white-sand beaches and brightly colored resorts, the mangrove swamps along Mexico’s Yucatán coast are a paradise for birds and the people who enjoy watching them. The densely wooded swamps, located along a major avian migration route, offer safe haven to the millions of birds that make fraught intercontinental flights between the Americas each year.
My guide to this flyway rest stop, Luis Salinas-Peba, is a soft-spoken scientist at the local campus of the National Autonomous University of Mexico. A botanist by training, Salinas-Peba is also a master of bird identification who can name just about every species we see and hear packed among the mangroves. The mix of long-distance migrants and local species is dazzling: Blue-winged teals from Canada cross paths with endemic Yucatán wrens. Towering pink flamingos mingle with teacup-size hummingbirds.
The air pulsates with the guttural alarm cries of cormorants, which get louder and more insistent as our tiny boat glides closer to their nests. Several of the sleek black birds suddenly launch into the sky, pulling my gaze upward and my thoughts toward the past, when a visitor from space 66 million years ago turned a primeval paradise into a burning apocalypse.
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Roughly 30 miles to the east of this mangrove swamp sits Chicxulub Puerto, a calm seaside village at the center of an immense impact crater that stretches into the Gulf of Mexico. The ever shifting earth has softened the crater rim, which today is only visible using advanced satellite sensing. But its footprint on the planet tells an incredible tale. On one infamous day in the late Cretaceous period, an asteroid the size of a mountain slammed into what is now the Yucatán coast, gouging the earth and setting off a series of catastrophic events. Vaporized rock and noxious gases choked the atmosphere, forests were obliterated across the globe, and temperatures fluctuated dramatically. The impact and its aftermath ended the reign of the dinosaurs, wiping out a group of creatures that had ruled the planet for 135 million years.
Ask just about any paleontologist, and he or she will tell you that life found a way and that some dinosaurs survived the mass extinction. That’s because today’s birds are the last remaining twig on an otherwise demolished dinosaur family tree, grown from fierce predators and sculpted by evolution into an array of flapping, feathery fowl.
“There is no doubt that birds are dinosaurs,” says Luis Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles County. “The evidence is so overwhelming, I would put it next to whether you’re going to question if humans are primates.”
In the hellscape left by the asteroid, what gave the ancestors of modern birds an edge over their Cretaceous cousins? It’s a tough nut to crack, given how rare birds are in the fossil record. But some exceptional discoveries over the past dozen years, coupled with advances in genetic analysis, are starting to reveal how the Chicxulub impact shaped the modern-bird origin story. That, in turn, is offering some of the first plausible clues to how birds made it through the cataclysm and exploded into the more than 10,000 species that exist today.
The oldest known root on the bird family tree is the raven-size Archaeopteryx, a 150-million-year-old animal that bore a telling mix of attributes. While all of today’s birds are toothless, Archaeopteryx had jaws bristling with sharp teeth. It sported claws on its front limbs and a long, bony tail. These traits are lost in birds and instead reveal close ties to its more reptilian cousins, such as the Velociraptors of Jurassic Park fame. But Archaeopteryx had characteristics of modern birds too. Its fossils show prominent wings covered in aerodynamic feathers and a wishbone not unlike one plucked out of a chicken dinner.
Not long after its discovery in the 1860s, the species was hailed as a clear transitional step between dinosaurs and birds. But few new fossils emerged to fill in the evolutionary gaps, and details about its ancestors and descendants remained murky for decades.
That finally changed in 1996, when scientists unveiled the first known fossil of a feathered dinosaur unrelated to birds. Dating back nearly 130 million years, Sinosauropteryx prima was a game changer among the dozens of spectacular species being mined from early Cretaceous rock formations in China, mainly in Liaoning Province. There, primordial lakes and active volcanoes had created all the right conditions for exquisite fossil preservation. The result is a menagerie of non-avian dinosaurs and their primitive bird contemporaries, often accompanied by feathers, scales, and skin that are sometimes so detailed they even retain traces of pigment. Like Archaeopteryx, many of these animals are surreal mash-ups between the standard notion of a modern bird and classic images of a predatory dinosaur.
With feathers dark as an oil slick, the non-avian dinosaur Microraptor gui probably glided between branches using stiff feathers on all four of its limbs. Nearby, the primitive bird Longipteryx chaoyangensis was flitting along waterways, snapping up fish with its reptilian, tooth-studded jaws. And Anchiornis huxleyi, a charcoal-hued dinosaur with a crown of rusty fluff, was stalking the forest floor like a goth pheasant, unable to truly fly because of its stubby, three-clawed wings.
“If you didn’t see the fossils themselves, you wouldn’t think these things existed,” says Shannon Hackett, a bird curator at the Field Museum in Chicago.
Despite this bounty of finds from Liaoning, paleontologists still faced gaps in the fossil record, which they sometimes tried to fill based on patchy data from mere fragments of bone. Some DNA work put the origin of modern birds deep in the Cretaceous, with many of the avian groups that exist today springing up very early in the time line. This implied a tale of mega survivorship, with a bunch of modern bird ancestors somehow making it through the mass extinction in one giant flock.
Other experts argued that all birds living before the cataclysm were of the more primitive persuasion, like the ones whose fossils were found in China. Under that theory, a few ancient species survived the impact and gave rise to a “big bang” of modern bird evolution only after the rest of the dinosaurs died.
For years the debate was as contentious as asking whether cheesecake is cake or pie. But in 2005 bones from Antarctica threw an exciting new ingredient into the mix: a bird that lived just before the Chicxulub event and looked stunningly similar to a modern duck.
Julia Clarke at the University of Texas at Austin first described Vegavis iaai based on a fossil dated to around 67 million years ago, just before the asteroid strike. Traditional anatomical analysis and a digital reconstruction of the bones show that Vegavis seems to bear traits in its skeleton that exist only in today’s birds, signs it is indeed part of a modern line. Clarke and her team place it in the same group that includes present-day ducks and geese.
In 2016 they examined a second, more complete skeleton of Vegavis and found that the animal not only looked like a duck, it may have also quacked like a duck. The fossil contains the oldest known example of a vocal organ called a syrinx, a squawk box akin to the ones found in today’s waterfowl.
“Vegavis is easily one of the most important early fossils” for understanding the spread of birds, says Daniel Field, an evolutionary paleobiologist at the University of Bath. This evidence of a modern bird group popping up just before the asteroid’s impact added to a growing need to rethink the polarizing positions on bird evolution that reigned in the 1990s.
Closer looks at fragmentary bones, combined with more advanced methods of genetic backtracking, are fleshing out the story. In a study published in 2015, a team led by Yale University ornithology professor Richard Prum combed through the genes of 198 living bird species and calibrated their results against the latest fossil finds. Their detailed avian family tree suggests that only three modern groups got their start just before the asteroid strike.
The picture emerging now shows animals that looked very much like modern birds, flying and diving and pecking in the shadow of the dinosaurs. A select few made it through the mass extinction, and that’s when birds as we know them really took off.
Kemmerer, Wyoming, is built on bones. About a hundred miles northeast of Salt Lake City as the crow flies, the town sits among crumbling buttes packed with billions of fossils. Known for these riches since the 19th century, the land hosts about a dozen commercial quarries that supply trade shows and gift shops around the world. Whole family dynasties rise and fall and engage in understated rivalries over the discovery, preparation, and sale of animals that died some 52 million years ago.
In the heart of Kemmerer, a couple of doors down from the first store opened by J.C. Penney, Lacey Adams perches on a metal stool at her prep table inside Tynsky’s fossil shop. As her safety glasses pin down parts of her spiky blond hair, Adams uses a compressed air chisel and needles to painstakingly remove the pale rock surrounding tea-brown bones.
“I know offhand what species I’m looking at because of the ridges,” she says, turning a thin slice of stone under the light to reveal otherwise imperceptible bumps. “Like, that one’s obviously Knightia,” a member of the herring family commonly found in the region.
This dry, rugged land is mostly known for its abundant fossil fish. After all, we’re standing in what used to be a massive lake, a subtropical everglade that would have made a South Floridian feel at home. As with the much older wetlands in China, this lake left an entire ecosystem frozen in time, including a valuable collection of ancient birds. Quarry workers and scientists here have unearthed partial bones, stray feathers, and well over a hundred complete bird skeletons. “They’re the best picture we have of how Earth’s biota was recovering after the great extinction event at the end of the Cretaceous,” says Lance Grande, a curator at the Field Museum and my host at the dig site.
Black-billed scythebill (Campylorhamphus falcularius)
Grande has been coming to this paleontological wonderland, prosaically known as Fossil Lake, for more than 40 years. He usually spends a few weeks each summer mining privately held land leased by the Tynskys. The most scientifically valuable finds return with him to Chicago, and the Tynsky family keeps the rest for commercial sale.
For a blistering week in late June I join him up on the butte, where a cadre of enthusiastic high school students and museum volunteers teach me how to lift large slabs out of the earth and check them for signs of past life. About midmorning on one achingly bright day, Grande asks me to pause my sweaty efforts and come see something amazing: A worker from a neighboring quarry has brought us a bird.
Grande has built relationships with several other local families, and they often share their most interesting finds in case he wants to make a purchase on behalf of science; some they donate. During a tour of the town, I happened to see one long-time fossil hunter affably barter his most recent discovery—a stunningly detailed ancient shrimp—for a wad of cash and a copy of Grande’s latest book, simply titled Curators.
This time, though, the potential prize came to us, lovingly displayed from the back of a dusty pickup truck. The animal is only partially uncovered from its limestone tomb, but I can clearly see delicate bones and the impression of a feathered wing. Grande wants a closer look, so we wrap it for transit and race down to the local hospital to get an x-ray. The radiology techs greet us with less surprise than I would have expected—this is clearly not their first fossilized patient. It takes a couple of tries, but by the end of our impromptu visit, Grande is convinced there’s a complete bird skeleton inside the hunk of rock. He’ll later negotiate with its discoverer to buy the fossil for further study.
In this way, many of the birds from the hills around Kemmerer have come to roost in Chicago, held in the display cases and storage rooms of the Field Museum. During a visit to the storied institute a few weeks after our dig, I get a closer look at an early parrot, a perching songbird, and a type of mousebird that have all been recently described by scientists. These remains show that the postimpact ecosystem was an exceptionally diverse aviary. “Every time we find a new bird specimen, one out of two times, it’s something completely new,” Grande says. “It’s pretty exciting.”
The past few years have been a boon to researchers trying to capture an image of bird life recovering after planetwide disaster. In New Mexico, paleontologists recently extracted parts of a different mousebird that lived 62 million years ago. Tsidiiyazhi abini, Navajo for “little morning bird,” is now one of the oldest known avians on this side of the extinction event. It joins a 61-million-year-old giant penguin found recently in New Zealand that looks different from other penguins that lived around the same time.
All these fossils seem to fit with the latest genetic puzzle pieces. A number of papers released in 2014 looked at the full genomes of 48 living bird species and concluded that modern birds saw a rapid boom in diversity soon after the asteroid impact. The 2015 genetics study came to a similar conclusion. Even if the two teams don’t agree on some of the finer details, they each support a picture of survivors making a strong comeback.
“Evolution took tens of millions of years to produce a small dinosaur with wings that could fly by flapping its arms. And then that body plan proved really successful when the asteroid hit,” says Stephen Brusatte, a paleontologist at the University of Edinburgh. “Some of those birds made it through, and then, on the other side, there was a whole new world to conquer.”
Fish (Mioplosus labracoides) are the most common finds in the Kemmerer, Wyoming, lake.
The tougher question is why these particular ancestors of modern birds made it through. With more fossils and faster gene sequencing, theories about survivorship abound.
Examining the lifestyles of species that lived before and after the asteroid, Daniel Field and his colleagues think the widespread disappearance of forests may have had something to do with it. In the last days of the Cretaceous, the world as a whole was a warmer, wetter place than it is today. Lush forests were abuzz with all types of exotic birds, including many that might have passed for contemporary species at first glance.
Field’s research is revealing that when the asteroid hit, whole forests disappeared, and the world was plunged into an impact winter. One ancient bird group that didn’t survive was the Enantiornithes. Many of these once abundant birds have feet suited for perching in trees, suggesting they were largely arboreal. So far, not a trace of them has been found beyond the Cretaceous. Instead, the surviving bird species seem to be more at home on scrubland or at sea. All of today’s birds that depend heavily on thickly wooded zones arose well after the mass extinction, around the time forests would have been bouncing back too.
Another attractive notion is that certain birds were better at proliferating in a disaster zone. In 2017 a team led by Gregory Erickson at Florida State University presented evidence that egg-laying, non-avian dinosaurs took months to incubate and hatch their young. Since many modern-style birds generally reproduce quickly and mature in a matter of days or weeks, they might have had a competitive edge over their more reptile-like cousins in the grim aftermath of the asteroid strike.
Depending on whom you ask, smaller bodies, polar adaptations, seed-based diets, and even nest designs may have played roles in determining who lived and who died. Solving the mystery will almost certainly require exhaustive hunts for animals that lived even closer in time to the impact. Ongoing fieldwork in places like South America, New Zealand, and the frosty deserts of Antarctica already hint at fresh discoveries in the near future. And richer genetic clues should flood the field in the coming years. At the China National GeneBank in Shenzhen, scientists are using faster, more precise techniques to churn out drafts of entire genomes for all living bird species by 2020. Their work should help researchers not only to understand living birds but also to match useful traits in fossil animals to those in the animals’ living descendants.
“We’re going to see an explosion in the number of bird genomes,” Clarke says. “It’s going on right now, and it’s amazing, and I love it.”
The most likely answer to the question of survivorship is that it took a suite of characteristics for certain birds to be successful. That’s why it’s important to keep adding evidence and probing each new theory. “These are global, very complex patterns that we’re trying to piece together from more than 60 million years later,” Field says. But “trying to dive into all of those related questions is gradually improving our understanding of survival across one of the most severe mass extinction events in the history of the world.”
Back on the rim of the impact crater, Xavier Chiappa-Carrara, head of the academic unit of the National Autonomous University of Mexico in the Yucatán, is investigating how birds will cope with a more insidious kind of mass extinction. Mexico is home to more than a thousand bird species, and about half of them can be found in the Yucatán. Roughly 220 are migrants, spending the winter in the region or just passing through on their way between hemispheres. Today many of these birds are at risk due to habitat loss.
During our visit to the mangroves, we saw a large, plastic-lined shrimp farm cutting into the coastline. Vacation homes and hotels creep around the edges of the swamp. And all across the peninsula, increasing numbers of people are drawing water from the underground aquifers that support the entire coastal ecosystem. Chiappa-Carrara and his team are now racing to understand how humans are affecting the wildlife.
It’s a familiar refrain around the world. We are changing the environment so quickly, wiping out habitats and altering the climate, it’s like an invisible asteroid hit the planet.
But while the ancient space rock couldn’t have cared less, people can alter the course of this catastrophe, which is why Chiappa-Carrara has hope. He and his colleagues help organize a Yucatán bird festival every year, bringing the wonders of all things feathered to the public. Every year they inspire more visitors to appreciate and protect these survivors from a fallen kingdom—the dinosaurs that still share the land and sea and sky.