Fur, wings, and feathers

A mammal that ate a dinosaur

For most of the time that dinosaurs ruled the earth, we are told, our ancestors were mouse-sized creatures hiding in the undergrowth. That is partly true, and a fossil from Liaoning, in northeastern China, shows how partial. The animal is called Repenomamus, from rock about 125 million years old. One species was about 40 centimeters long, a badger-sized animal weighing 4 to 6 kilograms. Another, Repenomamus giganticus, was over a meter long and weighed 12 to 14 kilograms, as much as a medium-sized dog. In 2005 a team led by Hu Yaoming reported a skeleton of the smaller species with the bones of a baby dinosaur, Psittacosaurus, preserved in its stomach. Whether it hunted the young dinosaur or scavenged it, it ate one. Early mammals were not all mice.

How to build a mammal

The mammal body plan took a very long time to assemble. The line that leads to us split from the line that leads to reptiles and birds about 320 million years ago. For the next hundred million years our branch, the synapsids, were among the most successful animals on land: the sail-backed Dimetrodon, which is often mistaken for a dinosaur, is one of them, and it is more closely related to you than to Tyrannosaurus. Then, in the great extinction 252 million years ago, most of them were wiped out, and the survivors that led to mammals were small.

Mammals did not appear in a single step. Hair, milk, warm blood, a single jawbone, and the three tiny bones of the middle ear came one by one over perhaps 100 million years. Each is worth a look.

Ear bones from jaw bones. A reptile’s jaw joint is made of two small bones at the back, the articular in the lower jaw and the quadrate in the skull. In a mammal, the lower jaw is a single bone that meets the skull directly, and the two old joint bones have moved into the middle ear, where they became the malleus and the incus, which carry sound from the eardrum to the inner ear. Fossils caught the move in the act. Morganucodon, about 205 million years ago, had both a mammalian and a reptilian joint. And in 2011, Meng Jin and colleagues described Liaoconodon, a mammal from Liaoning about 120 million years old, whose ear bones were detached from the jaw but still attached to it by a rod of ossified cartilage, a stage paleontologists had been looking for since the 1800s. Your own malleus and incus form near your jaw in the embryo before they settle into your ear. Two of your ear bones are the old hinge of a jaw, itself rebuilt from a gill arch, as Lesson 1 said.

Fur and warm blood. Most reptiles warm themselves in the sun. Mammals make their own heat, which lets them stay active when it is cold or dark, and fur keeps it in. Fur is rarely preserved, but a lakebed in Inner Mongolia about 164 million years old has given us Castorocauda, a close relative of the mammals with a flattened tail, webbed hind feet, curved teeth for gripping fish, and the oldest known impression of a pelt. It was an otter-like swimmer weighing about half a kilogram, and it shows that mammals in the Jurassic already did more than hide. There were gliders, diggers, climbers, and swimmers.

Night work. Living in the shadow of the dinosaurs may have meant living by night. A 2017 study of the daily habits of 2,415 living mammals concluded that the common ancestor of all of them was nocturnal and that daytime activity appeared only after the dinosaurs were gone. An echo of that long night survives in your eyes: most mammals see with only two kinds of color receptors, where birds and many reptiles have four. Some monkeys, apes, and humans regained a third kind later.

Milk and live birth. The most important invention gives the group its name: the mammary gland. A mother mammal feeds her young on her own milk, which lets her produce small, helpless offspring and still raise them. How the offspring arrive varies. Monotremes, the platypus and the echidnas, still lay eggs, though their young drink milk. Marsupials such as kangaroos give birth to tiny young that finish growing in a pouch. Placental mammals, which include us, nourish the embryo through a placenta and an umbilical cord for a long pregnancy. Fossils from Liaoning show these branches were already separate: Juramaia, about 160 million years ago, is the oldest known relative of the placentals, and Sinodelphys, about 125 million years ago, of the marsupials.

The earliest mammal-like fossils are often little more than teeth and jaw fragments, so scientists argue about which count as true mammals. One group, the haramiyidans, has been moved in and out of the mammals more than once. The line is blurred because the invention was gradual.

Three ways to fly

The seventh turning point is flight. It has evolved many times in the history of animals. The first fliers were insects, more than 300 million years ago, which is why a sky full of food was waiting for any vertebrate that could get up there. Among the vertebrates, three groups made wings that fly under their own power, and each did it differently.

Pterosaurs were the first, around 228 million years ago, in the Late Triassic. They are not dinosaurs, though they are close cousins, both being members of a group called the archosaurs, the “ruling reptiles,” which also includes crocodiles and birds. A pterosaur’s wing was a membrane of skin and muscle stretched from a single enormously long finger, the fourth, which is the ring finger, down to the body and legs. The three other fingers stayed small, with claws. Some pterosaurs were the size of a sparrow; Quetzalcoatlus, with a wingspan of 10 meters or more, rivaled a small airplane. They disappeared in the same extinction that ended the dinosaurs.

Bats came last, at least 52 million years ago. A bat also has a skin wing, but a different one: four long fingers, the second to the fifth, spread like the ribs of an umbrella, with membrane in between. The thumb is a small claw. About one in five mammal species is a bat. One of the oldest known bats, Onychonycteris, found in Wyoming’s Green River rocks, could fly but, from its ears, could not echolocate, which tells us that flight came first and echolocation later.

Birds use feathers, which grow from the arm and hand. The bird’s wing skeleton is a streamlined version of the dinosaur’s arm, with a hand reduced to a few fused bones, and the flat surface of the wing is made of feathers, not skin. Archaeopteryx, from the limestone quarries of Solnhofen in Bavaria about 150 million years ago, was a magpie-sized animal with a bird’s feathers, but also teeth, claws on its fingers, and a long bony tail. The first skeleton came to light in 1861, two years after Darwin’s On the Origin of Species, and it seemed to be the intermediate form that Darwin’s theory predicted.

And then there are dinosaurs that tried the second design. In 2015, Xu Xing’s team described Yi qi, a pigeon-sized feathered dinosaur from about 160 million years ago in Hebei, with a long rod growing from its wrist that held up a skin membrane like a bat’s. In 2019, a second one, Ambopteryx, turned up in Liaoning. It is not certain that either could fly rather than glide, and they left no known descendants. But they show that when a feathered animal started to take to the air, there was more than one way to build a wing.

How does a wing hold up an animal? The explanation in many textbooks goes: air splits at the front of the wing, the air going over the curved top has a longer way to go, so it has to go faster to meet the other air at the back, and fast air has lower pressure. The last part is true; the first is not. The air over the top does not have to “catch up” with the air below, and in fact it gets to the back edge first. Lift comes mainly from the wing’s angle and shape turning the air downward, and, as Newton’s third law says, the wing is pushed up. The pressure difference is the result of that bending of the flow, not the other way around. NASA’s education pages list the “equal transit time” story as an incorrect explanation of lift.

Feathers before flight

Did feathers evolve for flying? The fossils say no. The oldest known feathers belong to animals that could not fly, and the first one found sparked a revolution. In 1996, in Liaoning, a small dinosaur about a meter long was found in the Yixian Formation, about 125 million years old, whose body was outlined by a fringe of fuzz. This Sinosauropteryx was not a bird, but a runner with a long tail, covered in simple, hair-like filaments. Since then, the same beds, the Jehol Biota of northeastern China, have given up dozens of feathered dinosaurs, and a study of the pigment cells preserved in Sinosauropteryx showed that its tail had alternating light and chestnut-red rings, like a red panda’s.

The feathers came in a surprising range of jobs. Warmth is a likely first use: the giant tyrannosaur Yutyrannus, described in 2012, was about 9 meters long and weighed over a ton, and carried a coat of feathers up to about 20 centimeters long. It lived in a cool climate, with an average temperature of about 10 degrees Celsius, so the feathers were probably insulation. Not every large dinosaur wore them: skin impressions of Tyrannosaurus rex and its close relatives show scales, so a fully feathered T. rex is not what the evidence suggests. Display is the second job. Epidexipteryx, from about 160 million years ago in Inner Mongolia, was a pigeon-sized animal with four long, ribbon-like feathers on its short tail, and apparently no wing feathers at all; its name means “display feather.” And in Caudipteryx, a Liaoning dinosaur that ran on the ground, wing feathers were symmetrical and too short for flight. Flight is the third job, and the last.

What makes a flight feather different? Its shaft is off center. The vane on the front edge is narrow and stiff, and the one behind it is wide, a shape that helps the feather hold up under the pressure of the air. The flight feathers of Archaeopteryx and of the four-winged Liaoning dinosaur Microraptor are asymmetrical in this way, and the feathers of Caudipteryx are not. Microraptor, about the size of a crow, also had feathers on its legs. Studies disagree on whether it flapped or glided; the pigment cells in its feathers show that they were glossy black and iridescent, like a starling’s.

Display has a long afterlife. The peacock’s tail is the best-known case: a heavy train that serves no purpose except to be admired. A popular explanation is that a large, bright train is an honest signal of health, because only a male with few parasites can grow one. Some studies of peahens found that they prefer males with more eyespots. Others found no such preference, so the story is a hypothesis, not a result.

The day the sky fell

Sixty-six million years ago, an asteroid about 10 kilometers wide struck the shallow sea on the edge of what is now Mexico’s Yucatán Peninsula. It left a crater about 180 kilometers across, known by the name of a nearby town, Chicxulub. The crater was first noticed in 1978 by two geophysicists searching for oil, Glen Penfield and Antonio Camargo, and was confirmed as an impact site by drilling in the early 1990s. Two years after Penfield’s data, the American physicist Luis Alvarez and his son, the geologist Walter, had proposed an asteroid when they found an unusually high concentration of the element iridium, rare at the earth’s surface but common in asteroids, in a thin clay layer at the very boundary between the Cretaceous and the Paleogene.

The impact threw rock, dust, and sulfur into the air. Sunlight dimmed and the surface cooled. The plants that feed everything else suffered first, and then the animals that ate them. About three-quarters of all species died. The dinosaurs that were not birds disappeared, along with the pterosaurs, the mosasaurs, and the plesiosaurs. The survivors included animals that could hide, burrow, dive, or live on seeds and the scraps of a dead world: crocodiles, turtles, frogs, small mammals, and a few kinds of birds.

That last point is the surprise of this lesson. Birds are dinosaurs. The ancestor of all living birds was a feathered dinosaur, the way the ancestor of all living primates was a small mammal. Strictly speaking, what died out 66 million years ago were the non-avian dinosaurs. The avian ones survived, and today there are about 11,000 species of them. The turkey in an American Thanksgiving dinner is a dinosaur; so is the egg in a Chinese tomato-and-egg stir-fry. The dinosaurs did not entirely die. They changed their clothes.

The disaster was also an opening. Many of the niches the dinosaurs had held were suddenly empty, and the mammals, once small and nocturnal, moved in. Within tens of millions of years there were whales, bats, horses, and primates. Whether our own branch would have gone as far if the asteroid had missed is something nobody can say. It is a fair reading of the record, though, that the mammals’ big opportunity came because of a disaster.