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Dinosaurs among us: an exhibition explores the link between birds--living dinosaurs--and their extinct ancestors.

THE NEXT TIME you dodge a pigeon on the sidewalk, watch a sparrow eat from a feeder in a backyard, or order chicken for dinner, know that you just had an encounter with a modern dinosaur. Dinosaurs never really vanished from Earth. Most did go extinct, but their evolutionary legacy lives on all around us--in birds.

"Dinosaurs Among Us" highlights the unbroken line between the charismatic dinosaurs that dominated the planet for about 170,000,000 years and modern birds, a link that is marked by shared features that include feathers, wishbones, enlarged brains, and extremely efficient respiratory systems. The fossil record of this story and the biological research it inspires grow richer by the day--so rich, in fact, that the boundary between the animals we call birds and those we traditionally called dinosaurs practically is obsolete.

"With this ... exhibition, we invite visitors to question what they think they know about dinosaurs--how they looked and behaved and even whether all of them actually became extinct," says Ellen V. Futter, president of the American Museum of Natural History. "While paleontology has been a proud part of this institution's legacy for more than 100 years, we live today in an exciting new era of advancement in dinosaur research. There has never been a more interesting time to enjoy dinosaurs or a more fascinating time to learn about their behavior, appearance, and connection to current life, specifically modern birds."

Living birds belong to a group, or clade, called the Dinosauria. It includes the extinct dinosaurs and all of their living descendants, which is why most scientists now agree that birds are a kind of dinosaur just like we are a kind of mammal. Fossils, genes, behavior, and the anatomy of living animals tell us that birds share a common ancestor with dinosaurs that were not birds--and that alligators and crocodiles are birds' closest living relatives.

"The idea that birds are dinosaurs isn't a new one--it was first proposed by Thomas Huxley about 150 years ago, but now it's taken on a whole new dimension as different technologies and, as a result, different ideas, are being applied to the field," explains Michael Novacek, senior vice president and provost for science at AMNH.

"Dinosaurs Among Us" features ancient, rarely seen fossils and lifelike models, including a 23-foot-long tyrannosaur (Yutyrarmus huali) with a shaggy coat of filaments called proto-feathers and a small dromeosaur (Anchiomis huxleyi) with a 22-inch wingspan and vivid, patterned plumage on all four limbs. Visitors will encounter a tiny dinosaur whose sleeping posture precisely echoes that of a living bird; a fossilized dinosaur nest containing remains of the adult that guarded the hatchlings; and a relative of Triceratops that had simple plumes on its body.

The exhibition, which comes on the heels of the unveiling of a 122-foot-long titanosaur cast on permanent display in the museum's Orientation Center, is part of a series of events, public programs, exhibitions, and digital offerings highlighting dramatic developments in paleontology. "This exhibition is based on lots of new evidence amassed over the last two decades," says exhibition curator Mark Norell, Macaulay Curator in the Division of Paleontology and the Division's chair. "I think this is really going to shake up the way people think of dinosaurs."

Making nests, laying eggs, and tending to babies are not just bird traits. Crocodiles do these things, and to some extinct dinosaurs did, too. Today, scientists use observations of living relatives to learn about the lifestyles of animals that have been extinct for more than 65,600,000 years. That includes Citipati osnwlskae, whose fossilized nest is on display as a cast.

The humble eggshell is the structure that allowed animals to colonize the land. A complete life-support system, the eggshell holds water and food for the developing embryo, while letting oxygen in and carbon dioxide out. Similarities between the eggshell structure of some groups of dinosaurs and living birds are another link in the chain of evidence connecting them.

One extraordinary fossil on view preserves a recently hatched Troodontid dinosaur--a member of a group of small, feathered, non-bird dinosaurs with large brains--atop the eggs of what would have been its nestmates. In that fossil, the eggs are not paired, suggesting the parent had only one egg tube--as modem birds do--as opposed to the two present in their ancient ancestors. The exhibition offers a climbable, full-scale model of a nest with 20 eggs discovered in China, likely laid by one of the largest oviraptorosaurs ever found, Gigantoraptor.

Museum-goers can look through a microscope to discover more about eggshell layers (which can tell scientists what type of dinosaur laid the egg) and growth rings (which allow researchers to track maturation in dinosaurs and their relatives). Learning about prehistoric eggs like these and how they differ in size, shape, composition, and more can teach researchers about the transition from nonbird dinosaurs to modem birds.

Feathers come in different colors, sizes, and shapes and serve a wide range of functions: peacocks display them to attract mates; penguins rely on them to reduce drag underwater; and herons create shade with them. Today, birds are the only feathered animals alive, but 150,000,000 years ago, it was a different story. Early birds had feathers, but so did dinosaurs of all shapes and sizes. One reason is that feathers are one of the most useful skin coverings ever to evolve.

Thousands of feathered dinosaurs have been discovered, with most belonging to the theropod branch of the family tree. Researchers also have found evidence of feathered ornithiscians--a branch only distantly related to birds that includes familiar dinosaurs like Triceratops and Stegosaurus. Some scientists think all dinosaurs, including sauropods like Titanosaur, had feathers, just as all mammals have at least some hair. Mammals like elephants, though, have very limited hair. Similarly, sauropods may not have had many feathers, making them unlikely to be preserved in fossils.

Paleontologists continually are discovering new examples of feathered fossils. Discoveries in the Liaoning Province of China, which tend to be exquisitely well preserved, have transformed our understanding of the transition from feathered dinosaurs to birds. Thousands of feathered dinosaurs have been discovered there in the last 15 years, and fossil casts and models of some of the most important ones are on display in "Dinosaurs Among Us."

Finding fossils in the field is just the beginning of the discovery process for modem paleontologists. Using tools ranging from particle accelerators to genetic analysis, researchers are figuring out what color feathers were in life, investigating the relationship between scales and feathers, and learning more about the genes that control feather development. Fossilized dinosaur feathers and eggs are clear signs that these extinct animals and living birds are tightly linked, but this kinship goes much deeper. High-tech views inside fossils and close study of birds and crocodiles reveal that the insides of dinosaurs were a lot like those of living birds and crocodiles.

The size and shape of an animal's brain tells us a lot about how that animal experiences--and gets around in--the world. Birds have very large brains for their body size--six to 11 times bigger than those of equivalent-sized reptiles. Much of the increase in size is in the cerebrum, the part of the brain responsible for learning. In birds, the cerebrum and optic lobe--which governs sight--tend to be large and advanced, while the olfactory region, which is connected to the sense of smell, is less well developed.

Using techniques like CT scans, modem paleontologists can compare the braincases of modern birds to their relatives, including theropod dinosaurs. This research has shown that one group of theropods displays the trend toward inflation of the "thinking" brain we see in living birds, suggesting that some non-bird theropod dinosaurs probably were capable of advanced learned behavior.

The brain is not the only place where an unbroken line between birds and dinosaurs is on display. The large hearts, high body temperature, and powered flight of birds all are driven by a set of extremely efficient lungs. However, birds are not the only ones to have such "super lungs." Extinct dinosaurs and living crocodilians have them, too. This likely means the last common ancestor of birds and crocodiles, which lived more than 240,000,000 years ago, also had birdlike lungs, suggesting the trait evolved at least 100,000,000 years before the oldest known bird.

Even the position of some fossil dinosaurs thought to be sleeping in a birdlike position--sitting on folded hind limbs, forearms held close to the body, and head tucked under one arm--makes us wonder. Birds sleep this way to preserve warmth. Was this birdlike dinosaur warm-blooded, too? The evidence seems to point that way.

Making comparisons to living birds can help researchers draw new conclusions about extinct dinosaurs. Visitors will be able to see two fossilized specimens of Khaan mckennai known as Sid and Nancy. These animals are oviraptorids, a group of fairly small, birdlike dinosaurs with toothless beaks, wishbones, and skulls filled with air pockets. While these animals are nearly identical, scientists suspect that one is a male, based on the presence of large structures beneath its tail that have a triangular, spearheaded shape. Those structures are smaller in the other animal, and lack the triangular, or "chevron," shape, suggesting that the larger structures could have supported the muscles used in a tail-feather display, much like those still put on by the male sage grouse and peacock.

Birds have hollow bones, and some have assumed this trait evolved along with flight: lighter bones should make it easier to fly. Yet, studies have shown that the primitive theropod Allosaurus also had hollow bones. Allosaurus was a big animal with tiny arms, so it was not flying anywhere. Like so many other bird traits, hollow bones appear early in the dinosaur family tree. Hollow bones are among several traits that made early birds well prepared for flight before they could take to the skies. Another is the development of the furcula, or wishbone. Once thought unique to birds, wishbones now are known to occur in some bipedal, meat-eating nonbird dinosaurs. A cast of a Tyrannosaurus rex wishbone, makes it clear that the mere presence of a furcula does not mean an animal could fly.

Once you start seeing the resemblance between nonbird dinosaurs and living birds, it is hard to stop. The similarities especially are striking when it comes to legs, feet, and claws. The four-inch talons of the harpy eagle, for instance, are as large as those of a Velociraptor, and enable this modem predator to carry off prey weighing up to 20 pounds.

How did feathered dinosaurs finally take to the air and become birds? This dramatic transition did not happen all at once, and feathers were just the beginning. Those awkward in-between stages include some of the most fascinating animals that ever lived: bushy, feathered tyrannosaurs; birds with lizardlike tails, teeth, and claws; and even some small, leaping creatures with four wing-like limbs.

Full-size wings evolved in stages, each of which must have served a useful function to persist but, before flight evolved, early wings might have been used to glide from tree to tree or make a gentle descent--and small wings could provide a big boost when leaping out of the reach of predators, pouncing on prey, or running up steep slopes and tree trunks.

Animals capable of true flight can keep themselves in the air by their own power, supporting their weight by flapping; in contrast, gliding is more like a slow, controlled descent, as if guiding a parachute. Many so-called flying animals--including frogs, lizards, and squirrels--really are just gliders. Wings capable of supporting true, powered flight evolved in three vertebrate groups: birds, bats, and pterosaurs. All have light, flexible wings that can support the animal's weight in the air.

The dynamics of flight can be explored through a large-scale media interactive called "Will It Fly?" By varying the wings, breastbone, and body size, visitors can build eight different dinosaurs, ranging from Archaeopteryx lithographica, often called the "first bird," to Velociraptor mongoliensis, and digitally launch them to see whether they will fly.

Fully modem birds already filled the skies at least 50,000,000 years ago, and many were almost indistinguishable from today's birds. Their modernity means that the key adaptations for powered flight already were present, including full-size wings, shoulders that permit them to flap to a near 180-degree arc, and fused skeletons to transfer energy from flapping into flight.

More types of dinosaurs live on Earth today than ever have been described by paleontologists. We call them birds, and there are perhaps up to 18,000 living species. The mass extinction that erased most dinosaurs about 65,000,000 years ago left a few bird lineages unscathed and, in only 15,000,000 years--an evolutionary heartbeat--all of the familiar groups we know today were flourishing. "Dinosaurs Among Us" highlights some of these amazing species.

Hoatzin are the only living representatives of one of the most ancient lineages of birds, with origins about 64,000,000 years ago. Young hoatzin have two claws on the bones that support their flight feathers--that is, on their hands. If a chick falls from the nest, which is a common cause of death among many baby birds, it can claw its way back to safety.

The shaggy-throated giants of the crow family, common ravens are social, highly intelligent hunters and scavengers. Some ravens and crows even have been observed using tools and solving complicated problems in the wild.

White-bellied Storm Petrels frequent the world's wildest oceans, rarely approaching land except to breed on remote islands. These little birds often feed by surface pattering, hovering with their feet touching the water while picking at the surface for small crustaceans and fish.

These extraordinary living dinosaurs provide a vivid link to the ancient past in ecosystems all over the planet, from tropical forests to frozen tundra. Their diversity and success across the planet can mean only one thing--the new Age of Dinosaurs is now.

"Dinosaurs Among Us" is on view through Jan. 2, 2017, at the American Museum of Natural History, New York.

The Changing Science of Paleontology

"The field ... has become less geological and more biological."

In the last couple of decades, the field of paleontology has become less geological and more biological. Although excavation work around the world continues to be a key driver of new discoveries, access to advanced scientific tools like computed tomography (CT) scanners, electron microscopes, and high-throughput computing allows researchers to go beyond just identifying species and approximating when they went extinct. Now, paleontologists are asking more complex questions like: "What did dinosaur brains look like?" and 'What color were their feathers?" and "How did flight evolve?."

Mark Norell, the Macaulay Curator and chair of the American Museum of American History's Division of Paleontology, and curator of "Dinosaurs Among Us," has led this effort through his laboratory's work--which focuses on the evolutionary relationships of dinosaurs and modern birds--as well as that of the many scientists he has trained since he joined AMNH in 1989.

With the discovery of extraordinarily well-preserved fossils in Mongolia, China, and other locations around the globe, Norell and his collaborators have generated new ideas about bird origins and the groups of dinosaurs to which modern birds are most closely related.

Among these discoveries are the first embryo of a meat-eating dinosaur; theropod dinosaurs Shuvuuia, Tsaagan, Citipati, and Khaan; and Citipati specimens sitting on top of their eggs in a brooding position. Norell was part of the team that, in 1998, announced the discovery in northeastern China of two 120,000,000-year-old dinosaur species, both of which show unequivocal evidence of true feathers.

In recent years, Norell's research group has focused on using or developing new paleobiological techniques, with the goal of answering questions like those discussed below:

How intelligent were dinosaurs? Today, CT scanning is used for much more than making medical diagnoses. Norell's research group uses high-resolution CT to peer inside the braincases of extinct dinosaurs, allowing scientists to create digital endocranial casts, called endocasts--detailed, 3D reconstructions of the interiors of fossilized skulls--which they use to examine the size and shape of various brain regions, including the brainstem, which regulates heartbeat, respiration, and other functions; the cerebellum, which coordinates muscle movements; and the cerebrum, which is involved with complex sensory functions and memory.

A number of Norell's current and former students and postdoctoral researchers are using this technique to compare the endocasts of extinct dinosaurs to the brains of modern birds in order to explore the evolution of the so-called "bird brain." Contrary to the cliche, the term describes a relatively large brain that provides the superior visual and motor coordination required to fly. This "hyperinflation" of the brain distinguishes birds from other living reptiles. Scientists increasingly are finding that features once considered exclusive to modern birds, such as feathers and the presence a wishbone (fused clavicle bones), first appeared in nonavian dinosaurs.

Amy Balanoff, a former student of Norell's, who currently is an instructor at Stony Brook University, provides more evidence to add the hyperinflated brain to that list of features. Her studies suggest that dinosaurs evolved the brainpower necessary for flight well before they actually took to the air as birds.

Eugenia Gold, who recently received her Ph.D. in comparative biology from AMNH's Graduate School and also is an instructor at Stony Brook, has worked on examining how brain shape changes through the evolution of flight in theropods, a group of carnivorous dinosaurs closely related to birds. Gold uses data from CT and positron emission tomography (PET) to reveal the areas of the brain that "light up" during flight in modern birds, and then compares those areas to the equivalent structures in extinct dinosaurs.

How did dinosaurs fly? There are two traditional theories in terms of understanding how flight evolved: "ground-up" (the idea that dinosaurs living on the ground started flapping their wings either to run faster or jump higher) and "trees-down" (which predicts that arboreal dinosaurs started flying by using their wings to slow their descent upon jumping or to help steer them from tree to tree).

Recently, though, researchers have started realizing that these two ideas likely are not mutually exclusive. To explore the complexity of flight evolution, Ashley Heers, a postdoctoral researcher in AMNH's Division of Paleontology, studies how modern juvenile birds use their underdeveloped wings, which, in some cases, look very similar to the wing anatomy of their dinosaur ancestors. By examining bird behavior, and using methods like X-ray reconstruction of moving morphology, or XROMM (essentially an X-ray movie), Heers has found that developing birds employ their wings in lots of different ways before they fully can fly--for instance, to run up steep slopes and escape predators. Her work suggests that theropod dinosaurs, many of which also had underdeveloped wings, might have moved in similar ways.

What color were dinosaur feathers? Feather color is produced partially by arrays of pigment-bearing organelles called melanosomes, the structure of which is constant for a given color. In 2008, researchers discovered these microscopic structures within fossilized feathers of an ancient bird. By comparing the imprint patterns of fossilized melanosomes to those in living birds, scientists can infer the color of dinosaurs that lived many millions of years ago.

This was done for the first time in 2010, when two teams of researchers reported finding melanosomes preserved in feathers of two small, Chinese, nonavian dinosaurs--Sinosauropteryx and Anchiornis. In 2012, Norell and his colleagues were part of the research group that used this technique to find the earliest record of iridescent feather color, in a pigeon-sized, four-winged dinosaur known as Microraptor.

How fast did dinosaurs grow? Until recently, paleontologists only could guess the maturity rate and life span of dinosaurs. Recent research involving microscopic analyses of the cellular structure of dinosaur bone has revolutionized the answers to these mysteries. Dinosaur bones contain growth rings, somewhat like the rings in tree trunks, which reveal yearly periods of rapid and slow growth.

A relatively recent use of this feature, involving AMNH paleontologists and Gregory Erikson at Florida State University, found that Archaeopteryx, often called the "first bird," grew much more slowly than living birds, taking about 970 days to reach the adult stage. For comparison, modern birds reach adult size in a matter of weeks.

How did dinosaurs breathe? Birds have a highly efficient respiratory system powered by four sets of air sacs that pump air throughout their body. This adaptation allows the air to flow in one direction (as opposed to mammals, who breathe in and out through the same tube) and is vital to birds' ability to fly high over long distances. During bird development, finger-like projections from these air sacs invade bone to create networks of internal chambers, a condition called pneumaticity.

To explore how this unique feature evolved in dinosaur ancestors, Aki Watanabe, a doctoral candidate in AMNH's Graduate School, and colleagues recently used high-resolution CT, for the first time, on the backbone of a dinosaur fairly closely related to birds: 70,000,000-year-old Archaeornithomimus. They found a remarkable network of pneumatic chambers in the vertebrae of the neck and chest, suggesting that key components that power the avian respiratory system already were in place in Archaeornithomimus, and therefore likely evolved in the common ancestor of birds and Archaeornithomimus more than 150,000,000 years ago.

How do dinosaur eggs compare to those laid by modern birds? Dinosaur eggs and nests are a unique source of data about the transition between nonbird dinosaurs and birds, providing insight into behavior and physiology that researchers cannot acquire from bones alone. Daniel Barta, a student in the Comparative Biology Ph.D. program at AMNH's Graduate School, uses different types of advanced microscope techniques to get an up-close look at the organization of calcite crystals that make up dinosaur eggshells.

These studies tell researchers what kind of dinosaur laid the egg and, possibly, the nesting environment. In the microstructure of eggshells, researchers have found evidence of the direct transition between nonbird dinosaurs and birds: primitive theropod dinosaurs have a single layer of calcite; oviraptor dinosaurs, which are more birdlike, have two layers; and troodontid dinosaurs, which are even closer to birds on the evolutionary tree, have three layers, the same number of layers seen in eggs laid by modern birds.

The Proof is in the Paleontology

"[The American Museum of Natural History] leads expeditions to some of the most remote corners of the Earth and is home to the largest collection of vertebrate fossils in the world."

The American Museum of Natural History has a long and venerable history of international paleontological research and exploration. Museum paleontologists study the history of life on Earth through the discovery, analysis, and description of fossil remains of dinosaurs and other reptiles, mammals, birds, fishes, and invertebrates. The Division of Paleontology traces its history to AMNH's Department of Vertebrate Paleontology, founded in 1892 by future museum president, Henry Fairfield Osborn.

AMNH leads expeditions to some of the most remote corners of the Earth and is home to the largest collection of vertebrate fossils in the world. Early expeditions carried out by museum scientists include Roy Chapman Andrews' Central Asiatic Expeditions (1921-30), which revealed a treasure trove of fossils in the Gobi Desert of Mongolia and in China, and Barnum Brown's expeditions to the American West (1900-10), which resulted in the discovery of Tyrannosaurus rex.

Today, fieldwork continues across the planet. Since 1990, curators Michael Novacek, provost of science, and Mark Norell, the Macaulay Curator, chair of the Division of Paleontology, and curator of "Dinosaurs Among Us," have led expeditions to the Gobi Desert with colleagues from the Mongolian Academy of Sciences. These yearly expeditions have yielded discoveries of dinosaur, bird, and mammal fossils.

Curator Jin Meng's work focuses on Asian mammals from the critical Paleocene/Eocene boundary, a time of major global climate changes and reorganization of terrestrial ecosystems, and he actively is pursuing work in Antarctica as well.

Curator of Fossil Mammals and dean of AMNH's Graduate School, John Flynn has pursued fieldwork in South America and Madagascar for several decades, and has helped revise the paleontologic, geologic, and tectonic history of these continents.

Axelrod Research curator John Maisey concentrates on rare shark fossils to learn more about the origin of these predatory fish, which have a pedigree extending back more than 400,000,000 years.

Invertebrate paleontologists at AMNH likewise have been instrumental in advancing our understanding of early life as well as the evolutionary processes leading to the present day. Curator Neil Landman is one of the world's foremost specialists on Cretaceous marine mollusks, and the work of assistant curator Melanie Hopkins strives to unlock the evolutionary history of animals over vast stretches of geologic time, with a particular focus on trilobftes--extinct arthropods that lived for almost 300,000,000 years.

AMNH's paleontology collection currently contains an estimated 5,480,000 specimens, including more than 5,020,000 fossil invertebrates and approximately 500,000 fossil vertebrates.
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Title Annotation:Science & Technology; "Dinosaurs Among Us" exhibition
Publication:USA Today (Magazine)
Geographic Code:1USA
Date:Jul 1, 2016
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