Stegouros elengassen.
Credit: Luis Enrique Pérez López (licensed under CC BY-SA 4.0)

Looking at plant-eating dinosaurs is like browsing a Medieval armory. Helmets (dome-headed pachycephalosaurs), whips (sauropods), shields (Stegosaurus and ceratopsians—though those of the latter probably weren’t used for defense), spears (Stegosaurus, Triceratops), clubs, and thick armor (both ankylosaurs). Now, paleontologists (scientists who study ancient life) have added a sword to the list of prehistoric weaponry. 

A team led by Sergio Soto-Acuña, a paleontology student at the University of Chile, discovered a dinosaur they named Stegouroselengassen. The dinosaur lived between 75 and 72 million years ago in what is now Patagonia. The team published their findings in the scientific journal Nature. 

The new dinosaur is an ankylosaur, a member of the group that includes Ankylosaurus and other armored plant-eating dinosaurs. Stegouros was lightly-armored for an ankylosaur, possessing several rows of knobby plates—called scutes—that ran down its back and sides. It was relatively small, only about 6 feet (2 meters) long. It also had thin limbs and an unusually short tail. 

The tail of Stegouros has attracted the most attention. Some ankylosaurs possessed tail clubs to defend themselves against predators. But Stegouros had a different tail weapon. Its scutes became larger and sharper down the tail, fusing into a large, bladelike weapon at the tip. 

Stegouros’s tail resembles a macuahuitl, a weapon used in the Aztec empire. The macuahuitl was a heavy wooden club lined with several sharp obsidian blades. As with the macuahuitl and the European broadsword, the sheer weight of the swinging tail made it dangerous, more so than the sharpness of the blade. A well-placed swing would have dealt a slicing, shattering blow to the legs of any predator that was trying to catch it. 

In paleontology, as in other sciences, one discovery often sheds light on others. The ankylosaur Antarctopelta was discovered in 1986 on James Ross Island, just off the coast of Antarctica. It had unusual vertebrae at the base of its tail, but the rest of the tail was not found. These basal tail vertebrae closely resemble those of Stegouros, so scientists now think Antarctopelta had a similar tail sword.  

Soto-Acuña and his colleagues suggest that these two strange dinosaurs, along with another named Kunbarrasaurus from Australia, formed a part of a separate group of ankylosaurs. This group diverged from the main ankylosaur lineage early in its history and lived for tens of millions of years in relative isolation near the South Pole. 

October 18, 2019

Recent excavations in central Thailand have led to the discovery of a new type of predatory dinosaur, Siamraptor suwati. The ancient creature belonged to a group of giant meat-eaters called carcharodontosaurs, which means shark-toothed reptiles. Siamraptor dates from the Cretaceous Period, a time in Earth’s history from about 145 million years ago to 66 million years ago. The Cretaceous was the last of the three periods that make up the Mesozoic Era, the time when the dinosaurs lived.

Fossils found in Thailand helped scientists reconstruct the skull of the newly identified Siamraptor suwati dinosaur. Credit: Duangsuda Chokchaloemwong, et al/Nakhon Ratchasima Rajabhat University

Siamraptor (Siam is the previous name for Thailand; raptors were carnivorous, bipedal dinosaurs) is the first example of a carcharodontosaur to be found in Southeast Asia. Previous examples came from northern Africa and Europe, and close cousins have been found in Argentina (Giganotosaurus) and the United States (Acrocanthosaurus).

The fossilized bones of Siamraptor were found in 115-million-year-old rocks in the central Thai district of Ban Saphan between 2008 and 2013. Paleontologists from Thailand’s Nakhon Ratchasima Rajabhat University found the fossils—22 in all—while working on a project with Japan’s Fukui Prefectural Dinosaur Museum. The announcement that the fossils belonged to a new type of carcharodontosaur came in October 2019 after years of study. Siamraptor was not the first find for the Japan-Thailand Dinosaur Project. The team earlier identified two new types of plant-eating dinosaurs and an ancient relative of the alligator and crocodile.

Twenty-two fossils aided in the skeletal reconstruction of Siamraptor suwati. Credit: Duangsuda Chokchaloemwong, et al/Nakhon Ratchasima Rajabhat University

The Siamraptor fossils include parts of a single animal’s feet, hands, hips, spine, and skull. The animals’s bones were somewhat porous, containing air sacs that would have made the creature a lighter and more agile hunter. Nearby were also many Siamraptor teeth, suggesting that this animal had not been alone. Like sharks, dinosaurs shed teeth throughout their lives, particularly when they ate, and the Ban Saphan site (a floodplain during the Cretaceous Period) may have been a common feeding ground. Siamraptor probably preyed on plant-eating dinosaurs in the area, using its bladelike serrated teeth—measuring up to 6 inches (15 centimeters) long—to slice through tough dinosaur flesh.

Scientists classify Carcharodontosaurus with other meat-eating dinosaurs in a large group known as theropods. These animals make up one of two main groups of saurischian (lizard-hipped) dinosaurs. Saurischians include such famous dinosaurs as Tyrannosaurus and Velociraptor. Although Carcharodontosaurus is classified in the same main group as Tyrannosaurus and Velociraptor, it was not closely related to them.

Tyrannosaurus and Carcharodontosaurus shared such traits as big heads, long bodies, and short arms, but the animals’ snouts were quite different. Tyrannosaurus had a broad head and a wide mouth with teeth made for crushing bone and pulling away flesh. Carcharodontosaurus had a much narrower head and a more precise bite with sharper teeth meant for slicing flesh. The two apex predators (at the top of the food chain) appear to have coexisted in several areas, which probably made for some rather interesting confrontations.

April 28, 2017

For decades, dinosaurs have been grouped into two broad categories: long-necked sauropods and meat-eating theropods (along with birds) in one group, and the remaining plant-eaters, such as Stegosaurus, Triceratops, and Iguanodon, in the other. In March, a group led by Matthew G. Baron from the University of Cambridge in the United Kingdom challenged that view. They published their findings in the prestigious scientific journal Nature.

Tyrannosaurus is a fearsome example of a saurischian (reptile-hipped) dinosaur. Credit: © Science Picture Company/SuperStock

To study dinosaur relationships, paleontologists (scientists who study prehistoric life) use a type of analysis called cladistics. In cladistics, researchers evaluate anatomical traits of at least three related species. These evaluations help them construct a cladogram, a kind of family tree illustrating how the species are related to one another. Groups of species that are related to one another are called clades. For dinosaurs, skeletal features are used, such as the absence or presence of a particular bone or a certain feature on a bone.

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The accepted dinosaur family tree describes two major kinds of dinosaurs: ornithischians and saurischians. Credit: WORLD BOOK illustrations by George Fryer, Bernard Thornton Artists

For 130 years, paleontologists have grouped dinosaurs into two large, confusingly named clades based on the arrangement of their pelvic, or hip, bones. All tetrapods (animals with four limbs) possess three paired sets of hip bones: two ilia (singular: ilium), two ischia (singular: ischium), and two pubes (singular: pubis). In one group of dinosaurs, the pubes run from the hip socket down and away from the center of the animal, close to the ischia. Because these hips looked similar to those of birds, early paleontologists named the group Ornithischia, which means bird-hipped. But ornithischians are not closely related to birds.

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This illustration shows the difference between bird-hipped ornithischians and reptile-hipped saurischians. Credit: WORLD BOOK illustrations by Alex Ebel

In sauropods and most theropods, the pubic bones project straight down from the hip socket or at a slight forward angle. This is closer to the condition seen in other reptiles, so they were included in the clade Saurischia, which means reptile-hipped. In some small theropods, however, the pubis is “reversed,” angling away from the center of the body at an angle similar to the ischium. Birds evolved (developed over time) from these theropods, making them members of the “reptile-hipped” clade, even though they truly have bird hips.

Baron studied 73 dinosaur species and catalogued 457 skeletal characteristics with them. He coded these characters into a computer program, which returned new controversial results. Theropods were found to be more closely related to ornithischians, with sauropods and some primitive meat-eaters remaining in Saurischia. Despite their different-shaped hips, theropods and ornithischians were found to be united by more than 20 unique characters. To refer to the clade, Baron and company revived the name Ornithoscelida (meaning bird-legged), which had been coined by the famous British zoologist Thomas Henry Huxley in 1870. Fortunately for the sake of clarity, birds would be considered bird-legged dinosaurs under this system!

The new placement would resolve some puzzles about the origins of dinosaurs. The earliest known dinosaur skeletons are about 230 million years old and have been found in South America. But still older reptiles that are regarded as the ancestors to dinosaurs have only been found in northern regions far from South America. To further muddle things, footprints widely attributed to early dinosaurs have been found in eastern Europe and are some 5 million to 9 million years older than the South American skeletons. With the proposed reorganization, those earliest-known skeletal remains would be considered early relatives of sauropods, suggesting that the earliest dinosaurs have yet to be discovered somewhere in the Northern Hemisphere.

While the proposed rearrangement would solve some problems relating to the origin of some dinosaur features, it would create problems relating to the origin of others. For instance, sauropods and theropods both had air sacs connected to their lungs. These air sacs had long chambers that tunneled through their bones, making them lighter. Based on the new family tree, the first dinosaur would have had to possess these air sacs. Why would ornithischians lose a trait that was apparently so useful to sauropods and theropods and remains vital to birds today? Scientists will continue studying them to try to answer this puzzling question.

Though paleontologists commended Baron for the meticulousness of his work, they were quick to point out that more data would be needed to convincingly rearrange the family tree. The Saurischia-Ornithischia division has been upheld for decades by dozens of cladistics analyses, so it will take more than one new analysis to convince paleontologists to overturn 130 years of study. But the bird-legged dinosaur clade will have a leg to stand on if more fossil discoveries and cladistic analyses back up Baron’s findings.

December 27, 2016

Lida Xing, a paleontologist (scientist who studies fossils) from the China University of Geosciences in Beijing, made a spectacular find just by going to the market. There, he found part of a tiny dinosaur—complete with feathers—perfectly preserved in amber.

A lump of amber has preserved this feathered dinosaur tail (along with ants, a beetle, and bits of foliage) for 99 million years. Credit: © Ryan C. McKellar, Royal Saskatchewan Museum

Amber is a hard, yellowish-brown fossilized resin. It comes chiefly from the resins of pine trees that grew millions of years ago. These resins were gummy materials mixed with oils in the trees. When the oils oxidized (combined with oxygen), hard resins were left. These pine trees were then buried underground or underwater, and the resins slowly changed into lumps of amber. These lumps often contain insects trapped as the resins flowed from the trees. But finding larger animals such as small vertebrates (animals with backbones) is incredibly rare.

Xing found the remarkable fossil at an amber market in the Southeast Asian nation of Myanmar (also called Burma). The seller thought the bushy object in the amber was part of a plant and had polished the piece to be made into jewelry. But it was in fact part of the feathery tail of a small dinosaur. Xing promptly brought the specimen to fellow paleontologist Ryan McKellar of the Royal Saskatchewan Museum in Regina, Canada, where they studied it with other scientists using computed tomography (CT). Computed tomography involves taking many X-ray images of an object from a number of directions. A computer combines the set of X-ray “slices” of the object to create a three-dimensional image called a volumetric model. The scientists recently wrote about what they found in the journal Current Biology.

Paleontologists have discovered feathered dinosaur fossils in the past, but the heavy weight of the sediments and rocks deposited above them had flattened the carcasses during the fossilization process. Because this tail was trapped in amber, however, it was preserved in three dimensions, allowing scientists to see the exact layout of the feathers. Even traces of the pigments that colored the feathers have been preserved, showing that the animal would have been light brown with a whitish underside.

The tail likely belonged to a juvenile (young) bipedal dinosaur related to Tyrannosaurus rex. Despite the specimen’s prominent plumage, paleontologists know that the tail belonged to a nonbird dinosaur (all birds are technically dinosaurs). The fossil dates back 99 million years, some 50 million years after the appearance of early birds such as Archaeopteryx. A tail from Archaeopteryx would have been short and stiff. The tail from this new fossil, however, is long and flexible. It is bent a great deal in the amber, and scientists estimate that only a third of the tail was preserved. Furthermore, the beautifully preserved feathers trapped in amber would have been useless for flight. The feathers possess simple barbs and barbules that would hold strands of the feathers together, but they have much thinner central shafts than modern flight-worthy feathers. These ancient feathers would have been soft and downy, and they probably kept the young animal warm.

The loss of the dinosaur’s tail in amber probably cost it its life. Dinosaurs could not detach their tails the way some lizards can. A dinosaur could likely survive without the tip of its tail, but this fossil is from the mid-section of a dinosaur’s tail. Once the hapless youngster got stuck in resin, it probably either starved to death or became an easy snack for a predator—except for the part stuck in resin, of course.

Lida Xing has a knack for making spectacular finds of amber-entrapped animals. Earlier this year, he announced the discovery of a pair of 100-million-year-old bird wings. His remarkable discoveries are shining a new light on the evolution of feathers in birds and dinosaurs.

April 27, 2016

In this illustration, a group of dinosaurs and flying reptiles flee a vast fire. Such an event, connected to the eruption of a volcano, could have caused the extinction of the dinosaurs at the end of the Cretaceous, some 65 million years ago. Credit: © Mark Garlick, Photo Researchers

Dinosaurs reigned supreme on Earth for tens of millions of years. Other animals, such as mammals, lived in the shadows, eking out an existence among giant plant-eaters and ferocious predators. Dinosaurs were so dominant that, if it hadn’t been for some extremely bad luck 65 million years ago in the form of a giant asteroid, they might still populate Earth today. Right? Maybe, maybe not.

A recent study published in the journal Proceedings of the National Academy of Sciences casts doubt on this picture of dinosaur supremacy. Paleontologists Manabu Sakamoto and Chris Venditti of the University of Reading and Michael Benton of the University of Bristol analyzed parts of the dinosaur family tree over time. They compared speciation (spee see AYE shun) events to extinction events. Speciation is when one species evolves from another—for example, human beings, chimpanzees, and gorillas evolved from a common ancestor that lived between 4 million and 10 million years ago. That split that occurred was a speciation event. Sakamoto, Venditti, and Benton made comparisons of speciation and extinction events within five groups of dinosaurs: meat-eating theropods, long-necked sauropods, horned ceratopsians, duck-billed hadrosaurs, and other plant-eating dinosaurs.

The scientists found that the rate of extinction began to exceed the rate of speciation for theropods, sauropods, and other plant-eating dinosaurs 50 million years before the asteroid hit. This means that dinosaur species in these groups were going extinct faster than new species evolved to take their place as much as 115 million years ago, in the middle of the Cretaceous Period.

Does this mean that dinosaurs were already on their way out, and a huge rock from space only hastened their demise? Absolutely not! While the dinosaurs may have been experiencing a downturn in diversity, this study does not indicate that they were at death’s door. Whether dinosaurs gradually declined before their extinction has been a hotly-debated topic in paleontology for many years. In fact, long before the Chicxulub impact crater (created by an asteroid that struck Earth 65 million years ago) was discovered, most paleontologists thought the dinosaurs had slowly died out and that there was no mass extinction. The question of dinosaur decline is very complicated and touches on such paleontology issues as the completeness of the fossil record. Because of many differences in the layers of Earth, some animals in some periods have nearly complete fossil records, while others from other periods may have few or no fossils. This study will not be the last word on the matter.

The study does, however, highlight a few important things about dinosaur diversity and extinction. First, the extinction of the dinosaurs was a complex event. The environment of the Late Cretaceous was challenging. Enormous volcanoes called the Deccan Traps raged in what is now India. The climate was getting colder. Drifting continents and changing sea levels were fragmenting land habitats, making it more difficult for land-dwelling dinosaurs to move from place to place. These factors may have affected the group’s species diversity.

Second, the study highlights how diverse dinosaurs were. There are about 600 named dinosaur species, and scientists estimate that another 600 or more remain to be discovered. Some of these species may have lived in regions where fossilization rarely occurs (such as mountain environments) and thus will never be found. Some paleontologists involved in assessing dinosaur diversity point out that the astonishing diversification that occurred in the Jurassic Period could never have lasted forever, particularly in the challenging environment of the Late Cretaceous.

Finally, the study holds great relevance for us today. If the results are borne out by future analyses, it would suggest that a modest loss of species diversity may have made the dinosaurs more susceptible to random cataclysmic events such as the Chicxulub asteroid. Today, many species are going extinct, and the planet is warming rapidly. The loss of diversity might make today’s ecosystems more susceptible to collapse from random events such as volcanic eruptions or asteroid impacts. Like the dinosaurs, humans might have built their global dominance on an unstable foundation. Unlike the dinosaurs, however, we have the ability to make changes to strengthen ecosystems and reduce global warming, giving us more of a fighting chance against extinction.

Other World Book articles

• Drilling for Answers (March 9, 2016) – Behind the Headlines article
• Geology (2007) – A Back in Time Article
• Global warming

March 23, 2016

A recreation of a horse-sized tyrannosaur, Timurlengia euotica, stalks through its natural environment of 90 million years ago. Two other reptiles (Azhdarcho longicollis) fly nearby. The newly-discovered fossilized remains of a Timurlengia euotica reveal how Tyrannosaurus rex and its close relatives became top predators. Credit: © Todd Marshall (Smithsonian)

Rex means king, and, sure enough, Tyrannosaurus rex was the king. As top predator at the end of the age of dinosaurs, T. rex hunted down prey with its heightened senses and tore them apart with its powerful jaws. It wasn’t always like that, however. Tens of millions of years earlier, the smaller ancestors of T. rex cowered from other predators called allosaurs (like the well-known Allosaurus). Little is known about how ancestors of T. rex evolved (changed over time) from small, nimble hunters into the huge predators of the Late Cretaceous (80-66 million years ago). Last week, a team of paleontologists, led by Stephen Brusatte of Scotland’s University of Edinburgh, announced a discovery, Timurlengia euotica, that helps fill in this gap in the Tyrant Lizard King’s family tree. The paleontologists announced their findings in the journal Proceedings of the National Academy of Sciences. 

Tyrannosaurus rex was a large, meat-eating dinosaur that lived about 68 to 65 million years ago in what is now western North America. It was one of the largest meat-eating dinosaurs, measuring about 40 feet (12 meters) long and about 12 feet (3.7 meters) high at the hip, and weighing about 7 short tons (6.3 metric tons). Similar animals roamed Asia and Europe. These monsters were the last of a group of meat-eating dinosaurs called tyrannosaurs, which came into existence some 150 million years ago.

Unlike the giant T. rex, most early tyrannosaurs were modestly sized hunters. They probably had hairlike feathers for warmth, camouflage, and display. Timurlengia reveals tyrannosaurs’ humble origins, but it also hints at their future dominance. It was a horse-sized tyrannosaur that lived in what is now Uzbekistan some 90 million years ago. The dinosaur is named for Timur (sometimes called Tamerlane, as in a poem by Edgar Allan Poe), a conqueror who ruled that region with an iron fist from the late 1300’s through the early 1400’s. Its specific name, euotica, references its keen sense of hearing. While Brusatte’s team only found a little over a dozen fragments of Timurlengia’s skeleton, some of these pieces contained a large inner ear, similar to later giant tyrannosaurs like T. rex. The fragments revealed that the dinosaur had a relatively large brain, also like its later cousins. Apparently, tyrannosaurs had all the smarts and sensory equipment to become top predators 90 million years ago, but they still lived in the shadow of the more primitive allosaurs.

At some point 80 to 90 million years ago, the allosaurs were replaced as apex predators by the tyrannosaurs, which eventually reached the giant proportions of T. rex. Although Stephen Brusatte and the team’s findings reveal that tyrannosaurs were already smart hunters with keen senses, it still isn’t known why exactly this change in dominant dinosaur occurred. Tyrannosaurs could have slowly replaced allosaurs because of their superior intelligence and senses. Or, an environmental disaster could have caused the allosaurs to go extinct, making room for the tyrannosaurs to thrive. More finds like Timurlengia will help scientists determine how tyrannosaurs emerged from the shadows to become the kings of the dinosaurs.

March 9, 2016

Next month, in April, a deep-ocean drilling project will begin off the Yucatán coast in the Gulf of Mexico. Most such oceanic drill projects are concerned with oil exploration. This isn’t your usual drill team, however. The drillers in this case come from the International Ocean Discovery Program, the National University of Mexico, and the University of Texas. And they will be drilling into Chicxulub Crater, an impact crater formed by a giant asteroid that helped kill the dinosaurs some 65 million years ago. They won’t be looking for oil; they’ll be looking for answers.

The Chicxulub Crater along the northern coast of Mexico’s Yucatán Peninsula formed when an asteroid hit the earth about 65 million years ago. Debris from the impact may have led to the extinction of the dinosaurs.
Credit: WORLD BOOK map

At the end of the Cretaceous Period, dinosaurs were the dominant land animals on the planet. Other reptiles filled the seas, and birds—descendants of dinosaurs—roamed the skies. Mammals existed, but they were far smaller and less common than today. Then, an asteroid at least 6 miles (10 kilometers) wide slammed into the Yucatán Peninsula, in present-day Mexico. The impact threw up large amounts of gas and dust into the atmosphere. This material would have blotted out the sun for many years. Plants that use sunlight to make their food would have died out, followed by the animals that ate them. With no more prey animals, large carnivores starved as well.

When the dust settled, about half of all species on Earth had gone extinct. All the dinosaurs—except birds—were dead. Only a few kinds of other reptiles survived. Mammals survived too and over time evolved to become the dominant large-bodied animals on land and in the sea.

The Yucatán asteroid formed a large impact crater, which is now partly on the peninsula and partly in the Gulf of Mexico. Tens of millions of years of plate tectonics and erosion have taken their toll on the crater, and it is barely visible in satellite images today. But the mark that it left in the rocks should still be clear.

The team plans to use a drilling ship to sample rock deep beneath the ocean floor. In drilling into the crater, scientists hope the presence (or lack) of microfossils (tiny preserved remains of ancient organisms) will teach them more about the nature of the asteroid impact and how quickly life returned to the area afterwards.

The drilling evidence may also better explain how responsible the impact was for the mass extinction at the end of the Cretaceous Period. Most scientists agree that the asteroid did most of the damage, but others argue that other causes, such as massive volcanic activity in present-day India, were more to blame. Such extinction hypotheses are not mutually exclusive, however: it may have taken more than one disaster to knock out the dinosaurs. In fact, they may even be linked. Recent studies have suggested that the Yucatán impact caused the spike in volcanism on the other side of the globe in India. Whatever the case, the drilling team will help get to the bottom of this and other stories, as well as to the bottom of the impact crater itself!

September 5, 2014

Paleontologists have unearthed fossils in the Patagonian Desert in Argentina that represent the most complete giant dinosaur ever discovered, the journal Scientific Reports announced today. According to Kenneth Lacovara of Drexel University in Philadelphia, his team has uncovered 70 percent of the key bones needed to fully describe the creature, one of a group of dinosaurs known as sauropods. Sauropods were immense, long-necked, plant-eating dinos that were the most massive beasts ever to roam the surface of Earth. The scientists estimate that the newly found sauropod measured 85 feet (26 meters) in length and weighed approximately 60 tons (54 metric tons).

Seismosaurus, the huge sauropod depicted in this illustration, was one of the largest dinosaurs that ever lived. Scientists believe it measured about 150 feet (45 meters) in length. The giant fossil Dreadnoughtus recently found in Argentina may not have been as large as Seismosaurus. But, the recently found skeleton of Dreadnoughtus is the most complete ever discovered for the sauropods, and it will allow scientists to learn much about how these giant dinosaurs moved and lived. ( World Book illustrations by Tim Hayward, Bernard Thornton Artists)

Lacovara and his team named the 77-million-year-old fossil Dreadnoughtus schrani after the massive battleships introduced at the turn of the last century. “Dreadnoughtus was huge, and in its environment there would have been nothing that could have preyed on it; it was essentially impervious to attack,” explained Lacovara. “And that evoked in my mind those turn-of-the-last-century battleships—the first really big steel battleships—that were also impervious to attack from the other ships that existed at that time. So, what better name than ‘dread nought’—’fears nothing’.”

The Dreadnought battleship was introduced by the British Navy in 1906. The first modern battleship, the Dreadnought was more powerfully armed and more heavily armored than any earlier warship.  On the seas, it “feared nothing.” (World Book illustrations by George Suyeoka)

Additional World Book article:

• Fascinating Facts About Fossil Feces (a special report)

May 19, 2014

Fossils from a massive dinosaur that was longer than two tractor-trailer trucks parked end-to-end and weighed more than 14 African elephants have been unearthed in the Patagonia region of Argentina. The 95-million-year-old fossils were found near La Fletcha, about 800 miles (1,300 kilometers) south of Buenos Aires, the capital. The yet-unnamed dinosaur may have been the biggest of the big–a new species of titanosaurus, the largest of the gigantic, long-necked dinosaurs called sauropods.

Sauropods walked on four stout, strong legs, much like those of an elephant. Nearly all sauropods had a long neck, a small head, a long tail, and a huge, deep chest and stomach region. Sauropods were the largest plant-eaters, feeding on the leaves of tall shrubs and of such trees as conifers. During the Cretaceous Period, they declined in importance in the Northern Hemisphere. But they remained the dominant plant-eaters in what are now South America, India, and Africa. Other supersized saurpods include Seismosaurus and Supersaurus.

A new sauropod discovered in Argentina may have been larger than Supersaurus (above), one of the largest dinosaurs to ever walk on Earth. (World Book illustration by John Francis, Bernard Thornton Artists)

The newly discovered sauropod was an estimated 130 feet (40 meters) long, weighed 88 short tons (80 metric tons), and stood 65 feet (20 meters) tall at the hips.  (By way of comparison, Tyrannosaurus rex was about 40 feet (12 meters) long, weighed about 7 short tons (6.3 metric tons), and stood about 12 feet (3.7 meters) high at the hips.)

The Argentine and Spanish paleontologists who excavated the fossils based their estimates of its gargantuan size on the largest of the thigh bones found. About 150 bones from at least seven individual dinosaurs have been found at the site. They include 10 vertebrae (bones of the spine) from the torso, 40 vertebrae from the tail, parts of the neck, and complete legs. Scientists cannot yet say with certainty that the newly discovered sauropod is the largest dinosaur–and, therefore, the largest animal–to ever walk Earth. Fossil skeletons from sauropods are generally incomplete. In addition, sauropod tails, which accounted for much of the dinosaurs’ length, are particularly rare.

Additional World Book articles:

• Prehistoric animal
• Tyrannosaurus Rex: The Tyrant Still Reigns (a Special Report)
• Paleontology (2003) (a Back in Time article)
• Paleontology (2006) (a Back in Time article)
• Paleontology (2008) (a Back in Time article)
• Paleontology (2013) (a Back in Time article)

March 27, 2014

To scientists’ surprise, 70-million-year-old dinosaur fossils found some years ago in Alaska actually belonged to a previously unknown miniature cousin of Tyrannosaurus rex. This new species of pygmy dinosaur, Nanuqsaurus hoglundi, had a skull that was only 23 to 27 inches (60 to 70 centimeters) as an adult. The skull of an adult T. rex could be up to 60 inches (150 cm) long. (Note: The words pint-sized and pygmy here are relative. N. hoglundi was about the length of a polar bear, compared to which, students are pint-sized.)

The fossils—fragments of the top of the skull and jaw—were found in 2006 on Alaska’s North Slope, inland from Prudhoe Bay. The discoverers, who were involved in analyzing other fossils, shelved the fragments in a museum. When they recently examined the find, they discovered that the bones represented a new species of tyrannosaur. The tyrannosaurs, which lived during the late rank part of the Cretaceous Period, rank among the most frightening meat-eaters of their time. Tyrannosaurus rex, whose name means king of the tyrant lizards, is the most famous—but not the only—member of the genus.

Tyrannosaurs were fast, active carnivores. They roamed what are now western North America and east-central Asia. (c) Jan Sovak

A study of T. rex‘s mini cousin revealed that the part of N. hoglundi‘s brain devoted to smell was particularly enlarged. This suggests, the scientists said, that the dinosaur stalked its prey mainly by smell. A keen sense of smell would have been important in N. hoglundi‘s challenging environment. Although temperatures in what is now Alaska were much warmer 70 million years ago, the area still experienced long periods of darkness and challenging seasonal changes in the amount of available food. Scientists think the small size of this dinosaur predator was an adaption to its harsh environment. A smaller body is favored by natural selection because of the limited food sources available.

Additional articles in World Book:

• Tyrannosaurs rex: The Tyrant Still Reigns (a Special Report)
• What Has Caused Mass Extinctions? (a Special Report)