Brontosaurus.
Credit: © Emiliano Troco

Sauropods were the most spectacular of the dinosaurs. Their long necks supported small heads that took in needles and leaves. Despite such a nutrient-poor diet, they reached sizes and lengths unparalleled in any other terrestrial (land-dwelling) animals. How—and why—did they get so large? A recent study may have discovered a lead to unraveling the physiology of these amazing animals. 

Over tens of millions of years, the arrangement of the continents has shifted through the action of plate tectonics. Geologists can trace how a location has moved over the face of Earth to determine its paleolatitude. The paleolatitude is the position of a point on the Earth’s surface in relation to the equator at a time in the distant past. Both latitude and paleolatitude are measured on a scale of 0° (the equator) to 90° (the poles). Higher latitudes experience cooler temperatures and less sunlight in winter. 

Dinosaurs reigned during the Mesozoic Era—a time of warmer climates. Despite the planet being largely ice-free, regions near the poles still faced cold winters and weeks or months without sunlight. Nevertheless, dinosaurs have been found at high paleolatitudes, including in Antarctica and Alaska. At least some of these dinosaurs remained there through the winter. 

Sauropod fossils, however, are conspicuously absent from these polar locations. No sauropod fossil has been discovered from a paleolatitude higher than about 65°. Instead, these chilly climates were strictly occupied by meat-eating theropods and some plant-eating dinosaurs called ornithischians. 

Some paleontologists (scientists who study prehistoric life) suspected the absence of fossils suggest that sauropods preferred warmer climates. But others thought that sauropods might not have fossilized well near the poles for some reason, or that the fossils are still waiting to be discovered. 

A team led by Alfio Alessandro Chiarenza of the University of Vigo in Spain analyzed the paleolatitude of all the places where sauropod fossils have been found. The team published their findings last month in the scientific journal Current Biology. They determined that the absence of sauropods at high paleolatitudes was not due to incomplete sampling. Chiarenza’s team used models of the Mesozoic climate and found that sauropods preferred savanna-type habitats. Sauropod ranges were tightly constrained by the lowest predicted temperature. 

Why didn’t (or couldn’t) sauropods brave the cold? They might have cooled down too quickly, despite their massive sizes. Many theropods—and possibly some ornithischians—had downy or hairlike feathers that could be used to keep them warm. Sauropods lacked any such insulation. Furthermore, a sauropod’s long necks and tails might have lost heat quickly when exposed to bitter-cold winds. 

Sauropods buried their eggs in the earth to keep them at a stable temperature. But this method probably would not have kept the eggs warm enough in cold climates. In contrast, theropods sat on their eggs and ornithischians covered their eggs in rotting plants. Both of these approaches could keep the eggs warm even in cold climates. 

Chiarenza’s team proposes that sauropods did not possess as high of a metabolism as theropods and ornithischians. Historically, scientists have classified animals as endothermic (“warm-blooded”) or ectothermic (“cold-blooded”). Endothermic animals tend to produce more of their own body heat, while ectothermic animals tend to rely more on their environment for heat. This is a false division, since every animal is somewhat reliant on its environment for heat. But animals classified as endothermic can usually survive in cooler temperatures. Reptiles, the classic endotherms, are concentrated near the equator. 

In many ways, this makes a lot of sense. A lower metabolism would have enabled sauropods to survive on less food. Their huge size would cause them to lose less heat to their environment, much like a well-insulated building. Growing evidence suggests that the ancestor of dinosaurs and crocodilians (a group of reptiles) had a high metabolism, but crocodilians “slowed down” when dinosaurs took over most environmental niches. A similar changed might have occurred in sauropods as well, enabling them to attain colossal sizes. Sometimes slow and steady wins the race. 

March 30, 2020

Some 15 million years ago, long after the non-bird dinosaurs and the beasts they lived alongside had gone extinct, giant creatures still walked (or swam) the Earth. Recently, the discoveries of a team of scientists led by Edwin Cadena of Del Rosario University in Colombia have been putting more of a face—or a shell, in this case—on one of these ancient giants. And, not only did the giant have a shell, but the shell had spikes! Meet Stupendemys, the titanic turtle.

Stupendemys lived during the Miocene Epoch, around 23 million to 5.3 million years ago. credit: © Jaime Chirinos

The giant aquatic (water-dwelling) Stupendemys had a big head with a sharp beak. It had four paddlelike limbs and a short tail. It probably could not withdraw its head or limbs into its large shell, as can many turtles. Its shell alone measured about 8 feet (2.4 meters) long. Stupendemys weighed about 2,500 pounds (1,150 kilograms). It and the prehistoric marine turtle Archelon were the largest turtles known to have ever lived.

Stupendemys had been known since the 1970’s from huge pieces of shells and limb bones, but no cranial (skull) material had been positively identified. Cadena’s new trove of Stupendemys fossils included skull fragments, however. Found in Colombia and neighboring Venezuela, the skull pieces matched those of previously unidentified ancient turtle remains discovered in other parts of South America, showing that Stupendemys was fairly widespread.

Some­ of the Stupendemys shells found by Cadena and his team had large forward-pointing horns at the shoulders. The scientists believe this is a case of sexual dimorphism. Sexual dimorphism is a difference in body size or shape between males and females of the same species. Males probably used these horns in combat over mates and territory. Deep gouges were often found near the Stupendemys shell horns, suggesting that males literally locked horns when fighting.

Stupendemys was not the top boss of South American waterways during the Miocene Epoch, a time in Earth’s history that lasted from 23 million to 5.3 million years ago. The turtle was likely a gentle giant, surviving on a diet of hard-shelled mollusks, fruits, and seeds. And its huge size did not grant it complete protection from predators. Giant crocodilians prowled the region at that time, including the 40-foot (12.5-meter) giant caiman Purussaurus. One Stupendemys shell found by Cadena’s team contained an embedded crocodilian tooth! It’s hard to say who came out on top in that encounter, but it was likely an epic struggle.

April 13, 2017

For thousands of years, indigenous (native) people of Western Australia knew about giant ancient footprints along the shore of the Indian Ocean. But only recently have scientists learned about, and been able to study, the tracks, which were made by dinosaurs some 100 million years ago. A team of scientists led by Steven W. Salisbury of the University of Queensland studied the collection of fossilized footprints—which includes the largest ever discovered—for five years. Salisbury and his team recently published their findings as a memoir in the Journal of Vertebrate Paleontology.

Richard Hunter, an elder of the Goolarabooloo community, lies alongside a massive sauropod track in the Walmadany area of Dampier Peninsula in Western Australia. Credit: © Steve Salisbury, University of Queensland

A fossil is the mark or remains of an organism that lived thousands or millions of years ago. Most people think of bones or shells when they hear the word fossil. But tracks, trails, and burrows left by ancient organisms are also extremely important in paleontology (the study of prehistoric life). These marks, called trace fossils, give paleontologists a rare glimpse into the lives of prehistoric animals. The scientists can use trace fossils to answer many questions about an animal’s behavior, such as how it moved or how many animals moved together at a time. Scientists cannot usually pair a trace fossil to an exact species (kind) of animal, but they can often determine broadly what type of animal left the mark.

The fossilized tracks in question are on the northern shores of Western Australia. About 130 million years ago, the region was a sandy floodplain covered with braided rivers. Braided rivers have numerous channels separated by small temporary islands. After the tracks were made, floods rapidly covered them in sediment, preserving them from destruction. Thousands of tracks are scattered over several dozen sites in the area, and about 150 are in excellent condition. The findings give scientists a valuable snapshot into life during the early Cretaceous Period in Australia.

Salisbury and his team identified several types of prints coming from ornithopods (plant-eating dinosaurs that could walk on two or four legs), sauropods (large plant-eating dinosaurs with long necks and tails), stegosaurs (relatives of Stegasaurus), and theropods (meat-eating dinosaurs) of different sizes. One of the tracks measures a whopping 5 ½ feet (1.7 meters) long. This print was made by the hind foot of a huge sauropod some 18 feet (5.5 meters) tall at the hip.

The indigenous people of the Western Australia coast had known of the tracks for thousands of years and had incorporated them into their belief system. In one story, the Dreamtime figure Marala, also known as the Emu Man, makes the three-toed footprints that today are believed to have been made by theropods. (The emu is an Australian bird that has three toes on each foot.) The Dreamtime is a fundamental spiritual concept that connects traditional beliefs and practices among the Aboriginal people of Australia.

In 2008, the state government of Western Australia—unaware of the ancient tracks—proposed that a natural gas processing facility be located near the site. Fearing that the tracks would be damaged or destroyed, the Aboriginal people contacted Salisbury to assess the tracks’ scientific importance. As word spread of the natural gas plant and the damage it could cause to the tracks, environmental groups, paleontologists, and local citizens campaigned for the area to be preserved. The company planning to build the processing plant eventually withdrew its application. Now the tracks, with their important connections to prehistory and the Dreamtime will remain protected.

September 28, 2016

When you see models or illustrations of dinosaurs, have you ever wondered how accurate they are? Bones, skin impressions, and tracks can tell scientists and artists a great deal about the shape, size, and movements of these animals, but how do people know what color patterns the beasts adopted? Most of the time, artists make educated guesses based on what is known about each dinosaur and its habitat. But researchers led by Jakob Vinther of the University of Bristol in the United Kingdom recently collaborated with artist Bob Nicholls to create what may be the most accurate representation of a dinosaur to date. What’s more, the finished model provided clues about the animal’s habitat (rather than the other way around).

The recently created Psittacosaurus model shows the dinosaur with countershading camouflage (darker on top, lighter on bottom). Many modern-day animals exhibit countershading. Credit: © Jakob Vinther, Robert Nicholls, Stephan Lautenschlager, Michael Pittman, Thomas G. Kaye, Emily Rayfield, Gerald Mayr, Innes C. Cuthill/Univerity of Bristol

Psittacosaurus was a small plant-eating dinosaur that lived 130 million to 100 million years ago in what is now Asia. Its name, which means parrot lizard, comes from its large, sharp, parrotlike beak. The dinosaur measured up to about 2 feet (60 centimeters) tall at the hips and 6 1/2 feet (2 meters) long.

Vinther’s team examined an exquisitely preserved Psittacosaurus from China that included much of its skin. They found that darker areas of skin were full of melanosomes, tiny structures that produce pigments, but the lighter areas were not. This discovery showed that the color pattern seen in the fossil was real, and not the result of discoloration during fossilization.

With that knowledge, the researchers turned to Bob Nicholls, a Bristol-based paleoartist (an artist who specializes in depicting prehistoric life). He carefully examined the specimen, taking into account every minute pattern on the animal’s skin. He then created two sculptures of the animal, painting one a uniform shade of gray, and decorating the other with the intricate patterns seen in the fossil.

From the sculpture, it became clear that Psittacosaurus exhibited countershading. Countershading is a type of camouflage in which the parts of an animal exposed to sunlight are darker colored and the animal’s underside is lighter colored. In sunlight, the exposed, darker-colored sections appear brighter and the lighter-colored sections are shaded by the rest of the body. This effect flattens the appearance of the animal and makes it more difficult to see. Many kinds of animals, from sharks to antelope, exhibit countershading. In the case of Psittacosaurus, this small herbivore (plant eater) was countershaded to try to escape the sight of large meat-eating dinosaurs—relatives of the infamous Tyrannosaurus rex—that lived in the area at the time.

Vinther and his team then took the two sculptures to ecosystems with different lighting characteristics and observed how well camouflaged the countershaded model was in relation to the gray one. They found that the countershaded sculpture was best camouflaged in areas with moderate tree cover, suggesting that this species of Psittacosaurus lived in woodland environments. This evidence also jibes with other fossils that have been found along with the Psittacosaurus, such as petrified tree trunks and remains of shade-loving plants. This is the first time that a dinosaur’s color pattern was used to infer the type of environment in which it lived.

This study showed that determining the colors and color patterns of dinosaurs isn’t just a goal in and of itself, but it can also help define a dinosaur’s ancient environment. It’s not just good for art; it’s good for science!

August 17, 2016

Eons ago, a monstrous beast stalked an ancient South American floodplain. One of its enormous footprints in the soft clay was covered with layers of silt and was preserved for some 70 million years. Last month, the track was discovered, revealing that huge predatory dinosaurs lived in South America up until the extinction of the group about 65 million years ago.

The record-setting dinosaur footprint was found at Maragua Crater near Sucre, Bolivia, a site already known for other, smaller dinosaur tracks. Credit: WORLD BOOK map

Dinosaurs are a group of prehistoric reptiles that ruled Earth for about 160 million years. Most of these animals died millions of years ago, but their direct descendants—birds—continue to flourish today. Dinosaurs have fascinated people ever since they were first described in the early 1800′s as having strange appearances and huge sizes. Scientists now know that not all dinosaurs were large. Many, such as the microraptor and compsognathus, were, in fact, quite small.

The South American footprint, however, belonged to something gigantic with really big feet. It was found about 45 miles (60 kilometers) outside of Sucre, the official capital city of Bolivia, by a local tour guide. At some 45 inches (115 centimeters) wide, it is the largest carnivorous (meat-eating) dinosaur footprint ever discovered. The previous record was nothing to sneeze at either: a 40-inch (110-centimeter) wide track from New Mexico, a state in the southwestern United States.

The animal that made the South American print probably belonged to a group of dinosaurs called abelisaurids, large meat-eaters with short skulls and tiny arms that lived in South America, Africa, and India. Based on the enormous size of the footprint, scientists think the dinosaur could have been up to 40 feet (12 meters) long.

The discovery of this footprint helps paleontologists fill in the history of large meat-eating dinosaurs in South America. Giganotosaurus, one of the largest meat-eating dinosaurs known, stalked the continent some 95 million years ago. But Giganotosaurus probably died out after 5 million years or so, and paleontologists had not found fossil evidence of any large carnivores taking its place. The print was dated at 70 million years old, showing that abelisaurids took over after the demise of Giganotosaurus. With any luck, paleontologists will soon find the bones of these giant hunters and better understand the ecology of South America at the end of the age of dinosaurs.

July 19, 2016

Late last month, paleontologists (scientists who study fossils) announced an amazing discovery. Researchers led by Lida Xing at the China University of Geosciences in Beijing had discovered two bird wings preserved in amber. They published their findings in the journal Nature Communications.

Amber preserved this 100 million-year-old wing tip featuring bones, feathers, and soft tissue.
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. In the 1993 science fiction film Jurassic Park, dinosaur DNA (deoxyriboneucleic acid) is discovered in the blood of an ancient mosquito fossilized in amber. Movie scientists then used the DNA to recreate dinosaurs—an improbable, yet intriguing, plot line.

The wing fossil subjects of last month’s report were formed about 100 million years ago, in the Cretaceous Period, in what is now Myanmar (also called Burma). Two birds apparently became stuck in the sticky resin of a tree and died. The amber preserved the three-dimensional structure of the birds’ wings, as well as the wings’ feathers, skin, and bones—even the color patterns!

Xing and his team think the wings came from a group of birds called enantiornithines, which means opposite birds in Greek. These birds had claws and teeth, and they went extinct along with the nonflying dinosaurs about 65 million years ago. The fossils showed that the wings were from young birds and that the birds hatched as miniature adults, ready to fly. This is different from modern birds, which must develop for weeks or months before they can leave the nest.

The structure of the wings and the arrangement of feathers are similar to modern bird wings. Birds evolved (developed over time) about 150 million years ago from meat-eating dinosaurs, so they must have quickly developed modern-looking wings, before enantiornithines and the ancestors of modern birds split.

Unlike science fiction, these fossils won’t resurrect the extinct enantiornithines, even if they do contain DNA. The technology to create entire animals from bits of ancient DNA does not yet—and might never—exist. The fossils do, however, offer paleontologists a treasure trove of information that will help us better understand early birds and their world.