Dressing up dinos

Adding soft tissue to bone helps scientists, paleoartists bring ancient creatures to life

Fossils of an ancient animal don’t typically include much more than the creature’s hard parts — sometimes intact, but often shattered to smithereens. Lucky paleontologists may stumble upon a well-preserved, nearly complete skeleton that offers a rough idea of an animal’s size and shape. But fossils that preserve soft tissues — skin, flesh, feathers — are the rarest of the rare. These geological treasures form and survive only under special environmental conditions that scientists are just now beginning to understand.

DRESSING UP DINOS Conditions resulting in the near-complete preservation of a dinosaur are rare. This artist’s illustration depicts a young hadrosaur that was discovered in 2001 after becoming mummified 77 million years ago. copyright Julius T. Csotonyi/www.csotonyi.com
HOLEY HINTS Scientists often rely on bones to figure out how dinosaurs looked. The number of holes called neurovascular foramina in a creature’s facial bones (part of T. rex upper jaw shown) can hold clues about soft tissues. A. Morhardt
THAT’S CHEEKY Counts of neurovascular foramina suggest that Triceratops (model of skull at upper left) could have had one of two possible types of tissue around its mouth. The large herbivore could have had flexible, snakelike lips (modeled at upper right) or small, fleshy cheeks (modeled at lower right). T. Keillor
ADDING MEAT Computer models put virtual flesh onto bones, providing estimates for how much dinosaurs might have weighed (hefty T. rex at top, slender version at bottom). K. Bates et al./PLoS One 2009

Bones give just an overall hint of what an animal such as a fearsome Tyrannosaurus rex or a three-horned Triceratops might have looked like. Soft tissues, though, disclose more details. The recent discovery of a Triceratops fossil that included skin impressions provided surprising information: The creature may have sported bristlelike structures. “The skin is unlike anything we’d expected,” says Bob Bakker, a paleontologist and a visiting curator at the Houston Museum of Natural Science.

And sometimes soft tissues offer hints about how a creature might have functioned, lived and behaved. The size, shape and arrangement of feathers on an ancient dinosaur, for example, can suggest whether those structures aided flying and gliding, as in Microraptor, or whether they played a role in display, as in Caudipteryx.

Scientists are increasingly turning to new techniques to make up for the dearth of soft tissues preserved in the fossil record. Some investigate dinosaurs’ closest modern-day relatives, including crocodiles and birds. These studies hold clues to not only appearance, but also function — the extent and type of flesh present around a dinosaur’s teeth and jaws could suggest whether a creature could seal its mouth while chewing and, in turn, what types of food the animal might have eaten.

Other researchers use computers to add virtual flesh and muscle to bones. By laser-scanning museum specimens, scientists can now more accurately construct 3-D models of dinosaurs that provide a reasonable range of weight estimates for the creatures. Information garnered from other studies — how much cartilage a dinosaur’s knee might have had, perhaps, or the amount of muscle that once surrounded its bones — further flesh out cyberdinos. When fed into biomechanical simulations, such data show how quickly or efficiently a dinosaur might have moved.

Ultimately, these findings make it into the hands of paleoartists, the sketchers, sculptors and animators who work independently or with scientists to generate a picture of the past and bring long-extinct creatures back to life.

Signs of attachment

A dinosaur’s bones, regardless of how they’re found in rocks or mounted in museums, weren’t isolated structures. Like all vertebrates’ skeletons, they were once nourished by blood vessels, riddled with nerves and linked to each other with ligaments and to muscles with tendons. Each of these interfaces leaves a trace on a well-preserved fossil, says Ashley C. Morhardt, a vertebrate paleontologist at Western Illinois University in Macomb.

Small openings called neurovascular foramina pepper the facial bones of vertebrates. These holes — typically around 0.2 millimeters across, about the diameter of the body of a sewing pin, but sometimes larger — make space for the arteries that supply blood to the lips and cheeks and for the nerves that allow for sensation, Morhardt says. The number of neurovascular foramina on the bones around a creature’s mouth relates to the type and amount of overlying soft tissue, she and her colleagues reported in September in Bristol, England, at the annual meeting of the Society of Vertebrate Paleontology. By counting the foramina on a fossil’s facial bones, the team suggests, scientists can start to reconstruct a dinosaur’s countenance.

During the study, the researchers dissected modern animals with different types and amounts of facial tissue and then counted foramina. Most creatures that had a toothy smile with exposed teeth and little if any soft tissue around the mouth, such as crocodiles and their relatives, had more than a hundred small foramina on each bone around the jawline. For those with beaks made of nonpliable material, such as birds, turtles and tortoises, average foramina counts on each facial bone ranged from 50 to 100.

Most mammals, which have flexible cheeks and lips, typically had fewer than 50 foramina per facial bone. Sea lions were an exception to this rule, however, probably because they forage in deep water where it’s dark and therefore depend on whiskers and other sensory structures in the lips to locate food, Morhardt says. Lizards and snakes, which have liplike, fleshy tissues that cover the teeth and make an airtight and watertight seal, usually have no more than a dozen or so foramina.

Based on these trends, the researchers proposed at the vertebrate paleontology meeting that Herrerasaurus, an early predatory dinosaur with few neurovascular foramina, had lizardlike lips. Camarasaurus, a large herbivorous sauropod with peglike teeth, probably had fleshy cheeks that could form a seal and hold a mouthful of vegetation. For Triceratops, another vegetarian, the data aren’t so clear: These dinos could have had either lizardlike lips or small cheeks. “It’s pretty obvious that they had some kind of extra-oral covering,” Morhardt says. “We just haven’t been able to tease out what kind.”

Knowing the type and amount of soft tissue surrounding the mouth helps scientists picture the dinosaurs and provides clues about how dinosaurs fed and what they ate, says Matthew Bonnan, a paleontologist at Western Illinois University and Morhardt’s graduate adviser. By counting neurovascular foramina on Aardonyx, a recently described ancestor of sauropods, he and his colleagues inferred that the creature had lips but didn’t have fleshy cheeks. Without that constraint on its gape, the dino could have opened wide to gather large mouthfuls of browse — an ability that may have set the evolutionary stage for subsequent sauropod species to grow exceptionally large (SN Online: 11/10/09), the team reported online November 10 in Proceedings of the Royal Society B.

Such insights aren’t limited to faces, Bonnan notes. The surface texture of well-preserved limb bones holds information about where muscles attached and how big — and powerful — those muscles might have been.

Know what’s missing

Though most of an animal’s soft tissues decompose after death, some partially mineralized bits have a head start on fossilization and become preserved. These remnants may provide hints about tissue that’s missing, Bonnan says.

While it’s easy to measure the cartilage in a fossil, for example, it’s often difficult to estimate how much uncalcified — and now missing — cartilage was present to begin with. That, in turn, makes it difficult to ascertain the spacing and positioning of limb bones and, from that, the efficiency of a creature’s movement.

Previously, scientists looked to general similarities among modern creatures and employed a lot of informed guesswork and trial and error to fit bones together. Now, Bonnan and his colleagues have come up with detailed methods to better estimate how much cartilage may have been present in a dinosaur’s joints. Once again, the team reported at the paleontology meeting in Bristol, the technique stems from analyses of modern-day dino relatives.

For their study, the researchers looked at joints from the front and hind limbs of alligators, as well as those in the wings and legs of ostriches and helmeted guinea fowl. First, the team measured the total amount of cartilage present in recently dissected joints. Simmering the bones at 60° Celsius for 24 hours removed the uncalcified cartilage but left calcified tissue intact. “The lab smelled like bad chicken soup for quite a while,” Bonnan admits.

Removing the uncalcified cartilage from the joints of alligators shortened the humerus, the bone analogous to the one in the human upper arm, by about 5 percent, the team found. In the helmeted guinea fowl, removing that tissue shortened the limb bones by around 8 percent, and in juvenile ostriches, it trimmed the bones’ length about 15 percent. With this information, the team suggests, scientists can estimate the amount of uncalcified cartilage missing from a fossil limb bone and reconstruct the creature accordingly.

“If you want to be accurate, you need to account for these things,” says Tyler Keillor, a paleoartist at the University of Chicago. “What is the missing joint surface? How much space is there going to be? How would that have affected the range of motion?” Answering such questions helps scientists get a well-developed idea of how a dinosaur functioned as a living creature, he notes.

Scientists have been attempting to reconstruct ancient creatures by studying modern ones since the 1790s, says Bakker. The general arrangement of muscles, he notes, is the same for creatures as diverse as the salamander and the rhino. “We can reconstruct about 95 percent of their body with great precision,” he adds. “The scalpel is still one of our best tools.” New studies such as Bonnan’s and Morhardt’s are fleshing out some of the remaining details, he says.

A possibly more important finding relates to the distribution of tissue in joints, not just to the length of what’s missing, Bonnan says. For the adult animals that the team studied — including all of the helmeted guinea fowl, all alligators and one ostrich — the shape of the uncalcified cartilage in each joint showed little difference from the shape of the underlying calcified tissue. That was especially true for those creatures’ weight-bearing joints, such as a biped’s hind limbs, Bonnan notes.

Analyses also show that the overall distribution of cartilage provides insight into the forces that joints experience on a day-to-day basis. This finding should enable scientists to more accurately assess the posture of ancient creatures, and paleoartists to more accurately depict them, Bonnan says.

Model behavior

The posture of a dinosaur, as well as the size, shape and weight of its limbs, would have substantially affected how the creature moved through its environment. Until recent decades, most scientists thought that dinosaurs such as Triceratops were slow, lumbering creatures with a sprawling, crocodile-like posture. Evidence now indicates that Triceratops stood, walked and maybe even ran with its limbs beneath its body (SN: 11/4/00, p. 300).

Using detailed analyses of modern creatures, such as those by Bonnan and Morhardt, scientists can virtually add soft tissue to a dino’s skeleton. Then, with sophisticated computer models similar to those that evaluate the performance of cars and aircraft, paleontologists can readily assess just how much a dinosaur might have weighed or how fast it might have moved. Recently, Phil Manning, a paleontologist at the University of Manchester in England, and his colleagues used such models to analyze T. rex, among other dinosaurs.

First, the team scanned museum specimens with lasers to create high-resolution, 3-D models of the dinosaurs’ skeletons. Data from studies of living creatures, Manning notes, allowed the team to account for low-density spaces such as lungs, air sacs and even sinuses.

One of the specimens the team analyzed — the second-largest T. rex ever discovered, a beast nicknamed Stan — is 11.9 meters long. Stan weighed somewhere around 7.6 metric tons (about 40 percent more than the average African bull elephant), the researchers reported last year in PLoS ONE. Each of Stan’s legs weighed more than 1 metric ton, the team estimates.

Long muscular limbs can give a dinosaur power, but at the cost of speed: The fleshier a limb is, the more energy the creature must expend moving it back and forth, Manning says. Previous studies have suggested that T. rex could run only slowly if at all (SN: 3/2/02, p. 131), and Manning agrees: “The smaller, more gracile theropods will have the maximum running speed. When you get to something the size of T. rex, they’re not going to be the top sprinters.… They were bloody powerful organisms, but they were not built for speed.”

Real-world evidence can lend credence to such computer models. A good simulation of a walking dinosaur, for instance, should be able to reproduce a set of footprints with the same size and spacing seen in the fossil record. If the two don’t match, the researchers can tweak the model, thereby improving its performance, Manning notes.

Knowing the type and amount of soft tissue that cloaked a dino’s bones helps scientists sift through hypotheses about dinosaur anatomy and behavior. Whether a creature could have closed and sealed its mouth has profound implications, says paleoartist Keillor. “If you don’t have a sealed mouth, how can you efficiently sniff?” he asks. “How do you keep from drying out in an arid environment?… It’s an interesting mental exercise.”

Such scientific findings are a boon for paleontologists trying to understand the past and for paleoartists trying to depict it, whether they represent the dinosaurs in a hyperrealistic style or in a highly stylized fashion.

“Pictures really help paleontologists communicate with the general public,” says Robert Walters, a paleoartist based in Philadelphia. “You can write and describe it any way you like, but you really need to see the pictures to understand the structure of the animals.”

And new technologies such as virtual reconstructions allow scientists to reexamine things in ways that couldn’t be done before, he adds.

“Without the information that scientists develop, we’d be dead in the water,” Walters says. “We’d just be making up dragon creatures.”