Dinosaurs have been extinct for approximately 65 million years, yet they continue to fascinate and awe people all over the world. We have all seen movies and pictures of dinosaurs as huge lizards covered with skin similar to a crocodile. Since we only have some very old fossils to work with, how do we know how muscular these animals were? And how do we know what type of skin they had or how they moved?
Scientists have utilized centuries of biological and paleontological work to determine the most reasonable estimates for dinosaur characteristics by primarily studying fossilized skeletons. By studying dinosaurs’ nearest surviving relatives (birds) as well as large animals of all types, scientists can reverse engineer reasonable estimates for dinosaur mass, musculature, and gait. Like all good scientists, paleontologists make use of all available information to draw their conclusions; in the case of dinosaurs, there are also preserved footprints and fossils detailed enough to make out skin patterns and feathers. With a fossil showing the outline of skin, paleontologists can model the animal’s volume; combined with an estimate of tissue density this yields a weight estimate. Of course, knowing skin patterns also tells us a great deal about how dinosaurs looked and some functional details such as if the animal was covered in hard plates.
From the depth of footprints, researchers can determine another estimate for how much the animals weighed. From a series of footprints, we can deduce important information regarding their gait in the same way a high tech running store will fit shoes to an athlete after observing the patterns from their footprints. Combining the fields of biology and paleontology, paleobiology is the study of ancient plants and animals. Other key fossil discoveries include those of dinosaur dung, which sheds light on their diets. In a few rare cases, fossilized dinosaurs have been discovered where we can still distinguish their last meal. Scientists have uncovered fossilized dinosaur nests and eggs which tell us about their development and behavioral patterns. In 2014, research was published identifying the first record of fossilized pigmentation in marine dinosaurs; this means we can even know the animal’s skin color.
The Fossil Record
Given the millions of years that have passed since the last dinosaur died, it was believed that no actual dinosaur remains exist today. All of the evidence left over by dinosaurs is in the form of fossils. Fossils are formed by mineral saturation over many years after the animal or plant has died. This can happen on a molecule by molecule basis or can be a rock cast of the outline of the specimen. In the second and more common scenario, a bone is encapsulated with sediment to form a perfect mold of the original; over time the sediment hardens into rock and the original bone decays completely to leave an empty cavity. As minerals percolate through the soil they collect in this cavity and form the fossil, which will be composed of rock different than its surroundings.
Since fossils are actually made of rock and not the original material, fossilized dinosaur bones can only tell us some physical properties of the original bones. Fossilized bones are perfect for determining the physical shape and size of the dinosaur’s skeleton. Based on the shape of a bone’s surface and knowledge of living animal anatomy, scientists can determine with reasonable accuracy at what angle the bones fit together as well as where muscles attached and approximately how large the muscles were. However, that is about all that can be determined from bones alone. Depending on the condition of the fossil, it can even be challenging to estimate the weight of the original bones themselves. Bones can be very dense or very porous, which would lead to significant deviations in the weight of the skeleton as well as the weight of muscle and soft tissue that those bones could support. Only in well-preserved specimens, formed on a molecule by molecule basis, do fossils maintain the structure of the bone interior where density can be determined.
A showcase specimen of a 98% complete theropod fossil including the impressions of scaly skin and hair-like feathers (Credit: blogs.nature.com 2011)
Even though you can find beautiful dinosaur fossils in museums and collections all over the world, the exact processes required to form them occur very rarely in nature. If you keep in mind that dinosaurs dominated the earth for about 160 million years, the number of surviving fossils is a minuscule percentage of the number of animals that lived. Fossilization most often occurs in a watery environment as this allows for sediment to quickly build up on top of an animal after its death. Since most dinosaurs lived on land they were rarely preserved; most dinosaur fossils exist due to an animal (or some of its bones) ending up underwater after death.
Revolutionary New Breakthroughs
Fossils, since they are not actually bones, do not contain genetic information about the animals that formed them. At least this was the accepted scientific truth until 2005 when the research was published by the paleontologist Dr. Mary Schweitzer confirming blood cell structures and proteins from a particularly well preserved 68 million year old T. rex fossil. She actually first identified preserved molecular structures in dinosaur fossils in the early 1990s, but her work has only been widely accepted recently.
“The reason it hasn’t been discovered before is no right-thinking paleontologist would do what Mary did with her specimens. We don’t go to all this effort to dig this stuff out of the ground to then destroy it in acid […] It’s great science.” – Dr. Thomas Holtz Jr. quoted in Smithsonian Magazine
Dr. Schweitzer continues her research today and is constantly discovering more detailed information about dinosaur soft tissues, as well as inspiring other scientists to do the same. By comparing birds and dinosaurs, Schweitzer has even been able to identify fossilized medullary bones, which allow us to tell the animal was a pregnant female; dinosaur gender is typically very difficult for scientists to determine. Before Schweitzer’s work, the oldest soft tissue ever recovered was approximately 1 million years old, and her breakthroughs have begun to fundamentally alter how scientists view tissue decay and preservation. Although this does not mean we can fully sequence dinosaur DNA (sorry Jurassic Park fans), the discovery of preserved soft tissue is an incredible breakthrough in the field and will certainly open up new possibilities previously inconceivable.
The Largest Ever
The title of “biggest ever” is highly sought after in many fields, with paleontologists constantly on the lookout for new, larger dinosaurs. Dinosaurs, as well as living animals, are typically compared based on their weight. With only a skeleton, a weight measurement depends on several educated estimates made by paleontologists. All of the top record holders come from the same group of behemoths known as titanosaurs. Titanosaurs are a group of sauropod dinosaurs whose fossils have been found on all seven continents. Although weight is an estimate, dinosaur length can be determined exactly if a complete skeleton is recovered.
In 2013, some largely complete fossils of a new species of titanosaur named Patagotitan mayorum was discovered in the Patagonia region of Argentina. After a careful analysis paleontologists announced earlier this year that they believe it to be the largest dinosaur ever discovered, which in turn makes it the largest known land animal. Based on the diameter of its femur, an adult patagotitan is believed to have weighed a gargantuan 52 to 86 tons (100,000 to 172,000 pounds) and stretched up to 121 feet long; for perspective, the largest modern elephant ever recorded weighed about 12 tons (24,000 pounds). Scientists also used a secondary technique to estimate the size of a patagotitan: 3D computer modeling which resulted in a range of 45 to 77 tons. Although these may seem like large ranges – the high-end estimate is almost double the low end – there is simply no way to recreate this value with greater certainty. Dr. Charlotte Brassey of Manchester Metropolitan University, quoted in The Atlantic, explains that these titanosaurs “were so unusual, with long, slender necks and tails, and an air-filled skeleton, that we have no convincing analogues in the modern animal kingdom […] by virtue of being extremely large, we need to extrapolate our understanding of how animals function far beyond the upper limits of living land animals, such as elephants. The more we need to extrapolate, the less confidence we have in our reconstructions.”
The traditional way of measuring a dinosaur’s size was based on the length and diameter of its femur; this is typically the largest and strongest bone in the body. This is the most reasonable choice of metric since the femur length and diameter must be proportional to the animal’s size, or else it would not be able to support its own weight. In some cases, however, there are only a handful of specimens from what is believed to be a new species of dinosaur. If there is no femur present in the discovery, researchers must examine what fossils they have and make the best estimate based on a similar species with a more complete fossil record. If a complete femur is present then we can obtain a size estimate, but this is based on only one sample – bad news to any statistician. What if the one example we found happens to be the Shaquille O’Neal or Andre the Giant of the species? Or what if it appears adult but is really the fossil of a juvenile? Of course, paleontologists are aware that these estimates are not 100% accurate; but there are very few alternatives. Recent work in the field has centered around computer modeling and simulations to determine the most reasonable weight estimates based on kinetic analysis of how the dinosaur walked. There are other estimation methods as well, but there is really no way to verify which works best since the results cannot be compared to a real dinosaur.
There has been a notable handful of discoveries of remarkable fossils which single-handedly provided great insight to our understanding of dinosaurs. The best-preserved dinosaur fossil is that of a nodosaur (Borealopelta markmitchelli) uncovered in Canada in 2011 by a mining crew. This specimen is a complete version of the front half of the dinosaur, showing even minute details. It is incredibly rare for soft tissue to be fossilized as it typically decays before fossilization begins. In order for soft tissue to be preserved, an animal needs to be encased in a natural sealant (typically mud or clay) before decomposition or scavengers destroy this tissue. Then after this encapsulation, all other requirements of fossilization must also be met.
Museums all over the world have dinosaur skeletons on display, usually posed in a kinetic stance to quickly attract crowds. What is typically not mentioned on the information stand is that almost all of these skeletons are not complete skeletons of the same animal. Images of fossils, including dinosaurs, often show specimens where the entire skeleton is preserved in the same arrangement as when the animal was alive. These rare cases are the most dramatic and quickly convey to the viewer how the dinosaur appeared, so it should be no surprise that they are often used as examples. It is exceedingly rare for one dinosaur fossil to contain the entire skeleton of the animal. For reference, the most complete T. rex (which is famously well studied) fossil ever discovered contained approximately 80% of the complete skeleton. After an animal dies, its remains are typically picked apart by scavengers and can be further scattered by acts of nature; if an animal’s remains happen to fossilize in one cohesive rock, that rock still needs to remain intact for millions of years and appear at ground level in order for us to discover it.
Museums have the option of displaying true fossilized bones or merely castings of real fossils. A trained observer can tell the difference based upon the coloration and surface finish of the display. There are advantages and disadvantages of either decision that must be weighed by the museum when they install a new display. Sometimes, missing pieces of a certain species have never been discovered, leaving no option but to display a casting of paleontologists’ best guess as to how the skeleton appeared. The main goal of most natural history museums is to educate the public and inspire interest in the topic; displays of castings are just as good as fossil displays for meeting these objectives. It is hard to fault a museum for displaying castings as this is typically a safer and more affordable option. Castings weigh less than real fossils, which allows for smaller support structures. By making castings, the same remains can be displayed in multiple locations in order to reach a wider audience; plus if a casting is destroyed it can easily be replaced. Even when the skeleton is comprised of real fossils, they are almost always an aggregation of different animals due to individual discoveries of partial skeletons.
How Old Are They Really?
We know how old dinosaur fossils are thanks to the technique of radiometric dating. With the greater understanding of nuclear physics that began in the 20th century, paleontologists realized that the steady decay of radioactive isotopes can be used to gauge the age of any matter containing a known isotope. Matter is composed of atoms, and every atom has a nucleus composed of protons and neutrons. The number of protons in an atom determines its element, but among all atoms of the same element there can be variations in the number of neutrons; these are known as different isotopes of the same element. Some of these isotopes are unstable and will undergo radioactive decay where they emit certain radioactive particles. The rate at which a group of isotopes decays is called its half-life. Given a large enough sample of isotopes, its mass will be reduced by half after every half-life period. For most isotopes, this decay rate is independent of external influence such as pressure, temperature, chemical reactions, or magnetic fields.
As radioactive “parent” isotopes decays, they transition to a new isotope (often through several iterations) until they reach a stable “daughter” isotope. Given a sample of an isotope, scientists can compare the mass of unstable isotopes to that of the stable isotopes and, given the known half-life, determine how long ago that sample began decaying. In order to ensure that these parent and daughter isotopes are from the same original parent, it is preferable to perform this analysis on rocks that were formed all at once and have remained intact. Due to this, volcanic rocks are perfect for radiometric dating as they are formed from well-mixed magma, and once formed the isotopes are hardened in place. Since the rock samples are uniform and unaltered, it is ensured that the ratio of parent to daughter isotopes had not been contaminated. By comparing ratios of unstable and stable isotopes to other stable elements in the rock, scientists can account for the mass of daughter isotope present when the rock originated to determine very accurately when the rock initially formed. This proven mechanism gives scientists a clock that can be used to trace back the approximate age of almost anything by examining a sample itself or the rock directly surrounding the sample.
In order to accurately date fossils from the time of the dinosaurs, scientists must use isotopes with very long half-lives, typically over one million years. These long lasting isotopes are found in igneous rocks formed by volcanic activity; fossils, on the other hand, are typically found in sedimentary rocks, not igneous. To get around this, paleontologists typically identify layers of volcanic rock in the sediments, ideally above and below the fossils. By testing rock that is older and newer than the fossil, they can accurately bound the time frame at which the animal died. Since this sort of comparison is done at multiple locations for dinosaurs of the same species, this gives a statistically reliable window as to when each species was alive. Although other dating methods exist, radiometric is the most common and well established in the field. Using this technique scientists can say definitively that dinosaurs were alive from about 230 million years ago until the great extinction event around 65 million years ago.
Even though dinosaurs fossils were first discovered centuries ago, and some of the celebrity dinosaurs such as T. rex have been studied for decades, there are still many new discoveries being made in the field. By utilizing an all-around approach of more field exploration, a closer examination of existing fossils, comparisons to existing animals, and applying technological advancements, paleontologists continue to revolutionize our understanding of these ancient giants. In the last two decades (and after Jurassic Park premiered), scientists have found the largest dinosaur ever, the best-preserved dinosaur ever identified skin pigmentation, and have even confirmed the existence of dinosaur proteins. Two decades is a blink of the eye compared to the millions of years that dinosaur fossils have been buried underground. These exciting advancements in dinosaur research lead us to believe that these ancient monsters still have many secrets left to be discovered.