What are dinosaurs?
Dinosaurs are a group of ancient, extinct reptiles that served as the dominant form of life on Earth for millions of years. They were particularly successful because of their upright posture – a hole in their hip socket permitted them to stand upright, with their legs straight under their bodies. This compared to the splayed legs of lizards and crocodiles, allowed them to move much faster and have greater endurance. All dinosaurs belong to the taxon (a group of organisms that have a common ancestor) known as “dinosauria.” This term, which means Terrible Reptile, was coined in 1842 by Sir Richard Owen. At the time, only three dinosaurs were known: Megalosaurus, Iguanodon, and Hylaeosaurus. Since then, dinosaur fossils have been discovered all over the world and more than 700 species have been identified. Dinosaurs were a very diverse group that came in all shapes and sizes. Traditionally, scientists have divided them into two major groups: 1) ornithischia/“bird hipped” dinosaurs, such as ornithopods (two-legged plant-eaters), stegosaurs (plated dinosaurs), ankylosaurs (armored dinosaurs), pachycephalosaurs (dome-headed dinosaurs), and ceratopsians (horned dinosaurs); and 2) saurischia/”lizard-hipped” dinosaurs, such as theropods (two-legged meat-eaters, and birds) and sauropodomorphs (two and four-legged plant-eaters with long necks). Dinosaurs are then broken down into numerous genera, for example Tyrannosaurus or Triceratops. Each genus is then divided into one or more species, for example Tyrannosaurus rex or Triceratops horridus.
When did dinosaurs live?
Dinosaurs lived roughly between 252 and 66 million years ago, during the Mesozoic Era. They first appeared during the Triassic period (252 to 201 million years ago). During this time, all of the Earth’s continents were connected in a single land mass, known as Pangaea, and surrounded by one enormous ocean. The climate was hot and dry, with much of the land covered with large deserts. These first dinosaurs were small, bipedal (moved on two legs) meat eaters like the Eoraptor – a small carnivorous/meat-eating Theropod and one of the earliest known dinosaurs. Dinosaurs continued to thrive into the Jurassic period (201 to 145 million years ago). During this time, Pangaea split and the Atlantic Ocean was created. Temperatures fell, plant life flourished, and dinosaurs became the dominant terrestrial vertebrates on land. In fact, the Jurassic period is known as the Age of the Reptiles. Large herbivorous (plant eating) dinosaurs, such as Diplodocus and Brachiosaurus, flourished. They were preyed upon by large carnivorous dinosaurs like Allosaurus and Megalosaurus. During the Late Jurassic, the first flying dinosaurs, like Archaeopteryx, evolved. The Cretaceous period (145 to 66 million years ago) is the last and longest portion of the Mesozoic Era. Development of plant life, especially flowering plants, continued, resulting in dramatic changes to the landscape. Many different dinosaur species still flourished while avian/bird life diversified and spread. However, by about 66 million years ago, nearly all these large vertebrates became extinct.
What happened to the dinosaurs?
There were two large extinction events during the time of the dinosaurs. The Triassic-Jurassic Extinction Event occurred between 199 million to 214 million years ago. The exact cause of this event is debatable. Some scientists theorize that an increased period of volcanism released a large amount of carbon dioxide into the atmosphere and triggered an episode of global warming as well as rising sea levels. Approximately 76% of all marine and terrestrial species and about 20% of all taxonomic families went extinct during this time. The Cretaceous-Paleogene Extinction Event occurred about 66 million years ago and was one of the most dramatic mass extinctions in the history of the Earth. One major factor in this event was speculated to be a large asteroid that entered the atmosphere, creating a massive impact. A huge crater 112 miles in diameter was discovered buried beneath sediments of the Yucatán Peninsula near Chicxulub, Mexico, known today as the Chicxulub Crater. Furthermore, scientists have found a layer of grey clay all over the world that tests positive for high concentrations of iridium – an element common in most asteroids. The effects of this impact were catastrophic and included global forest fires and clouds of dust and smoke that were so thick that they blocked sunlight – this cloud may have engulfed the Earth for an entire year. With no sunlight able to penetrate the cloud, the climate dramatically changed and plant life died out. Nearly all the dinosaurs and many marine invertebrates went extinct. However, the lineages that led to modern birds and crocodilians survived.
What are fossils?
Most of the data about past extinctions comes from the fossil record. The fossil record refers to the preserved physical evidence in the form of the fossilized remains of prehistoric organisms. It is the history of life as documented by fossils. The study of the fossil record provides important information for three main purposes: 1) Scientists are able to observe progressive changes within a species over time. These changes are used to describe the evolutionary development of a certain species. 2) Fossil organisms provide information about past climates and environments. Certain species may require specific conditions to live, such as corals who live in warm, shallow water. 3) Some fossil groups are indicative of certain time periods, which allows scientists to quickly assign an age to the fossil as well as the rock surrounding it. It is impossible to know how many species have lived on the Earth.
Fossils are the remains or impressions of ancient, living organisms preserved either in a petrified form, as a mold, or as a cast in rock. They are the direct, physical evidence left behind by ancient life and provide significant clues about past environments. There are two main types of fossils: body fossils and trace fossils. Body fossils are the physical remains of the actual living creature, such as bones, while trace fossils (or ichnofossils) are records of the creature’s activities without preservation of the actual body, such as footprints. The process in which an object becomes a fossil is called fossilization. There are many modes of fossilization. Most fossils form when the physical evidence is rapidly buried in sediment. This quick burial protects the evidence from environmental (weather) and biological (scavengers) disturbances. If the fossil is organic, like the body of an animal, the body will eventually rot away and leave behind a hollow opening. In trace fossils, like a footprint or an animal burrow, the hollow opening already exists at the time of burial. Over the course of many years, these hollow openings fill with groundwater and then minerals. The minerals inside the openings eventually solidify, creating a fossil.
How can you date a fossil?
Once a fossil has been identified, the time interval in which it lived can be estimated, based on the span of time between the earliest and latest observed specimen. There are two main ways to date a fossil specimen: absolute dating methods and relative dating methods. Absolute dating methods give the date in a calendar year/based on a fixed timescale. Nearly all absolute dating methods use the breakdown of radioactive elements to determine age. An example of absolute dating is Uranium-Lead Radiometric Dating. This method measures the amount of radioactive decay of the chemical elements, which occurs at a consistent rate. This method can be used to age levels of rocks that formed between a million years to over 4.5 billion years ago with high precision. Relative dating methods only determine the sequence of age/whether an object is older or younger than another object. This type of dating is helpful in structuring a chronological sequence of fossils in a given area. An example of relative dating is Stratigraphy. Stratigraphy is the study of rock layers, also known as strata. According to the Law of Superposition, the strata that are lower are older than the strata that are closer to the surface. This means that fossils that are discovered in lower strata are older than the fossils that are found closer to the modern ground surface.
On display in the Paleo Lab
Coelophysis is one of the earliest dinosaurs that ever lived on Earth. They first appeared in the Late Triassic period (200 million years ago) in what is now southwestern North America. During this time, dinosaurs were not at the top of the food chain. They were carnivorous/meat-eating theropod dinosaurs that had to compete with crocodiles and archosaurs for their meals. The Coelophysis’ limb bones were hollow, making them very light weight. This, combined with long legs and a bipedal stance (moved on two legs), made this dinosaur extremely agile and fast. Due to their small size of approximately 50 pounds and 8 feet from head to tail, they were likely opportunists that only hunted small animals, fish, and even insects. They also had sharp serrated teeth, a double hinged lower jaw, grasping claws, large eyes, and a well-honed sense of smell. Scientists have discovered two body types: small/slender and robust. This likely represents a difference in the females and males, but it is unclear which is female and which is male. It may seem like the males would be more robust; however, in many other species (like birds) the females are more robust than males. There have been thousands of Coelophysis fossils found, making them one of the best-represented theropods in the fossil records. The largest concentration of Coelophysis remains were discovered at Ghost Ranch. The Ghost Ranch is located in north-central New Mexico approximately 60 miles northwest of Santa Fe. It was once the home of famed artist Georgia O’keefe and contains four quarries full of fossil dinosaurs, fish, and reptiles. These life-size, adult models of Coelophysis were manufactured by P.R. Gulley Studio located in Williamsport, Indiana, and purchased by the museum in 2013.
In 1897, Walter Granger (a paleontologist from the American Museum in New York City) was prospecting for fossils in east-central Wyoming. He had heard local tales of a sheep-herders cabin, made of strange building materials. He decided to visit the cabin and discovered that it was actually constructed from local rocks that contained dinosaur bones. The landowner told Granger that he found the bones just over the hill roughly a quarter mile away. Granger, accompanied by his field crew and the landowner, traveled to the location of the bones, and he identified them as the remains of dinosaurs, including Diplodocus, Apatosaurus, Allosaurus, Barosaurus, and Camarosaurus.
This area was named Bone Cabin Quarry and Granger determined it was part of the Morrison Formation. A formation refers to a packet of rocks in which the dinosaur bones were found. The Morrison Formation was a basin wherein alluvial plains and wetlands once existed. Drainage from mountains to the west, during this period, caused periodic flooding events leading to a “log jam” of dinosaurs too weak to ford the rivers. Granger excavated at this location for six years. He removed all the bones that he could find. The bones were sent back to New York for preparation and were later displayed all over the world, including Dayton, Ohio. The diplodocus bones you see in this gallery was excavated by Granger. They were given to the Dayton Society of Natural History in 1931. The bones were articulated and mounted special for display in our Paleo Lab.
Diplodocus was a long-necked dinosaur or sauropod with an extremely long, whip-like tail made up of about 80 vertebrae. While no longer the longest dinosaur (a title now taken by Supersaurus), its entire body would have been in excess of 80 feet long and would have weighed 15 tons. For reference, the average weight of a care is 1.5 tons, meaning you would need 10 cars to equal the weight of a Diplodocus! Their bones were adapted for weight with dozens of pluerocoels (openings in the vertebra used to reduce weight and structurally enhance the strength of the bone) invading the vertebra. This made them very light of their feet. To fuel their bodies, they would have eaten ferns, club mosses, seed ferns cycads, conifers, and horse-tails. They had a battery of peg-like teeth in its skull and strong jaws for chewing. A large alimentary canal (the whole passage along which food passes through the body) would have aided in absorption of the needed minerals from these food sources.
The first Tyrannosaurus rex was found in Montana in 1902 by famous paleontologist Barnum Brown. It has gone on to become the most famous dinosaur in the world. T-rex lived during the Late Cretaceous Period during roughly 68 to 66 million years ago in what is today the western United Stated, including Montana and Wyoming. At the time T-rex lived, this area was a floodplain with lush plant life. T-rex is a theropod, meaning it was a bipedal (walked on two legs) carnivore (meat eater). It weighed up to 15,000 pounds, was 40 feet long, and had very powerful jaws with as many as 60 extremely sharp teeth. T-rex had short arms; each arm that two powerful claws. There are different theories for these short arms. One idea that that its arms became short to prevent it from being top heavy as a result of the animal’s extremely large head. Another hypothesis is that head evolved to replace the grabbing function of the arms so the arms were no longer needed. Scientists maintain that T-rex did not have feathers as an adult, but may have had some feathers in its youth. This is known as ontogeny recapitulates phylogeny – which means that as youngsters and embryos, the body retains ancestral traits that disappear when the animal gets older. Since we have direct evidence from fossils found in China and Germany of feathers in therapod ancestors, therefore it is plausible that the T-rex chicks retained feathers as well.
This replica skull is on loan from the Cincinnati Museum Center. It is a replica of the well-known Wankel’s Rex, which is sometimes called “The Nation’s T-rex.” The original was found in 1988 by rancher and amateur fossil hunter Kathy Wankel near Fort Peck Reservoir. It was later excavated by a team of paleotologists from the Museum of the Rockies in Bozeman Montana from 1989 to 1990. They were able to recover about 85% of the skeleton. It was the first Tyrannosaurus rex ever recovering with a fully intact arm. In 2014, it made its way to the Smithsonian Museum of Natural History. In 2022, new research come out suggested that it might be a new species of tyrannosaur called Tyrannosaurus regina; this is a hotly debated topic among paleontologists.