Agatized Dinosaur Bone

Associated Smithsonian Expert: Matthew T. Carrano, Ph.D.

Dr. Matthew Carrano touches the Cretaceous/Paleogene boundary in the field near Hell Creek, Montana

Photo by Matthew Carrano, Smithsonian Institution

Dr. Matthew Carrano’s interest in dinosaurs was sparked when, as a second grader, he read a National Geographic book with illustrations by Jay Matternes. As an undergraduate at Brown University, he studied the functional morphology of dinosaurs, and went on to earn his Ph.D. at the University of Chicago. Since 2003, Matthew has been Curator of Dinosauria at the Smithsonian National Museum of Natural History. He currently studies the evolutionary history, functional morphology, and ecology of dinosaurs. His fieldwork has taken him to the Western Interior of North America, Chile, and Madagascar in search of new dinosaurs and the Mesozoic ecosystems of which they formed an important part. He is also studying the quality of the dinosaur fossil record, and is particularly interested in filling in the "gaps" of certain regions and time periods. Over the last 20 years, he has conducted research in museum collections around the world, including England, France, Germany, Belgium, Portugal, Argentina, Canada, Australia, Zimbabwe, and South Africa.

Meet our associated expert

This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.

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Fossil skeleton of a Permian synapsid that predated the dinosaurs (Dimetrodon sp.)
Photo by Smithsonian Institution, Department of Paleobiology

About Dinosaurs (Dinosauria): Paleobiology

Dinosaurs are land reptiles that include some of the biggest terrestrial life forms ever on Earth. We know dinosaurs from fossilized bones, teeth, footprints, eggs, and occasionally even soft tissues, although complete skeletons are rare. The earliest known dinosaurs are from the late Triassic (about 230 million years ago), when they were still overshadowed by other animals, such as synapsids (related to the ancestors of mammals). By the end of the Triassic, dinosaurs had begun their remarkable diversification, with several major lineages present. The Jurassic and Cretaceous periods saw dinosaurs dominate many land environments, reach enormous sizes, and diversify into thousands of species (including the first birds). Paleontologists have named more than 1200 species of dinosaurs, and that’s probably just a fraction of all the dinosaur species that existed. Most dinosaurs went extinct abruptly 66 million years ago (end of the Cretaceous), along with many other species. A period of severe environmental stress, punctuated by an asteroid impact, was likely responsible for this mass extinction.

Fossil Cretaceous tetrapod (a marine reptile called a Plesiosaur) displayed at the National Museum of Natural History
Photo by Chip Clark, Smithsonian, National Museum of Natural History

About Four-legged Animals (Superclass Tetrapoda): Paleobiology

Tetrapods (from tetra = “four”, and pous = “foot”) include amphibians, reptiles (including dinosaurs), birds, and mammals. The first well-documented tetrapod remains are known from the latest Devonian (about 360 million years ago) although tracks belonging to this group are known from rocks that are 395 million years old. Scientists agree that tetrapods evolved from a group called the lobe-finned fishes. Fins turned into legs even before the first tetrapods moved onto land. Lobe-finned fishes developed the first lungs, allowing them to breathe air. The first tetrapods still had both gills and lungs, suggesting a gradual transition from breathing under water to air-breathing. The move to land also required a stronger backbone to support the body while walking. Among present-day animals, lungfishes are the closest living relatives of tetrapods.