Fossil Crocodile

Associated Smithsonian Expert: Hans-Dieter Sues, Ph.D.

Dr. Hans Sues holding the skull of a lion hunted by Theodore Roosevelt

Photo by Smithsonian

Dr. Hans Sues is a senior research geologist at the Smithsonian National Museum of Natural History who specializes in the study of dinosaurs and other vertebrates from the Mesozoic Era. He first became attracted to dinosaurs when he was four years old. After earning his doctorate from Harvard University, he worked as a postdoctoral fellow at McGill University and the National Museum of Natural History. Later Sues was curator and senior manager at the Royal Ontario Museum in Toronto and Carnegie Museum of Natural History in Pittsburgh before returning to the Smithsonian. He has collected fossil vertebrates across the United States as well as in Canada, China, Germany, and Morocco. Among his many discoveries are several new species of dinosaurs.

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 Jurassic marine reptile (Icthyosaur)
Photo by Chip Clark, Smithsonian, National Museum of Natural History

About Reptiles (Class Reptilia): Paleobiology

Reptiles first appeared on Earth about 300 million years ago (during the Pennsylvanian). Before then, all back-boned animals (vertebrates) were tied to the water, living in or near swamps. Their eggs were laid underwater. A set of gradual adaptations, such as thicker, scaly skin, allowed life on land to emerge. The biggest change was the development of an egg containing layers of membranes (amniotic membranes) to retain water, supply food, and store waste for an embryo. Two further innovations – the eggshell and internal fertilization – marked the first reptiles. Fertilization inside the female and shelled, amniotic eggs freed reptiles from aquatic life. They diversified into dinosaurs, crocodiles, lizards, snakes, turtles, tuataras, and birds as they colonized a variety of land habitats. For more than 100 million years, reptiles dominated the Earth. The largest reptiles were wiped out in the extinction event at the end of the Mesozoic, but their smaller descendants live on today.

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.