American Mastodon

Associated Smithsonian Expert: Anna K. Behrensmeyer, Ph.D.

Dr. Kay Behrensmeyer working on describing strata at Olorgesailie, Kenya

Photo by Rick Potts, Smithsonian

Dr. Anna Katherine “Kay” Behrensmeyer grew up in western Illinois and hunted fossils with her brothers in bluffs of Paleozoic marine limestones along the Mississippi River. Kay started undergraduate school as an art major, but after a geology course at Washington University, St. Louis and then Indiana University’s Geological Field Camp in Montana, she decided to become a geologist/ paleontologist. She earned her doctorate in vertebrate paleontology and sedimentology from the Department of Geological Sciences, Harvard University. Kay’s Smithsonian career has involved paleontological and geological research in the field and laboratory, with a particular focus on the ecological context of human evolution in East Africa. She is co-director of the Evolution of Terrestrial Ecosystems program, which is working with ecologists to understand natural communities of plants and animals and how these change over time. Kay also is working on paleontological and archeological field projects in Kenya and Arizona, and she also continues research on Pakistan fossil vertebrates. One of her favorite research areas is Amboseli National Park, Kenya, where she continues long-term (35-year) research on processes that affect bone destruction and preservation, a sub-field of paleontology known as taphonomy.

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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Depiction of Triassic small mammal (Thrinaxodon) in habitat
Photo by Mary Parrish, Smithsonian, National Museum of Natural History

About Mammals (Class Mammalia): Paleobiology

On Earth today, mammals, particularly humans, are a common life form in many different habitats. But mammals were not always common or diverse. The first mammals, which evolved during the middle Triassic (about 240 million years ago), were small compared to many of their contemporaries, including the earliest dinosaurs. No larger than shrews or mice, these early mammals probably lived in burrows or other refuges when dinosaur and other reptilian predators roamed the Earth. Though they remained relatively small, mammals evolved to fulfill many ecological roles during the long reign of the dinosaurs. When dinosaurs (except birds) disappeared at the extinction event 66 million years ago, mammals survived and began to expand into ecological spaces (niches) vacated by the larger dinosaur herbivores and predators. By about 55 million years, ago, all modern groups of mammals had evolved. The Cenozoic (the Era in which we live) is known as the Age of Mammals, thanks to their successful radiation into a huge variety of habitats on Earth.

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.