Associated Smithsonian Expert: Douglas H. Erwin, Ph.D.

Paleobiologist Dr. Douglas Erwin studies invertebrate animals of the Paleozoic Era.

Photo by Smithsonian Institution

Dr. Douglas Erwin joined the Smithsonian National Museum of Natural History in 1990 and is the curator of Paleozoic invertebrates. His principal research interests revolve around large-scale evolutionary patterns in the development of life. He spent many years studying the great mass extinction that occurred about 252 million years ago and the recovery of life forms. More recently he has been focusing on evolutionary innovation, and in 2013 published The Cambrian Explosion: The Construction of Animal Biodiversity with Jim Valentine, about the origin and early evolution of animals. He is also a co-author of The Fossils of the Burgess Shale, a popularly acclaimed account of former Smithsonian Secretary Charles D. Walcott’s 1909 discovery and subsequent research into this vast repository of half-billion-year old fossil animals--65,000 specimens of which reside in the Natural History Museum’s collections.

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Fossil brachiopod shell (Mauispirifer hectorin) in limestone, along with other fossils (crinoids, bryozoans, and a bivalve).
Photo by Smithsonian Institution, Department of Paleobiology

About Articulate Brachiopods (Subphylum Rhynchonelliformea): Paleobiology

Articulate brachiopods get their name from the toothed hinge of their shell that allows the two halves to close snugly together (articulate = “jointed”). The fossil record shows that the earliest brachiopods had a smooth hinge. The toothed hinge likely evolved, along with strong adductor muscles, as an adaptation to lock the shell tightly closed when necessary. The first articulate brachiopod fossils date to the Cambrian (about 500 million years ago), when animals with shells were first becoming abundant on Earth. Articulate brachiopods flourished in the extensive, shallow, warm seas. During the 250 million years that followed, articulate brachiopods became widespread, but with the composition of species changing as some went extinct and new forms arose. Few survived the massive extinction at the end of the Permian (about 250 million years ago). However one group of articulate brachiopods, the Rhynchonellata, is still represented by living species on Earth today.

Heart-shaped fossil brachiopod (Paraspirifer clarkei)
Photo by Smithsonian Institution, Department of Paleobiology

About Brachiopods (Phylum Brachiopoda): Paleobiology

Brachiopods are small, shelled organisms that look superficially like bivalves (such as clams). Despite their resemblance, brachiopods have different evolutionary origins. Also, if you carefully compare a clam and a brachiopod shell, you will notice that the plane of symmetry differs by 90 degrees; two clam shells are mirror images of each other, while the plane of symmetry in a brachiopod passes through the middle of each shell. While species of brachiopods number in the hundreds today, they numbered in the thousands during the Paleozoic. They originated on Earth more than 500 million years ago, and proliferated through the Paleozoic. Scientists use fossil brachiopods as indicators of prehistoric climate change because gradual shifts in climate affected the distribution of brachiopod species. At the end of the Permian, most brachiopod species were wiped out in a massive extinction event that affected many other organisms, too. While they made a slow recovery, brachiopods have never achieved the incredible diversity and abundance they had during the Paleozoic.

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