Hemichordates and Chordates

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Monograptus priodon
Monograptus priodon
© 2004 UCMP

© 2003 Thomas Kammer

© 2003 Otto S. Zapecza

What are Hemichordates and Chordates? The group “Chordata” includes all vertebrates, but because vertebrates are such a large group in their own right, we discuss them separately. We limit our discussion here to the chordates without a backbone, namely the tunicates and lancelets. Fossil chordates can be found in rocks as old as the Cambrian; however, because they have few hard parts, their fossils are rare throughout the Phanerozoic. The Hemichordates, meaning “half-chordate,” share many of the characteristics of chordates, but differ in some aspects of development and morphology. Graptolites are the best-known group of fossil hemichordates. They first appear in the Ordovician, and later go extinct in the Devonian.

First known fossil occurrence: Cambrian.

Last known fossil occurrence: Quaternary. This group has living relatives.

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