heterostracan

fossil vertebrate order

Also known as: Heterostraci

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June 29, 2026

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The term "heterostracan" refers to an animal or fossil belonging to the order Heterostraci. This group is a type of ostracoderm, which are ancient, armored, jawless vertebrates that lived during the early Paleozoic Era. Heterostracans are characterized by their exoskeleton, which could be made of a few large plates or many small scales. They also had widely separated nostrils and eyes. While some heterostracans were bottom dwellers, others were adapted for mid-water life. This group is now extinct.1234

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agnathan
Form and function
The extinct heterostracans include obvious bottom dwellers (Drepanaspis) and others (Pteraspis) that were apparently adapted to mid-water, or nektonic, life. Some heterostracans, for example, had movable enamel plates inside the lower lip, probably to provide a biting or grazing mechanism. Osteostracans, anaspids, hagfishes, and lampreys have one…
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The extinct heterostracans include obvious bottom dwellers (Drepanaspis) and others (Pteraspis) that were apparently adapted to mid-water, or nektonic, life. Some heterostracans, for example, had movable enamel plates inside the lower lip, probably to provide a biting or grazing mechanism. Osteostracans, anaspids, hagfishes, and lampreys have one…
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agnathan
Annotated classification
†Order Heterostraci Head covered with 2 large bony plates, 1 above and 1 below, separated by a variable number of smaller plates. Single gill opening on each side, body covered with large scales, possibly paired nostrils. Bone lacks enclosed bone cells. 6 or 7 minor taxa…
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†Order Heterostraci Head covered with 2 large bony plates, 1 above and 1 below, separated by a variable number of smaller plates. Single gill opening on each side, body covered with large scales, possibly paired nostrils. Bone lacks enclosed bone cells. 6 or 7 minor taxa…
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fish
Agnatha: early jawless fishes
Heterostracan remains are next found in what appear to be delta deposits in two North American localities of Silurian age. By the close of the Silurian, about 416 million years ago, European heterostracan remains are found in what appear to be delta or coastal deposits.…
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Heterostracan remains are next found in what appear to be delta deposits in two North American localities of Silurian age. By the close of the Silurian, about 416 million years ago, European heterostracan remains are found in what appear to be delta or coastal deposits.…
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