heterostracan

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…

†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…

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.…