Pteraspidida
| Pteraspidiformes Temporal range: Upper Silurian-Devonian |
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| Lamiaspis longiripa | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Pteraspidomorphi |
| Subclass: | Heterostraci |
| Order: | Pteraspidiformes |
| Suborders and Families | |
| Synonyms | |
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Pteraspidiformes is an extinct order of heterostracan agnathan vertebrates known from extensive fossil remains primarily from Early Devonian strata of Europe and North America, and from Upper Silurian Canada.
A pteraspidiform heterostracan has the cephalothorax enclosed in armor, formed from several plates, including dorsal, ventral, rostral, pineal plates, a dorsal spine derived from a scale, and a large, scale-covered tail. Many genera were benthic, others were apparently active swimming nekton. Delicate, finger-like components of the anterior end of the ventral plate forming the edges of the mouth suggest that pteraspidiform heterostracans were filter-feeders that selectively filtered specific sized plankton from the water column.
If Weigeltaspis is not a psammosteid pteraspidid, then the Anchipteraspidids are the oldest-most known pteraspidid heterostracans, with three genera from Pridolian-aged strata from Upper Silurian Canada. The armor of anchipteraspidids superficially resemble those of cyathaspidids, though, anchipteraspidids already have the typical series of plates diagnostic of pteraspidids, and the growth habits of anchipteraspidids are diagnostic of pteraspidids, and not of cyathaspidids.
The Psammosteids are a group of pteraspidids that forms the sister taxon of Protopteraspididae and Pteraspidoidei, and are sometimes treated as a suborder, "Psammosteida," or as a family, "Psammosteidae." Anatomically, a typical psammosteid resembles a typical pteraspidid that has had the forebody greatly depressed, with a long, compressed tail. Some psammosteids, like Pycnosteus, have the ventral shield modified into a pedestal-like structure that some researchers compare to a sled's runner. The Psammosteids are the only heterostracans to survive beyond the Middle Devonian, as the last genera became extinct during the Late Devonian. The Late Silurian-Early Devonian genus Weigeltaspis may or may not belong to this group; species of this genus superficially resemble a small, elongated psammosteid.
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