Cyathaspidiformes
| Cyathaspidiformes Temporal range: Silurian-Devonian |
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| Reconstruction of Cyathaspis banksii | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Pteraspidomorphi |
| Subclass: | Heterostraci |
| Order: | Cyathaspidiformes |
| Suborders, Superfamilies and Families | |
Cyathaspidiformes is an extinct order of heterostracan vertebrates known from extensive fossil remains primarily from Silurian to Early Devonian strata of Europe, and North America, and from Early Devonian marine strata of Siberia.
Like their descendants, the pteraspidids, all cyathaspidiform heterostracans had the cephalothorax enclosed in armor, formed from several plates, including dorsal, ventral, a dorsal spine derived from a scale, and a large, scale-covered tail. Thus, the living animals would have resembled tadpoles encased in massive armor. The majority of taxa have the rostral and pineal plates fused or merged with the dorsal plate, and in the amphiaspidids, all the plates of the cephalothorax were fused together into a single "muff-like" unit. Unlike the pteraspidids, all cyathaspidiforms are thought to be almost uniformly benthic in lifestyle, though only the amphiaspids and the ctenaspids are thought to be burrowers.
The taxonomy of Cyathaspidiformes is a convoluted affair, as taking into account of several of the various problematic heterostracan taxa which strongly or superficially resemble cyathaspidiforms, such as Cardipeltis, Traquairaspis, Corvaspis and Tolypelepidida, Cyathaspidiformes appears to be polyphyletic. Phylogenetic analyses suggest that the tolypelepidid genera, including Athenaegis, Asketaspis, and in particular, Tolypelepis, are the sister-taxa of cyathaspidiforms.
Even within the order, there is much debate about organizing the taxa. Until recently, there were several families within the suborder Cyathaspidida, including Cyathaspididae, Irregularaspididae, Anglaspididae, Poraspididae, etc. In a phylogenetic survey in 2013 by Lundgren and Blom, the various families and genera within Cyathaspidida are reorganized into three families, Cyathaspidae, Ctenaspidae, and Ariaspidae. The suborder Amphiaspidida, traditionally thought of as the daughter or sister-taxon of Ctenaspididae/Ctenaspidae, was excluded from this particular survey, as, while the authors did not directly challenge this idea, they excluded Amphiaspidida from the survey, noting that that group was "extremely derived and virtually impossible" to be of any use to this survey.
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