Acanthodii
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Acanthodii or acanthodians is an extinct class of gnathostomes. They are currently considered to represent a paraphyletic grade of various fish lineages basal to extant Chondrichthyes, which includes
Three acanthodians from the Early Devonian of Great Britain: Mesacanthus (an acanthodiform), Parexus (a "climatiiform"), and Ischnacanthus (an ischnacanthiform)
Impression fossil of the diplacanthid Rhadinacanthus longispinus, at the Museum für Naturkunde, Berlin
Various acanthodians, from top left clockwise: Cheiracanthus, Acanthodes, Climatius, Ischnacanthus, Parexus, Gyracanthus. center: Diplacanthus.
Doliodus 22DB
Silurian
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The Silurian is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at 443.1 Ma to the beginning of the Devonian Period, 419.62 Ma. The Silurian is the thir
Acanthodii
…ambiguous, and may actually belong to jawless fishes such as thelodonts. The earliest unequivocal acanthodian fossils date from the beginning of the Silurian Period, some 50 million years before the first sharks appeared. Later, the acanthodians colonized fresh waters, and thrived in the rivers and lakes…
Ordovician-Silurian boundary on Hovedøya, Norway: the brownish late Ordovician mudstone and later dark deep-water Silurian shale layers have been overturned by the Caledonian orogeny.
Fossils of the late Silurian sea bed, England
Diorama of a Silurian seafloor
Cooksonia, the earliest vascular plant, middle Silurian