page_title: Acanthodii
section_title: Characteristics
hierarchical_section_title: Acanthodii / Characteristics
caption_attribution_description: Various primitive acanthodians from Early Devonian England and Scotland, Mesacanthus pusillus, Parexus falcatus, Ishnacanthus gracilis (C) Stanton F. Fink
context_page_description: Acanthodii or acanthodians is an extinct paraphyletic class of teleostome fish, sharing features with both bony fish and cartilaginous fish. In form they resembled sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteans. They represent several independent phylogenetic branches of fishes leading to the still extant Chondrichthyes.
  The popular name "spiny sharks" is a partial misnomer for these early jawed fishes. The name was coined because they were superficially shark-shaped, with a streamlined body, paired fins, and a strongly upturned tail; stout, largely immovable bony spines supporting all the fins except the tail - hence, "spiny sharks"; in retrospect, however, their close relation to modern cartilaginous fish can lead them to be considered "stem-sharks". Fossilized spines and scales are often all that remains of these fishes in ancient sedimentary rocks. Although not sharks or cartilaginous fish, acanthodians did, in fact, have a cartilaginous skeleton, but their fins had a wide, bony base and were reinforced on their anterior margin with a dentine spine.
context_section_description: The scales of Acanthodii have distinctive ornamentation peculiar to each order. Because of this, the scales are often used in determining relative age of sedimentary rock. The scales are tiny, with a bulbous base, a neck, and a flat or slightly curved diamond-shaped crown.
  Despite being called "spiny sharks," acanthodians predate sharks.  Scales that have been tentatively identified as belonging to acanthodians, or "shark-like fishes" have been found in various Ordovician strata, though, they are 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 throve in the rivers and lakes during the Devonian and in the coal swamps of Carboniferous. By this time bony fishes were already showing their potential to dominate the waters of the world, and their competition proved too much for the spiny sharks, which died out in Permian times (approximately 250 Million years ago).
  Many palaeontologists originally considered the acanthodians close to the ancestors of the bony fishes. Although their interior skeletons were made of cartilage, a bonelike material had developed in the skins of these fishes, in the form of closely fitting scales (see above). Some scales were greatly enlarged and formed a bony covering on top of the head and over the lower shoulder girdle. Others developed a bony flap over the gill openings  analogous to the operculum in later bony fishes. However, most of these characteristics are considered homologous characteristics derived from common placoderm ancestors, and present also in basal cartilaginous fish.
caption: Mesacanthus, Parexus, and Ischnacanthus of Early Devonian Great Britain
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