Acanthodii oracanthodians is an extinct class ofgnathostomes (jawedfishes). They are currently considered to represent aparaphyleticgrade of various fish lineagesbasal toextantChondrichthyes, which includes livingsharks,rays, andchimaeras. Acanthodians possess a mosaic of features shared with bothosteichthyans (bony fish) and chondrichthyans (cartilaginous fish). In general body shape, they were similar to modern sharks, but theirepidermis was covered with tiny rhomboid platelets like the scales ofholosteians (gars,bowfins).[1]
The popular name "spiny sharks" is because they were superficially shark-shaped, with a streamlined body, pairedfins, a strongly upturned tail, and stout, largely immovable bony spines supporting all the fins except the tail—hence, "spiny sharks". However, acanthodians are not true sharks; their close relation to modern cartilaginous fish can lead them to be considered "stem-sharks". Acanthodians had acartilaginousskeleton, but their fins had a wide, bony base and were reinforced on their anterior margin with a dentine spine. As a result, fossilizedspines and scales are often all that remains of these fishes in ancientsedimentary rocks. The earliest acanthodians were marine, but during theDevonian, freshwater species became predominant.[citation needed]
Acanthodians have been divided into four orders:Acanthodiformes,Climatiiformes,Diplacanthiformes, andIschnacanthiformes.[2] "Climatiiformes" is a paraphyletic assemblage of early acanthodians such asclimatiids,gyracanthids, anddiplacanthids; they had robust bony shoulder girdles and many small sharp spines ("intermediate" or "prepelvic" spines) between the pectoral and pelvic fins. The climatiiform subgroup Diplacanthida has subsequently been elevated to its own order, Diplacanthiformes. Ischnacanthiforms were predators with tooth plates fused to their jaws. Acanthodiforms werefilter feeders with a single dorsal fin, toothless jaws, and longgill rakers. They were the last and most specialized off the traditional acanthodians, as they survived up until thePermian period.[citation needed]
Thescales 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 asthelodonts. The earliest unequivocal acanthodian fossils date from the beginning of theSilurian Period, some 50 million years before the first sharks appeared. Later, the acanthodians colonized fresh waters, and thrived in the rivers and lakes during theDevonian and in thecoal swamps ofCarboniferous. By this timebony 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 inPermian times (approximately 250 million years ago).
Many palaeontologists originally considered the acanthodians close to the ancestors of the bony fishes.[citation needed] Although their interiorskeletons were made ofcartilage, a bonelike material had developed in the skins of these fishes, in the form of closely fitting scales (see above)[clarification needed]. Some scales were greatly enlarged and formed a bony covering on top of the head and over the lowershoulder girdle[citation needed]. Others developed a bony flap over the gill openings analogous to theoperculum in later bony fishes[citation needed]. However, most of these characteristics are considered homologous characteristics derived from commonplacoderm ancestors[clarification needed], and present also in basalcartilaginous fish[citation needed]. Overall, the acanthodians'jaws are presumed to have evolved from the firstgill arch of some ancestral jawless fishes that had a gill skeleton made of pieces of jointed cartilage.[citation needed]
In a study of early jawed vertebrate relationships, Daviset al. (2012) found acanthodians to be split among the two major cladesOsteichthyes (bony fish) andChondrichthyes (cartilaginous fish). The well-known acanthodianAcanthodes was placed within Osteichthyes, despite the presence of many chondrichthyan characteristics in its braincase.[3] However, a newly described Silurianplacoderm,Entelognathus, which has jaw anatomy shared withbony fish andtetrapods, has led to revisions of this phylogeny: acanthodians were then considered to be a paraphyletic assemblage leading to cartilaginous fish, while bony fish evolved from placoderm ancestors.[4]
Burrow et al. 2016 provides vindication by finding chondrichthyans to be nested among Acanthodii, most closely related toDoliodus andTamiobatis.[2] A 2017 study ofDoliodus morphology points out that it appears to display a mosaic of shark and acanthodian features, making it atransitional fossil and further reinforcing this idea.[5]
The oldest remains attributed acanthodian-grade chondrichthyans areFanjingshania andQianodus from the Early Silurian of China, dating to around 439 million years ago.[7][8] Compared to other contemporary groups of fish, acanthodians were relatively morphologically and ecologically conservative. Acanthodians rose in diversity during the Late Silurian, reaching their apex of diversity during theLochkovian stage of the Early Devonian, declining during thePragian but rising again during the followingEmsian, which was followed by a decline in diversity during middle-Late Devonian. The diversity of the group was consistently low but stable during the Carboniferous, slightly decreasing going into the Permian.[9] The youngest records of the group are isolated scales and fin spines from Middle-Late Permian strata in theParaná Basin of Brazil.[10]
^Maisey, John G.; Miller, Randall F.; Pradel, Alan S.; Denton, John S.; Bronson, Allison; Philippe, Janvier (2017-03-10).Pectoral morphology in Doliodus: bridging the 'acanthodian'-chondrichthyan divide. American Museum Novitates. Vol. 3875).hdl:2246/6701.
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