The acanthodians (Denison, 1979) were generally small fishes, mostly less than 200 mm long, that include the oldest known gnathostomes. The first acanthodians date from the Late Ordovician, but they became abundant only in the Devonian. A few lines survived through the Carboniferous and only one into the Early Permian.
Most acanthodians have slender bodies with one or two dorsal fins, an anal fin, and a heterocercal tail fin (Figure 3.16(a, b)). The pectoral and pelvic fins have been modified to long spines, and there may be as many
Fig. 3.16 Acanthodian diversity and anatomy: (a) Climatius in lateral view; (b) Euthacanthus in ventral view, showing the fin spines; (c) head region of Ischnacanthus in lateral view; (d) single scale of Acanthodes.( After Moy-Thomas base and Miles, 1971.)
as six pairs of spines along the belly of early forms. The other fins just noted (except the anal fin) are supported by a spine on the leading edge. The name 'acanthodian' refers to these liberal arrays of spines (akanthos=spine). The internal skeleton is rarely seen.
The acanthodian head is large (Figure 3.16(c)) and covered with light bony plates. The shoulder girdle, or scapulocoracoid, is ossified in some forms, but is separate from the skull in later acanthodians (Figure 3.16(c)), thus allowing greater freedom of movement. Acanthodians have large eyes supported by a number of sclerotic plates, and there are lightly-scaled sensory canals set into the crania in many.
The body is covered with small closely-fitting scales that are made from bone and dentine (Figure 3.16(d)). These show concentric lines that record the growth of the scale. It seems that young acanthodians had a fixed number of scales over most of the body, and each scale grew by addition of bone and dentine at the margins as the animal grew larger.
Most acanthodians lack teeth. Toothless forms probably fed on small food particles which they may have filtered from the water. Only some of the later forms may have taken larger prey. They had a wide gape and gill rakers, sharpened spikes in the throat region that are attached to the hyoid and branchial arches. One specimen has been found with a bony fish in its body cavity, presumably swallowed whole. The large eyes of acanthodians suggest that they lived in open deep water, and they may have fed at middle depths. The fin spines and other spines may have had a primarily defensive function in making acanthodians unpleasant for larger fishes to swallow. Later forms, such as Acanthodes, seem to have been able to erect their pectoral spines, which would have caused them to stick in the gullet of a would-be predator. Perhaps this was a useful defensive measure, as seen in modern sticklebacks.
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