A diverse group of early jawed fishes from the Paleozoic Era, usually classified as a distinct class of vertebrates. Most acanthodians have tiny body scales with a cross section like that of an onion, the layers reflecting incremental scale growth. Most acanthodians have strong spines at the leading edges of all fins except for the caudal (tail) fin. Acanthodians mostly were small fishes with large mouths and large eyes, suggesting an active, predatory lifestyle feeding on small prey, including other fishes. The oldest well-preserved examples are of Early Silurian age, and the last surviving examples are of Early Permian age. Primitive forms tended to be from marine environments and to have stouter bodies, larger stouter fin spines, and larger numbers of paired spines in front of the pelvic fins. Later forms are found also in freshwater and estuarine habitats and had more slender bodies with fewer and more slender spines.
Although experts do not currently consider acanthodians to be the most primitive of jawed vertebrates (the Placodermi have that distinction), acanthodians are the earliest jawed vertebrates to be represented in the fossil record by complete specimens. Superficially sharklike in appearance, they are instead currently grouped in the Teleostomi as the closest relatives of the bony fishes (Osteichthyes), with which they share characteristics such as small nasal capsules, large eyes, scales capable of continuous growth (in most species), and (in some specialized forms) three pairs of otoliths (ear stones). See also: Osteichthyes; Placodermi; Teleostomi
Most acanthodians are small fishes, with size ranging from a few centimeters to half a meter. A few species reached lengths of more than 2 m (6.6 ft). The body usually was streamlined, with a blunt head, a terminal mouth, and a long, tapered, upturned tail. The large eyes were set close to the front of the head, behind a pair of small nasal capsules. The head usually was covered by large, flattened scales (tesserae), between which ran sensory canals similar to those in bony fishes. The braincase was ossified in advanced forms but not in primitive forms, where it remains cartilaginous. The ear region included three pairs of growing ear stones in some advanced acanthodians; primitive acanthodians had an open connection (endolymphatic duct) between the inner ear and the external environment, through which tiny sand grains entered and were used for the sense of balance, as in many sharks.
All acanthodians had jaws (sometimes calcified) consisting of a pair each of upper palatoquadrate cartilages and lower meckelian cartilages. In some acanthodians, jawbones, usually with teeth, were attached to these jaw cartilages (Fig. 1). In many species, teeth were absent; where present, they were of several types: multiple cusps in a whorl attached to a single base, usually near the front of the mouth; separate teeth in whorl-shaped arrangements along the margins of the mouth; or permanent conical teeth and denticles fixed to the jawbones. Gills were positioned behind the head in a compact gill chamber, as in bony fishes; a few acanthodians appear to have had multiple external gill slits on each side, but most had ornamented, platelike bony armor that enclosed the gills, leaving only a single pair of gill slits for water flow.
Primitive acanthodians had two dorsal fins, an anal fin, and paired pectoral and pelvic fins, all with leading-edge spines (Fig. 2a–c). Later forms retained a single dorsal fin (Fig. 2d). The tail usually was long and flexible. In primitive forms, a series of paired spines, the prepelvic series, was located along the belly in front of the pelvic fins (Fig. 2a). Paired prepectoral spines were present anterior and ventral to the pectoral fins, or were united by bony bases to form a ventral armor between the pectoral fins. These additional paired spines and associated bony plates were reduced or lost in later acanthodians.
Scales of acanthodians were small and continuously growing. Most had an onionskin-like structure internally as successive increments of growth were added to the outside of the scale. The crown of the scale was rhombic in outline, made of dentine, and either smooth or ornamented with ridges, while the bulbous base was composed of cellular or acellular bone. Scales of acanthodians often are abundant and useful for dating and correlating rocks.
At one time there was controversy about whether the most primitive acanthodians were those with many fin spines or those with fewer, but more recently it has been recognized that the most primitive acanthodians were those with multiple pairs of stout, heavily ornamented spines, usually classified in the order Climatiiformes. More advanced acanthodians include those in the orders Ischnacanthiformes and Acanthodiformes. Many other acanthodian species are of uncertain relationships because they are known only from isolated scales or fin spines.
This order includes the most primitive acanthodians, typically with broad, heavily ridged spines, multiple pairs of prepelvic spines, and either prepectoral spines or well-developed pectoral armor. Teeth, where present, were numerous and arranged in whorl-like sets superficially similar to those of sharks. Examples are the well-known Climatius, Ptomacanthus, Euthacanthus, Brochoadmones, and Lupopsyrus (Fig. 2a). Diplacanthids, including Diplacanthus, Gladiobranchus, Uraniacanthus, and Tetanopsyrus, sometimes included as a suborder within Climatiiformes, had a pair of large dermal plates on the side of the head, and had their prepelvic spines reduced to one pair or none (Fig. 2b). Climatiiformes appeared in the Silurian; they flourished and then declined in the Devonian. The gyracanthid fishes, sometimes considered to be climatiiform acanthodians, have scales more like those of early chondrichthyans; gyracanthids survived into the Pennsylvanian.
These acanthodians had tooth whorls at the front of their mouth as well as tooth-bearing jawbones borne on the upper and lower jaw cartilages (Fig. 1). They had slender fin spines and no prepelvic or prepectoral paired spines. Some ischnacanthiforms were rather large predators. Many species are known only from their toothed jawbones, which often are found separately. Examples include Poracanthodes and Ischnacanthus (Figs. 1 and 2c). They are known from the Silurian to the end of the Devonian.
These are advanced, streamlined, toothless forms with only one dorsal fin and at most one pair of prepelvic spines. One of the best-known acanthodians of any age is Acanthodes, thanks to its ossified internal cranial structures. Later acanthodiforms had long gill rakers and are thought to have been plankton feeders. The group appeared in the Early Devonian and survived until the Early Permian. Examples include Cheiracanthus in Cheiracanthidae, Acanthodes and Homalacanthus in Acanthodidae, and Triazeugacanthus, Melanoacanthus, and Mesacanthus in Mesacanthidae (Fig. 2d).