Australia: The Land Where Time Began
Acanthodians - Acanthodii - Extinct
Spiny fins - "spiny sharks"
A class of bony fish which has bony spines preceding all fins, living between the Silurian and Permian. There are 3 main orders, Climatiiformes, Ischnacanthiformes, and Acanthodiformes.
They were a group of mostly small fish, a few cm in length, though the largest was 2 m long, that arose in the Silurian, reaching their peak of diversity in the Devonian. They were fusiform fish with enormous eyes, looking somewhat like modern fish. The small thin scales covering their bodies did not overlap. The pattern of the mosaic of dermal bones making up the skull appear similar to that found in modern bony fish, but when compared in detail it is difficult to see homologous structures. They had a heterocercal tail with the dorsal lobe larger than the ventral lobe, with the skeleton continuing to the very tip of the lobe. They had a number of solid fin spines, not only on the front of the paired fins, but also along the unpaired fins - 1 or 2 dorsal fins and an anal fin. They had pectoral fins and ventral fins, but also had up to 6 pairs in between the 2 usual pairs. Their vertebrae are of cartilage, the dorsal and ventral arches protecting the nerve cord and blood vessels. Bony ribs have never been found, so it is not known if they had ribs.
They show some similarities to more advanced fish, such as sharks and bony fish. The arrangement of the endoskeletal bones of the jaw is one of these features. These bones are preformed in cartilage then ossified with mesoderm-derived bone. Though the parts of the jaws have a similar origin, the teeth are very different from those of modern fish histologically, lacking enamel and apparently were not replaced, so had to last for the life of the fish. These teeth were of 2 types, laterally compressed with many cusps per tooth and fused to the jaw cartilage, or whirl-like structures that were held near the jaw cartilage by connective tissue.
Among fish they are unique in having ornamented bony spines in front of all fins, and very small scales with bulbous bases. They are distinctive in not having a pulp cavity and in having concentric layers of a dentine-like material. Dentine makes up the outer layer over a bone base. In primitive forms the bone is cellular, but in the more advanced forms it is acellular.
They seem to have been active swimmers, feeding in the mid-water to the surface. They probably got a head start because they were not in competition with the mostly bottom feeding fish of the time, the placoderms and agnaths. The evolutionary path they followed was to lighten the skeleton and make it more flexible. Over time, the spines became elongated and more firmly attached. They are thought to have had a defensive function, and it has been suggested that they may also have had a 'cutwater' role.
The Acanthodiforms were the longest surviving group. They were present in the seas from Late Silurian to the Early Permian. They were fast-swimming filter feeders that lacked teeth, straining food from the water with gill rakers.
The 3 main types distinguished by the presence or absence of bony armour bracing the pectoral fins, 1 or 2 dorsal fins the tooth and scale types.
The Climatiiforms are the earliest type had elaborate bony shoulder girdle armour and many sharp spines.
The Ischnacanthiforms were predators, the teeth of which were fused to the jawbones.
Detailed fossils of these fish are lacking, so not much is known about them.
The oldest known Australian acanthodians are found in 2 deposits near Canberra, the Mirrabooka and Silverdale Formations near Canberra, and the Broken Creek Formation in the Broken River area of north Queensland.
The scales found are from several different types of acanthodian, such as climatiids and ischnacanthids, some being very similar to those found in other parts of the world. Some of these are Nostolepis and Gomphonchus. Some of the small bony platelets found in Silurian deposits may actually be from placoderms.
They are nearly all slender, elongated fish with long heterocercal tails and short, blunt heads. Apart from a few specialised forms such as diplacanthids, the mouth is nearly always large, and the head has many small platelets called tesserae. All acanthodians had 2 dorsal fins, but the Acanthodiformes, had only 1 dorsal fin close to the tail. The eyes are surrounded by a varying number of sclerotic bones, and they had 2 pairs of nostrils, excurrent and incurrent nares, at the front of the head. The jaws are ossified as a single lower jaw cartilage, sometimes supported by a strip of external, ornamented bone (the mandibular splint), but lacks externally ornamented tooth-bearing bones. There are 5 gill slits at the sides of the head. They sometimes have branchiostegal plates preceding them, otherwise present only in bony fish.
Acanthodes was one of the most specialised acanthodians, it is the only one that gill arch bones and braincase are known from. Features of this acanthodian are a series of basibranchial bones, large hypohyals and ceratohyals, large epihyal and epibranchials, and short, rearward-facing pharyngobranchials. The upper jaw (palatoquadrate) may have a simple articulation with the brain case or have a complex double articulation (in some acanthodiformes).
The brain case of Acanthodes is composed of 4 bones that were held together by cartilage, as it was incompletely ossified. Most of the top of the head was covered by the large dorsal ossification that protected the brain. At the rear of the head there was a smaller ossification which also served as a site for trunk muscle attachment. There was a large anterior basal ossification, under the braincase, which was pierced by a canal for the hypophysis - a space where the pituitary is housed, and the internal carotid arteries converge from outside the braincase, and a small rear section below the occipital ossification. Overall, the shape and proportions of the braincase of Acanthodes looks similar to that of a primitive ray-finned fish. The inner ear canals of some acanthodians have been preserved. They had 3 semicircular canals, and some species had otoliths, ossified ear stones, in the inner ear, e.g., Carycinacanthus. The fish would have had an improved sense of balance, necessary for fast turns and maneuvers, as a result of these structures.
There was a shagreen of many tiny tight-fitting scales covering the body. The scales had swollen bases and no pulp cavity. The scales grew by the addition of concentric layers, like a an onion. There were 2 main scale types that are distinguished based on their histology.
The Acanthodes type had a crown composed of a true dentine layer on a thick acellular bone base.
The Nostolepis type scale had a dentine crown penetrated by vascular canals on a cellular bone base.
The crowns of acanthodian teeth are usually flat to weakly domed, and are separated a well-defined neck from the base. There are well-developed jaw bones with teeth in Ischnacanthid acanthodians, and sometimes developed complex dentitions with tooth "fields". Spiny individual teeth and and complex tooth whorls at the front of the lower jaws of climatiid acanthodians.
Acanthodiforms were toothless filter feeders. They had no teeth, but had gill rakers to strain food through the gill chamber. They were fast-swimming filter feeders. It is thought they probably went extinct at the close of the Permian because of competition with bony fish and sharks which were greatly expanding at the time, or through predation.
Several articulated, but incomplete, acanthodians from the Bunga Beds (late Givetian/early Frasnian) of the southern coast of New South Wales are tentatively identified as ischnacanthids. Heads are missing from all three prepared specimens. They exhibit the following characters: two dorsal fin spines; long, slender scapulocoracoids; slender, relatively deeply inserted, unpaired fin spines; minute scales with a fairly smooth, flat, crown; and an increase in size of normal body scales towards the tip of the tail. The fish are preserved in black, finely laminated shales, which were probably deposited as deep water, lacustrine sediments. The rarity, burial conditions, and headless state of the Bunga Beds acanthodians indicate that they might have died in shallow water, sunk to the bottom, refloated by gas-induced buoyancy, with the heads lost while drifting out to deeper waters, where the bodies finally sank to a scavenger-free, anaerobic substrate.
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