Australia: The Land Where Time Began

A biography of the Australian continent 

Chondrichthyan Radiation of the Late Palaeozoic

The placoderms and some other fish groups went extinct by the Early Carboniferous, and about this time the chondrichthyans had another burst of rapid radiation in the Early Carboniferous, possibly to fill empty niches vacated by the large numbers of fish types that had recently disappeared. Among the many new groups were the first chimaerids (holocephalans). Among these new shark forms were many with features that are not seen in living sharks.

From the Late Devonian to the Early Permian, there was a very unusual shark, the stethacanthids. These sharks lived from the Late Devonian to the Permian. On the top of the main dorsal fin of the males there was bony brush-like structure. The dorsal brush of Stethacanthus and Akmonistion was very large, and had many denticles that were sharp, but in other sharks of this type the dorsal brushes varied in size and shape. A very well preserved specimen of Stethacanthus was found in the Bearsden deposit near Glasgow by Stan Wood, a fossil collector from Edinburgh, and is now in the Hunterian Museum at Glasgow University. It was described by Michael Coats and Sandy Sequoia [Sequeira?]  in 2001. The dorsal fin spines of Damocles and Falcatus were narrow and cylindrical.

Stethacanthid teeth have a main cusp and many small cusps on either side of it on a broad root. The teeth are commonly found throughout the Palaeozoic. In North America, Early Carboniferous forms have teeth up to 7 cm wide, the cusps being almost 4 cm high. Based on the normal number of tooth rows assumed to be about 12, the estimated size of the mouth would have been about 1 m wide, and the shark would have been about 6 m long. At the time these large predators were in the ocean, in the inland water were crossopterygian (rhizodontiforms) predators 6-7 m long.  

Among other sharks present between the Carboniferous and the Permian was the ctenacanth group, the genus Ctenacanthus ("comb spine"). These had many rows of fine nodes on their fin spines, hence the comb-like appearance. In the Early Carboniferous sites near Edinburgh are Goodrichichthys and Ctenacanthus, both less than 50 cm long. Another group that was common from the late in the Palaeozoic to early in the Mesozoic was the Xenacanth sharks.

The teeth of Xenacanths are characteristic, having 2 main cusp and on the root, a well-developed bone button, the lingual torus. A xenacanth shark, Xenacanthus, that was well known from complete fossils that have been found in deposits dating to the Permian-Triassic of Western Europe, and from skulls and spines from the Texas redbeds, that were well preserved, had a large defensive spine that was serrated protruding from its neck. Unlike the condition in most sharks, that have a heterocercal tail, the tail was straight. They have been found predominantly in freshwater deposits. According to Long these predators invaded the freshwaters by moving into the river systems from the sea, their teeth being well-known from both marine and freshwater deposits. In the Somersby fish site, of Triassic age, near Gosford, New South Wales almost complete remains have been found of xenacanthid sharks

Among fish, the Edestoid sharks are some of the most bizarre. Some relatively complete body fossils have been found, showing that they were streamlined and fast-swimming, e.g., Fadenia. Some, such as Helicoprion and Edestus, had complex tooth-whorls that coiled about on themselves, having a shape like that of a snail shell, as in the ammonites of the time, and probably hung from the lower jaw in life. The function of these tooth-whorls is uncertain. There have been a number of suggestions to explain these bizarre teeth. One suggestion is that they were used as living sawfish (saw sharks) use their saw, charging into schools of fish and thrashing their heads from side to side, tearing the fish to pieces then eating the bits of fish.  Another suggestion is that they mimicked ammonites to attract prey if the shark used them in a particular way. Long prefers the suggestion that they were used like a saw shark’s saw.

This was a very successful group, spreading around the world during the Middle Permian. Helicoprion tooth-whorls have been found in Russia, North America, Japan and Australia.

Another type of shark, the orodontids, had an elongated body, no spines and crenulated crushing teeth that were characteristic of the group. The teeth of Orodus are common in Carboniferous deposits around the world.

There was also a group of flattened sharks that were ray-like, the petalodonts, from the Late Palaeozoic. Apart from some well-preserved specimens they are mostly known from their teeth.

A shark found in the Bear Gulch Limestone in Montana, Belanstea Montana, was a deep-bodied form that had large pelvic and dorsal fins that were feathery. Janassa also had broad, rounded pectoral fins that were ray-like. They had peculiar teeth that are strongly compressed, which indicates their prey were invertebrates with hard shells. Specimens that have articulated dentitions are present, the shark had only a few teeth in each jaw. Petalodus had characteristic teeth that were broad, flat and slicing, and had a collar of folded enamel at the base.

Typical genera are Ctenoptychius and Ageleodus, that have a flat, broad root with 4-30 small pointed cusps (Downs and Daeschler, 2001). Megactenopetalus was at the other extreme that had a single large upper tooth that was multicuspid, and in the lower jaw a single large shearing tooth, that together comprised the entire dentition of the animal (Hansen, 1978).

The teeth are largely used to identify most petalodonts.

Sources & Further reading

1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995
2. Long, John A., 2011, The Rise of Fishes - 500 Million years of Evolution, 2nd ed, University of New South Wales Press.