Australia: The Land Where Time Began

A biography of the Australian continent 

Chondrichthyans - Beginnings of Modern Faunas

Throughout the Mesozoic the sharks continued to diversify, and it was at this time that most of the modern shark families appeared. The hybodontids first appeared in the Early Carboniferous and by the early half of the Mesozoic were a prominent form in the deposits. A well-preserved complete fossil of Hamiltonichthys was found in Kansas. In Scotland Diploselache was found in a deposit from the Carboniferous. Hamiltonichthys is identified by the presence of enlarged head spines that were well-developed, dorsal fin-spines that were stout, teeth that were broad and multicuspid, and scale shapes that were peculiar. These anatomical features characterise the hybodontids.

Well-developed fin spines and small head spines, broad teeth and scale shapes, characterise the hybodontids. There were many species of Hybodus, a shark with a blunt head that grew up to 2.5 m from the Mesozoic of Europe, Africa, Asia, and North America. The teeth of most hybodontids had a broad root with many cusps, though some had crushing dentition consisting of flat plated teeth, as in Acrodus and Asteracanthus. In the front of the mouth they had smaller gripping teeth. A perfect 3-D cranium with the braincase and jaws, Triodus, is a hybodontid from the Cretaceous.

It was previously thought that the euselachians, true selachians, had been traced back to Palaeospinax, a form from the Lower Jurassic (Maisey, 1977), but is it is now believed to belong within the galeomorphs (Klug, 2009).

Order Hexanchiformes, the 7-gilled sharks, first appear in the Mesozoic. It is thought Macmurdodus from the Middle Devonian might possibly belong to this group. Some living sharks are primitive forms, Hexanchus, Heptranchias and Notorhynchus, the living cowsharks, have 1 dorsal fin and 6-7 gill slit pairs. Their teeth, with many flat cusps along a broad root, with marked differences between the dentition of the upper and lower jaw, are characteristic of the group. They are mostly fish-eaters inhabiting deep water. Fossil teeth of such groups as Hexanchus show that at least some of the forms have been present in the oceans since the Early Jurassic.

Many new forms of shark arose during the Mesozoic. In Cretaceous deposits some of the most common fossil teeth are those of sand tiger shark (Carcharias), narrow curved teeth. Others were Cretolamna and Cretoxyrhina, which were broader toothed lamnids. There were many others. At this time there are shark vertebrae, fossilised because of the high degree of calcification of the cartilage. In the Cretaceous very large lamnid sharks appear in the fossil record that are found as whole skeletons, up to 5-6 m long. They were alive at the same time as the giant marine reptiles, ichthyosaurs, plesiosaurs and mosasaurs. One of these was Cardabiodon that was found in Western Australia and described by Mikael Siverson that grew to 5-6 m. Another large lamnid shark from this time was the living megamouth shark, Megachasma that was first known from its teeth in the Cretaceous.

It was during the Tertiary that the last of the living sharks evolved. Among them were the rarer forms, such as the goblin shark, from the depth of the ocean, Mitsukurina, and many others that can be traced by the presence of their teeth among the fossils to their first appearance in the fossil record in this period.

By the start of the Miocene, about 23 MA, the largest ever sharks evolved. These sharks were previously thought to be the ancestors of the white pointer (great white) shark, now they have been assigned to their own family of lamnid sharks that is now extinct. Common in deposits from the Miocene and Pliocene around the world, the largest species of fossil macko, Isurus hastalis, up to 6-8 m, if it had developed serrations on its teeth it could have been ancestral to the great white (Ehret et al., 2009). It was previously believed it gave rise to the modern Carcharodon, the white pointer (great white), but this is now being debated. The teeth of the living species of Isurus, Isurus oxyrynchus, together with those of many extinct members of the genus, are found in rocks from the Miocene and younger rocks worldwide. From the Miocene and later rocks contain the teeth of Isurus, including those of Isurus oxyrinchus.

The largest ever known shark, Carcharocles megalodon, had teeth up to 17 cm high. Because improved  knowledge of how shark teeth increase in size proportionate to body size, the length of Carcharocles megalodon has been revised down from about 25 m to about 15 m, smaller, but still twice the size of the largest known white pointer. The teeth of this shark, large, triangular and serrated, are found in rocks from the Miocene and Pliocene from around the world.

Three extinct species of Carcharocles are C. megalodon, C. angustidens and C. auriculatus. It is suggested by those supporting the hypothesis that Carcharadon carcharias is a descendant of the giant C. megalodon and its related forms believe all these extinct species should be in the same genus, Carcharodon. Their argument is that the intermediate serrated teeth are predated by remains of Carcharodon carcharias, and that the molecular clock method based on DNA changes indicates that Carcharodon diverged from Isurus about 43 Ma. David Ward of Orpington, UK, has shown Long transitional teeth that are believed to represent the link from Otodus obliquus, to the serrated forms that are identical to those of Carcharocles species. O. obliquus was a large lamnid from the Late Palaeocene/Early Eocene with teeth up to 8 cm or more high that has been estimated to have grown to 6 m. According to Long, Carcharocles megalodon evolved from the earlier large species, Carcharocles angustidens, from the Oligocene.

The largest Carcharocles species evolved at the same time as the large filter-feeding baleen whales. It has been suggested that the whales started living, for at least part of the year around the Antarctic to escape the sharks. According to this suggestion the evolution of the large whales and of Carcharocles continued in step by the start of the Miocene.

There have been reports of great white sharks up to 10 m long, leading some scientists to suggest that Carcharocles might still be extant, though Long says these reports should be regarded with extreme caution as there has been no actual evidence so far. He suggests that the teeth of C. megalodon would still be present in the most recent marine deposits, but none have been found in deposits later than 2.5 Ma.

Long speculates that these giant sharks may have succumbed to extreme climatic changes, with the beginning of the cooling phase about 2.6 Ma in the Late Pliocene as the start of the Pleistocene ice age approached. The marine mammals may have been better able to adapt to the cooling because they were warm blooded. Fossil evidence of the presence of the great whales in the Antarctic waters began at about this time. Long asks the question “Was it just for feeding, or to escape the colder-blooded predators that could not cope with the near-freezing Antarctic waters?

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.