Australia: The Land Where Time Began
Chondrichthyans - Origin and Evolution
Sharks evolved early in the transition to jawed fish, the gnathostomes, about 420 Ma in the Early Silurian. The earliest records of sharks are only their scales, so it is not known if the first sharks had teeth, or even jaws. Because the only evidence of the earliest sharks is their scales it is difficult to do much more than guess at their ancestors. The scales of sharks are similar to those of agnathan thelodonts.
Sharks have retained their overall appearance and body plan since early in their evolution, most of the changes in their subsequent evolution has consisted of tweaking what must have been an ideal body plan. They have added increasingly sophisticated sensors, such as the ampullae of Lorenzini, that can detect fish buried in the sand by sensing their electrical fields. Some species have developed the equivalent of a mammalian placenta and give birth to live young. The white pointer (great white) Carcharodon carcharias, has become partially warm-blooded, being able to raise its body temperature above that of the surrounding water, so can maintain its speed.
Sharks appear to be closely related to placoderms. The scales from what is believed to be the earliest sharks, though as no teeth are known from the same fossil deposits it is uncertain if they had actually yet become jawed chondrichthyans, were from the Early Silurian of Mongolia. Based on the scales they have been given the names Mongolepis and Polymerolepis.
The earliest chondrichthyans to be known from their teeth are found at the beginning of the Early Devonian. The teeth are less than 4 mm, and the species was named Leonodus.
The earliest diverse assemblage of shark's teeth known are in the Middle Devonian Aztec Siltstone in Antarctica. This suggests the oldest true chondrichthyans, those with teeth of a characteristic form and tissue types, originated in Gondwana, from where the first great radiation of sharks may have spread from.
A small unusual tooth was found in a layer of a deposit in the Transantarctic Mountains, Macmurdodus in 1968. Since then the same type of tooth has been found in Early Devonian rocks in central Australia. The teeth, up to about 50mm wide, had several sharp, flat cusps spread approximately equally separated along the broad root. It had been suggested that its structure, a multilayered enameloid crown, made it the oldest known example of the neoselachian group, the group to which all modern sharks belong. Another modern feature in these teeth is its penetrating canals.
These teeth have also been found at other sites from the Antarctica Devonian, and the earliest confirmed shark's teeth from Arabia and Spain, both sites being on the margins of Gondwana, indicate that first big radiation of sharks occurred in Gondwana at the beginning of the Early Devonian.
Sharks had become cosmopolitan by the Late Devonian, more than 30 species being known to have a worldwide distribution. In the black shales of Ohio and Pennsylvania there are many complete shark fossils and isolated shark remains from about 24 different species. The most common genus in these deposits is Cladoselache, some up to 2 m long. This genus had a fusiform body with large wing-like pectorals and 2 triangular dorsal fins. It had a short, strong fin spine in front of each fin. It looked like modern sharks.
The quality of preservation is demonstrated by the shark fossils displaying muscle bands and in some specimens the stomach contents of the sharks. There were 5 log gill slits in Cladoselache, and many rows of small multicuspid teeth in its powerful jaws. The "cladodont" tooth has a central main cusp with a number of smaller cusps on either side. This type of tooth was adapted to swallow prey whole.
53 fossil sharks have been found with prey still in their gut regions, 64 % with ray-finned fish, about 28 % with Concavicaris, a shrimp-like animal, 9 % conodont animals, and in 1 specimen there was a smaller shark inside. In these fossils it can be seen that the prey were swallowed whole, tail first. Apparently it caught the prey by the tail, then swallowed it whole.
Another shark from the Late Devonian Cleveland Shale was Diademodus. It looked similar to Cladoselache but had different teeth with several equally-sized cusps on each root. Sharks were increasing as the end of the Devonian approached. They may have been pushed to make their swimming more efficient by the presence of large placoderms, in particular the giant placoderm Dunkleosteous.
Most of the Devonian sharks are known only from their teeth. There is a very great diversity of tooth types in the fossil record as they adapted into a wide variety of feeding strategies. Primitive sharks, such as Leonodus and Antarctilamna, often had 2 cusps on each tooth. Sharks from the Late Devonian mostly had multiple cusps on each tooth. Siamodus, ("tooth from Siam") from Thailand was an extremely unusual tooth had up to 8 cusps on a strongly arched root.
Phoebodus is another group of sharks that was common in the Late Devonian. There are well-developed roots on these teeth, which sometimes protruded prominently, on which the crown had 3 main cusps, sometimes with intermediate cusps on either side. Phoebodus were distributed around the world in Middle-Late Devonian rocks. In the Carboniferous and Permian the teeth attributed to Phoebodus have since been considered as in the wrong genera. Thrinacodus ferox, first described from Australia is now recognised around the world. In this species the teeth have the appearance of little grappling hooks, Chlamydoselachus, the living frilled shark, is believed by some to be a survivor from the Devonian because its teeth closely resemble those of phoebodonts. By the close of the Devonian sharks were present in marine and freshwater habitats around the world.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|