Australia: The Land Where Time Began

A biography of the Australian continent 

Chondrichthyans

Class: Chondrichthyes

Sharks have changed very little since their appearance in the fossil record at least 420 million years ago, changes that occurred since then have mostly been tweaking the shark specs.  Refinements that made the hunting more efficient and changes to the feeding mechanisms and making their bodies even more streamlined. The highly successful body plan of sharks underwent 2 major modifications - when the holocephalans (chimaeras and rabbitfish) evolved at the start of the Early Carboniferous from the basic shark body plan, and in the Jurassic, when the rays flattened the basic shark body. Just after the decline of the placoderms, the sharks reached their peak of diversity. At this time the basic shark body branched into many, often bizarre forms. Some Palaeozoic sharks had coiled whorls of teeth, others had huge bony structures on their dorsal fins. At the present there are more than 900 species of sharks, rays and Holocephalans. These include the largest fish in the world, the whale shark, longer than 7 m. Until 2 million years ago sharks like the predatory Carcharocles megalodon, the biggest known shark, with teeth twice the size of its modern relatives, was still terrorising the oceans during the early stages of human evolution.

The interactions between sharks and humans are usually thought of in terms of the number of people killed annually. The reality is quite different, while sharks kill around the world, about 20-30 humans per year, humans kill about 73 million sharks per year, mostly for food. So who eats whom? Of shark species that are often thought of in connection with attacks on humans the “great white” (previously known in Australia as the “white pointer”), the largest and most feared, actually prefers to eat seals, as their thick layer of blubber provides much more energy than bony humans, making seals more worthwhile as a prey species.

Sharks were among the earliest gnathostomes (jawed fish) to evolve. They are first found in the fossil record as teeth, spines and scales, with a few fossils skulls and in very rare cases, whole body preservation. Shark scales have been found as far back as the earliest part of the Silurian so must have been present in some form prior to that time. The oldest known possible shark scales have been found in Late Ordovician deposits.

The earliest known shark scales are found in deposits containing no teeth, suggesting to Long that the earliest protosharks had not yet developed teeth, or were possibly agnathans (jawless fish). The earliest fish in the shark line is not yet known. According to Long the evidence appears to be pointing towards the early acanthodians as the ancestral shark, but the evidence is far from conclusive.  

The oldest well-preserved fossil sharks may provide the best evidence from their peculiar anatomy, including the enigmatic forms from the MOTH locality in British Columbia of Early Devonian age, from genera such as Seretolepis, Kathemacanthus, and from the Emsian of Québec, Doliodus (Miller et al., 2003). It has been found that in these localities the oldest known sharks have a number of forms in which fin spines occur in front of some or all their fins.

According to Long, research on a number of species of living sharks has shown that sharks are far more advanced than previously believed. Various shark species have a number of ways of reproducing, some laying eggs that receive no further attention from the mother. Other species have the equivalent of a mammalian placenta, giving birth to live young. Still others produce infertile eggs that the young hatching from fertile eggs eat until their birth.

The snouts of all sharks are extremely sensitive, having many pores, each containing an ampule of Lorenzini, enabling the finding of prey buried in sand or mud by detecting the weak electrical fields produced by the body of the prey. The chemosensory organs in sharks have been refined to such an extent that they can detect blood in the water from more than 1 km away, even at concentrations in the PPB range.

It is now believed that the unusual head shape of hammer head sharks has evolved as a way of having their ampullae as widely spread as possible to detect more precisely the electrical fields of buried prey, one of their main foods, in a similar manner to the achievement of binocular vision by separating the eyes.

Sharks of various species now live in many habitats, rivers, reefs, estuaries, abyssal depths or near the surface. Those like the Zambezi River sharks travel hundreds of kilometres up rivers from the sea.

Some, such as the white pointer sharks (great white) have developed the ability to partially regulate their body temperature, keeping it slightly above that of the surrounding water, they are tachymetabolic. This enables their muscles to operate more efficiently so they can swim at greater speeds.

According to Long, the evidence suggests that sharks originally had at least the potential to develop bone, eventually losing this trait as their evolution progressed (Coats et al., 2002). They settled on a skeleton of mainly cartilage, retaining few bony tissues on their skeleton. Their spines, teeth and scales are always composed if dentinous tissues, semidentin or orthodentin, though there are remnants of perichondral bone that is acellular, occurring in some early forms such as Akmonistion.

Origin and Evolution

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.

 

 

 

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Last Updated 25/02/2011

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading