Hook's UT Place

**Hook** · Posted: Fri Dec 21, 2007 10:29 am Post subject: [ARTICLE] - Ancient Sharks

By SharkSense (aka Silvershark) - Listed at The Forum Directory

Article from my website introducing the section on Ancient Sharks.

*******

The land was bare, dry rock. The only greenery came from the pioneering ancestors of the plants, still forced to cling to moist environments. Away from water, the land was devoid of life. There were no insects, no millipedes, no spiders. It would be many millions of years before vertebrates would take their first steps on this uninhabited terrain colonising this empty world. The days lasted only 21 hours, the moon much larger. Without plants there was little oxygen and twice as much carbon dioxide in the atmosphere as there is today. This was a hostile environment.

Beneath the surface of the water though, the scene was different. In the oceans, life was thriving and had been for millions of years. Trilobites were abundant. Some were active predators, others sifted food out from the mud or filtered particles from the water. There were tiny trilobites just a few millimetres long, and giants that reached a metre or more. Some were equipped with good eyesight, others were totally blind. Hunting them in the shallow waters were huge eurypterids, also known as sea-scorpions, reaching two metres in length. These fierce predators preyed upon any trilobite unlucky enough to end up within reach of its deadly pincers. Though the sea scorpions were the top predator in the shallows, even they had to bewares of the even larger predators that lurked in the ocean deep. Away from the shallow continental shelf was the realm of the orthocones, ancient relatives of squid and octopus. Some of these reached lengths of eleven metres. Equipped with grasping tentacles and protected by a long, hard shell, these were the biggest predators alive.

It was in this world of the Late Ordovician around 450 million years ago, that the earliest ancestors of today's most feared marine predator arose. This was where the Chondrichthyes lineage began - the line leading to all the modern sharks and their relatives. Here though, the sharks were far from having top predator status. In these seas, they were the prey.

The following Devonian saw an explosion of life. Fishes of all kinds and sizes appeared in the oceans. Among them was an armoured fish that would have dwarfed the largest predatory fish alive today, the great white shark. This fish was Dunkleosteus, a ten metre armoured giant with jaws that could slice through it's preys toughest defences. No shark alive at this time was a match for these voracious predators.

Little is left of these earliest ancestors of the sharks. The only evidence that remains of their existence are tiny dermal denticles, also known as skin teeth. What they looked like, what they ate, how they lived, even how big they were remains a mystery. It wasn't until the Late Devonian that the first fossil shark teeth appeared, teeth that can give us an idea of what these early sharks might have eaten when they were alive. From this period though comes an amazing fossil find. A nearly complete fossil of Doliodus problematicus that provides an insight into the history of early sharks. It appears to have had a flattened body form like the angel sharks of today and was unusual in that it's pectoral fins (the first pair of fins used for steering) were equipped with a pair of spines. Such a feature is unknown in any other sharks for although a number do possess spines, these are found on their dorsal fins. This small shark would have needed these spines to defend itself in these oceans filled with deadly predators. At a length of only around seventy-five centimetres, it was a bite-sized snack for many of the larger carnivores that were around.

These early sharks survived the risks that faced them in these dangerous seas. While many of their predators such as Dunkleosteus and the sea-scorpions disappeared, the sharks continued to thrive. They diversified into a range of different forms from those similar to modern sharks around today, to those that were truly bizarre.

*******

The Natural World Forum

Silvershark Fossils

Forum Directory
_________________
{xCBx}Hook Hook Pirate

(Site Administrator)
Hopelessly Addicted to UT99 & Mapping for it!

The NEW Forum Directory at: www.forumdirectory.freeforums.org