Life on Earth. David Attenborough
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reality its legs. It is now thought that it may be the first known member of a strange group called the lobopods which today includes odd little creatures called velvet worms.
The great variety of creatures in the Burgess Shales is a reminder of how incomplete our knowledge of all fossil faunas actually is. The ancient seas contained many more kinds of animals than we can ever know. In this one site, conditions allowed a uniquely large proportion to be preserved, but even this is only a hint of what must have once existed.
The Burgess Shales also contain superbly preserved examples of trilobites like those in the Moroccan limestones. Their body armour was constructed partly of calcium carbonate and strengthened by a horny substance called chitin, a material that forms the external skeletons of insects. But chitin, unlike skin, does not expand, so any animal with such an external chitinous skeleton has to shed it regularly if it is to grow – as indeed insects do today. Many of the trilobite fossils we find are in fact these empty suits of armour. Sometimes they are concentrated in great drifts, having been sorted by sea currents, as shells sometimes are when they are swept up on beaches today. The underwater avalanches in the Burgess Shales Basin, however, swept down not just discarded armour but living trilobites and buried them. Mud particles filtered into the animals’ bodies and preserved the finest details of their anatomy. So in them we can still see the paired jointed legs that are attached to each body segment, the feathery gill associated with each leg, two feelers at the front of the head, and the gut running the entire length of the body. Even the muscle fibres along the back, which enabled the animal to roll itself up into a ball, are still recognisable in some exceptional specimens.
Trilobites, as far as we know, were the first creatures on earth to develop high-definition eyes. They are mosaics, a cluster of separate components, each with its own lens of crystalline calcite orientated in the precise position in which it transmits light most efficiently, much like the eyes of today’s insects. One eye may contain 15,000 elements, and would have given its owner an almost hemispherical field of view. Late in the dynasty, some species developed an even more sophisticated kind of eye and one that has never been paralleled by any other animal. Here the components are fewer but larger. Their lenses are much thicker and it is thought that these species lived where there was little light and needed thick lenses to collect and concentrate what light there was. However, the optical properties of a simple calcite lens in contact with water are such that it transmits light in a diffused way and cannot bring it to a sharply focused point. To do this, a two-part lens is needed which has a waved surface at the junction between its two elements. And this is exactly what these trilobites evolved. The lower element of the double lens was formed by chitin and the surface between the two conforms to the mathematical principle that human scientists did not discover until 300 years ago when they tried to correct the spherical aberration of lenses in their newly invented telescopes.
As the trilobites spread through the seas of the world, they diversified into a great number of species. Many seem to have lived on the seafloor, chomping their way through mud. Some colonised the deep seas where there was little light and lost their eyes altogether. Others, to judge from the shape of their limbs, may well have paddled about, legs uppermost, scanning the seafloor below with their large eyes.
In due course, as creatures of many kinds and varying ancestries came to live on the bottom of the seas, the trilobites lost their supremacy. Two hundred and fifty million years ago, their dynasty came to an end. One relation alone survives, the horseshoe crab. It’s a misleading name for it is not a crab and only half its shell bears any resemblance to a horseshoe. Measuring 30 centimetres or so across, it is many times bigger than most known trilobites and its armour no longer shows any signs of segmentation. Its front section is a huge domed shield, on the front of which are two bean-shaped compound eyes. A roughly rectangular plate, hinged to the back of the shield, carries a sharp spike of a tail. But beneath its shell, the animal’s segmentation is clear. It has several pairs of jointed legs with pincers on the end, and behind these there are plate-like gills, large and flat like the leaves of a book.
Tower-eyed trilobite (Erbenochile erbeni) from the Timrahrhart Formation, Morocco.
Horseshoe crab (Limulus polyphemus) group spawning at high tide at sunset, Cape May, New Jersey.
Horseshoe crabs are seldom seen, for they live at considerable depths. Some inhabit Southeast Asian waters, others are found in the seas along the North Atlantic coast of America. Every spring, they migrate towards the coast. Then on three successive nights, when the moon is full and the tides are high, hundreds of thousands emerge from the sea. The females, their huge shells glinting in the moonlight, move towards the shore, dragging smaller males behind them. Sometimes four or five males, in their anxiety to reach a female, cling to one another and form a chain. As she reaches the edge of the water, the female half buries herself in the sand. There she sheds her eggs and the males release sperm. For kilometre after kilometre along the dark beaches, the living tide of horseshoe crabs is so thick that they form a continuous strip, like a causeway of giant cobbles. The breakers sometimes overturn them and they lie in the sand, with their legs waving, their stiff tails slowly swivelling, in an effort to lever themselves right side up. Many fail and are abandoned by the receding tide to die as thousands more swim in the shallows, pressing forward to take their turn.
This scene must have been enacted every spring for several hundred million years. When it began, the land was without life of any kind, and on such beaches the eggs were safe from sea-dwelling marauders. Perhaps this is why the horseshoe crabs developed the habit. Today beaches are not quite so safe, for hordes of gulls and small wading birds congregate to share the prodigious feast. But many of the fertilised eggs remain buried deep among the sand grains where they will stay for a month until, once more, high water reaches this part of the beach, stirring the sand and releasing the larvae to swim freely in the sea.
Although the trilobites were so successful, they were by no means the only armoured creatures to develop from the segmented worms. So did a group that must have been among the most alarming of all marine monsters – the sea scorpions, called scientifically the Eurypterids. Some grew to a length of two metres and were the largest arthropods ever known to have existed. However, in spite of their appearance and huge claws, many of them were filter feeders. Presumably, their fearsome claws were used in fights between one another rather than in subduing prey. Like the trilobites, they disappeared at the end of the Permian period.
One group related to the trilobites did however survive and today is extremely successful. They differed in one seemingly trivial but nonetheless diagnostic characteristic. They have not one but two pairs of antennae on their heads. They lived alongside the trilobites, comparatively unobtrusively for hundreds of millions of years, and then, when the trilobite dynasty came to an end, it was they who took over. They are the crustaceans. Today there are about 35,000 species of crustacean – seven times as many as there are of birds. Most prowl among the rocks and reefs – crabs, shrimps, prawns and lobsters. Some – the barnacles – have taken up a static life. Others – the krill which forms the food of whales – swim in vast shoals.
Robber crab (Birgus latro) climbing coconut tree, Aldabra Seychelles.
An external skeleton is highly versatile; it serves the tiny water flea as well as it does the giant Japanese spider crab that measures over three metres from claw to claw. Each crustacean species modifies the shape of its many paired legs for particular purposes. Those at the front may become pincers or claws; those in the middle, paddles, walking legs or tweezers. Some have feathery branches, gills through which oxygen is absorbed from the water. Others develop attachments so that they can carry eggs.
The limbs, which are tubular and jointed, are operated by internal muscles. These extend from the end of one section, along its length, to a prong from the next section which projects across the joint. When the muscle contracts between these two attachment points, the limb hinges. Such joints can only move in one plane, but crustaceans deal with that limitation by