Rocky Mountains of British Columbia, the Burgess Pass crosses a ridge between two high snowy peaks. Close to its crest lies an outcrop of particularly fine-grained shales, and in these have been discovered some of the most perfectly preserved fossils in the world. The shales were laid down about 530 million years ago, close to the beginning of the Cambrian period in a basin of the seafloor at a depth of about 150 metres. It must have been sheltered by a submarine ridge, for there were no currents to disturb the fine sediments on the floor or to bring down oxygenated water from nearer the surface. Few animals lived in those dark stagnant waters. There are no signs of tracks or burrows. Once in a while, however, mud from the ridge above slipped down in a turbid cloud, carrying with it all kinds of small creatures, and dumped them there. Since there was neither oxygen to fuel the processes of decay nor any scavenging animals to feed on the bodies and destroy them, many of the tiny carcasses remained complete as the settling mud particles slowly entombed them, preserving even their softest body parts. Eventually the entire deposit became consolidated into shale. Earth movements elevated and folded great areas of these marine deposits during the building of the Rocky Mountains. Many parts of them were distorted and crushed until most traces of life in them were obliterated. But miraculously, this one small patch survived virtually undamaged.
Velvet worm (Peripatus novaezealandiae). Velvet worms are known as ‘living fossils’, having remained the same for approximately 570 million years.
The range of creatures it contains is far wider than that found in rocks of a similar age at any other site so far discovered. There are the jellyfish that Ediacara would lead us to expect. There are echinoderms, brachiopods, primitive molluscs and half a dozen species of segmented worms – further representatives of the lineage that stretches from the beaches of Ediacara to the Barrier Reef of today.
There are also several creatures which were rather more mysterious. Among the most abundant of these was a strange segmented creature with what seemed to be a line of legs on its underside. It looked rather like a shrimp, though mysteriously none of the species had a head. It was given the name Anomalorcaris: strange shrimp. There were also small disc-shaped fossils marked with lines radiating from its centre that looked somewhat like a tiny slice of pineapple, which was initially thought to be some kind of jellyfish. Perhaps strangest of all, there was an elongated segmented animal that appeared to have seven pairs of spiny stilt-like legs, and seven flexible tentacles along its back, each ending in a tiny mouth. It seemed so strange as to be almost nightmarish, and the researcher who studied it accordingly called it Hallucigenia.
Subsequent work, however, showed that these oddities were not the founder members of some wholly unsuspected animal groups. A very exceptional specimen of Anomalocaris showed that the ‘strange shrimps’ were not complete animals but just the forelimbs belonging to a much bigger creature that used them to grab its prey. And the pineapple slice was eventually shown to have in its centre minute teeth. It was a mouth that belonged to the same animal as the tentacles. Both these pieces of Anomalorcaris’ body apparently had a more heavily strengthened exoskeleton and so regularly became separated from the animal’s more easily decayed body. As for Hallucigenia, further research on other specimens showed that it had been reconstructed in an upside-down position. The spindly legs were in fact protective dorsal spines, and what had been considered tentacles were in 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
