Life on Earth. David Attenborough
in a mesh of filaments while the sea cucumber slowly inches itself away on the tube feet that protrude from its underside. Over the next few weeks it will slowly grow itself a new set of entrails.
The echinoderms may seem, from a human point of view, to be a blind alley of no particular importance. Were we to imagine that life was purposive, that everything was part of a planned progression due to culminate in the appearance of the human species or some other creature that might rival us in dominating the world, then the echinoderms could be dismissed as of no consequence. But such trends are clearer in the minds of people than they are in the rocks. The echinoderms appeared early in the history of life. Their hydrostatic mechanisms proved a serviceable and effective basis for building a variety of bodies, but were not susceptible in the end to spectacular development. In the areas that suit them, they are still highly successful. A starfish on the reef can crawl across a clam, fasten its tube feet on either side of its gape and slowly wrench the valves apart to feed on the flesh within. The crown-of-thorns starfish occasionally proliferates to plague proportions and devastates great areas of coral. Crinoids are brought up in trawls from the deep sea several thousand at a time. If it is improbable that any further major developments will come from this stock, it is also unlikely, on the evidence of the last 600 million years, that the group will disappear as long as life remains possible at all in the seas of the world.
Panamic cushion sea stars (Pentaceraster cumingi) group on seafloor, Galapagos Islands.
The third category of creatures on the reef contains those with segmented bodies. In this instance, we do have fossil evidence of even earlier forms than the trilobites found in the Moroccan hills. The Ediacaran deposits in Australia which contain the remains of jellyfish and sea pens also preserve impressions of segmented worms. One species, a 5-centimentre-long animal named Spriggina after Reg Spriggs who first discovered the Ediacara fossils, has a crescent-shaped head and up to forty segments, fringed on either side by leg-like projections. What exactly it was, nobody can agree. No legs have been identified, but this may be a limitation in the process of fossilisation. Some scientists think it may represent a completely extinct group. One widely accepted possibility is that it was a kind of annelid worm related to the bristle worms that are so common on a reef and the earthworms that you can find in your garden.
Annelids have grooves encircling their body that correspond to the internal walls that divide its interior into separate compartments. Each of these is equipped with its own set of organs. On the exterior and on either side, there are leg-like projections sometimes equipped with bristles, and another pair of feathery appendages through which oxygen is absorbed. Within its body, each segment has a pair of tubes opening to the exterior from which waste is secreted. A gut, a large blood vessel and a nerve cord run from front to end through all the segments, linking and coordinating them.
Fossils can only tell us so much. Even the exceptionally well-preserved remains of Ediacara offer no clue about the connection between the segmented worms and the other early groups. However, there is one further category of evidence to be looked at – the larvae. The segmented worms have spherical larvae with a belt of cilia round their middles and a long tuft on top. These are almost identical to the larva of some molluscs, a strong indication that back in time the two groups sprang from common stock. The echinoderms, on the other hand, have a larva that is quite different, with a twist to its structure and winding bands of cilia around it. This group must have separated from the ancestral flatworms at a very early stage indeed, long before the split between the molluscs and the segmented worms. Geneticists, analysing the DNA of each of these groups, now confirm these deductions and reveal that there are two major groupings of bilaterally symmetrical animals. Octopus, crabs and flatworms form one group, while echinoderms, tunicates and all the backboned animals make up the other.
Segmentation may have developed as a way of enabling worms to increase their efficiency as burrowers in mud. A line of separate limbs down each side is clearly a very effective structure for this purpose and it could have been acquired by repeating the simple body unit to form a chain. The change must have taken place long before Ediacaran times, for when those rocks were deposited the fundamental invertebrate divisions were already established The Ediacaran fossils, in Australia where they were first discovered and in Britain, Newfoundland, Namibia and Siberia, now confirm these deductions. Thereafter their history remains virtually invisible for a 100 million years. Only after this vast span do we reach the period, 600 million years ago, represented by the Moroccan deposits and others throughout the world. By that time many organisms had, as we have seen, developed shells from which we can deduce their existence and shape, but not much more.
However, there is one exceptional fossil site dating from only a little later than those of Ediacara that provides far more detailed information about the bodies of animals than can come from mere shells. In the 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