FOSSILS are ten a penny-a dime a dozen. Whole mountain ranges are made of the skeletons of snails and sea lillies and lamp shells. Every day, without thinking, we shovel more fossilised plants on the fire and breathe in their preserved spores with the smoke, squirt algal remains into our petrol tanks and mix coral skeletons into our concrete. Palaeontologists bemoan not a lack of material, but a lack of time to describe the millions of specimens stored in museum vaults.
Yet most of these bear little resemblance to the creatures they once were. Fossilisation usually takes place so slowly that the soft tissue almost always decays, leaving behind only shells and bones (see 鈥淭he fossil next door鈥�). True, people can be preserved in peat bogs and mammoths can be frozen in permafrost, but these survive only a few thousand years-a blink of an eye in geological terms. But researchers have now discovered a fossilisation process that can strike like lightning, preserving muscles, intestines and even eyes in spectacular detail for hundreds of millions of years. The conditions it requires are still mysterious, and very rare, but last year progress was made in explaining this bizarre form of immortality.
West of Recife in Brazil, a high plateau rises clear of the surrounding arid scrubland. Buried in its rocks is the Santana Formation, a fossilised lagoon some 110 million years old, home to a menagerie of giant pterosaurs, turtles, crustaceans, fish and many species of flowering plants. The handsome Santana fossils-particularly the lustrously scaled fish-have been widely known since the 1830s. But it took 140 years before an accident revealed that their soft interiors are preserved as faithfully as their hard skeletons.
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Soft-hearted rocks
Colin Patterson, a palaeontologist from the Natural History Museum in London, was trying to clean some Santana fossils when it happened. The fossils are encased in hard limy nodules, so are often dunked in an acid bath to dissolve the unwanted rock. In the scummy residue left behind, Patterson found microscopic crustacean fossils called ostracods. Each had two shells made of phosphate, which survived the acid attack, and between these shells were the miraculously preserved limbs of the animal, so perfect that even the fine hairs remained.
Nearly two decades later, David Martill of Portsmouth University came across this research while hunting for an explanation for unusual preservation in a fossil he had found. Intrigued, he began to search for soft tissues in the fish and other animals preserved in the deposit. He struck palaeontological gold. When he dissected the fish he discovered gill lamellae and muscles in pristine condition, as if the animals had only just been killed. Zooming in with an electron microscope, he found first the sheaths surrounding the muscle fibres, then the fibres themselves, then bands on each fibril-representing the original cellular structure. Here and there, cell nuclei stood proud of the muscle tissue.
Skin and stomach wall had also been preserved, the stomach鈥檚 epidermal cells sometimes still bearing erect microvilli. And there were eggs-indisputably the world鈥檚 oldest caviar. Even the decayers themselves, swarms of bacteria, had been petrified. All of the tissues had been replicated by apatite, which is calcium phosphate, the same mineral that gives hardness to our bones and teeth. How on earth did this miraculous level of preservation come about?
One clue lies in the strange, split world of the Santana lagoon. Its surface waters were warm, nutrient-packed and teeming with life. Below this, though, seems to have been a layer of dense, probably highly saline water, lethal to most of the lagoon鈥檚 creatures. Brine-tolerant algal films carpeted the lagoon floor, lying on sediment so soft it was more like thick pea soup than a solid seafloor. Periodically, and catastrophically, the dense brine probably broke through to the surface, perhaps after a storm or a temperature change that upset the lake鈥檚 delicate equilibrium. This would have convulsed any animal caught in its toxic grasp. Fish fell to the sea floor still alive, where they thrashed and struggled, many being swallowed by the soupy sediment. Excavated 110 million years later, some still show the contorted vertebral column that betrays death by hypersalinity.
The poisoned victims lay scattered on or within the limy sediment, protected from scavengers by the same deadly waters that had killed them. This shouldn鈥檛 have stopped the bacteria that cause decay, though. Bacteria are supreme generalists, flourishing in the most extreme environments, from the superheated waters of geysers to the frozen interiors of glaciers. They should have swung into action almost immediately, stripping down the soft tissues of each carcass, while, working at their side, the enzymes from the animals鈥� own cells should have been splitting molecules apart. The destruction would have been over in weeks, but the apatite must have got to the tissues-and to the bacteria-even faster.
The fine details of the mummifying apatite were exhaustively catalogued in the early 1990s by Philip Wilby, at the Open University. He found that individual crystals less than one ten-thousandth of a millimetre across had formed on the decaying tissue, fitting it like a template. The source of the phosphate ions is unclear-some may have come from the decaying tissue itself, with more released from the surrounding sediment.
Shrimp mummies
Fossils like these are vanishingly rare, and so the conditions under which they were created must be equally rare. Efforts to pin these conditions down prompted some grisly and rather smelly experiments in the early 1990s by Derek Briggs and Amanda Kear of Bristol University. They put freshly killed shrimps, worms and lancelets into tanks, added bacteria-laden seawater from the Tay Estuary, left some tanks open to the air and some sealed, some with oxygen and some without. The soft parts nearly all decayed within weeks and, counter-intuitively, tissue decayed just as quickly without oxygen as it did in aerated waters. But Briggs and Kear were excited to find that in one set of experiments apatite began to replace some of the soft tissues, including muscles and eggs, after only two weeks. This only occurred in sealed tanks, and only worked with shrimps. In the sealed tanks, regardless of oxygen content, the pH dropped slightly and these acidic conditions inhibited the more usual precipitation of calcium carbonate, which scarcely ever preserves replicas of soft tissue. This allowed the mummifying apatite to crystallise onto the tissues instead.
Presumably, then, something must have sealed in the Santana carcasses-but what? According to Martill, sinking into the soupy sediment may have helped. Another possibility is that algal films may have grown quickly onto the carcasses to form a hermetic barrier. Martill saw signs of the remnants of such films in layers in the rock. The later precipitation of calcium carbonate in coats of nodules around the petrified remains must have acted like a plaster cast, preventing the fossils from being crushed as the sediment layers were buried and compressed.
While Martill was working on the Santana formation, even stranger carcasses were being unearthed in South Africa. In 1984, geologists from the Geological Survey of South Africa were hunting for bauxite deposits in the rugged hills of the Cedarberg range north of Cape Town. They were chipping away at the dark layers of a 430-million-year-old fossilised sea floor called the Soom Shale when one of the team, Hannes Theron, split a slab and found a strange fossil that looked like a cluster of tiny fret saw blades.
Baffled, Theron sent the specimen around the world, from one group of fossil researchers to another. Some wondered if it could be the remains of graptolites, fossils of extinct plankton that are one of the best means to date such ancient rocks. The specimen was duly sent to graptolite expert Barrie Rickards of the University of Cambridge. No, came back the reply-but perhaps they鈥檙e imprints of tooth-like fossils made of phosphate, called conodonts.
And so the specimen arrived at the door of Dick Aldridge of Leicester University. Aldridge confirmed that the blades were conodonts, but they were extraordinary. Most conodonts are barely a millimetre across, but these were ten times as big. And most conodonts are scattered widely after the death and decay of the otherwise soft-bodied creature that bore them. These were still in their original position, like a complicated set of dentures. Intrigued, Aldridge set off for South Africa in 1990.
Time capsules
Working with Theron, he patiently split open layer after layer of shale and found a variety of other fossils, including more conodonts. When Aldridge took these ancient sets of dentures back to Britain to study them more carefully, he discovered to his amazement that they were accompanied by black, carbonised eye capsules. How could an eye be preserved for hundreds of millions of years?
With mounting excitement, Aldridge headed back to South Africa with one of us (Sarah). Joined by Theron, the team searched the Soom Shale deposit in earnest. Soon they had unearthed nautiloids-long, cone-shaped shells inhabited by squid-like creatures; small lamp-shells, which had hitched a lift on the outsides of the nautiloids, like barnacles clinging to a ship鈥檚 hull; giant sea scorpions-the hoodlums of the Ordovician seas; strange, multi-segmented chimeras, which still cannot be placed comfortably within any known phylum; and everywhere tangled strands of carbonised seaweed, hinting at algal-choked waters like the Sargasso Sea.
In the spring of 1994, Sarah split open a block of the Soom Shale to reveal the silvery remains of an entire conodont animal. It looked like an alien sardine. Close-set eyes topped a slender, eel-like body with silvery imprints of V-shaped muscle blocks, like those of a kipper. There were even traces of the muscles that enabled the eyes to rotate-a key biological feature which mean conodont animals can be securely classified as early vertebrates, our own distant relatives.
When this creature and the other Soom animals were examined more closely under an electron microscope and with chemical probes, the preservation of these creatures became even more bizarre. In the topsy-turvy world of the Soom Shale, the tooth-like conodonts themselves, originally made of bone-hard calcium phosphate, had largely disappeared, leaving empty spaces. The carbon had also mostly vanished, except around the toughened eye capsules. In its place were clay minerals, the stuff that makes up most of what we know as mud. And the clay hadn鈥檛 just moulded around the surface. The flesh had literally been turned to clay, with amazing fidelity, even down to individual muscle fibres. If the Santana lagoon held the Medusa鈥檚 petrified victims, the Soom sea played host to the first golems, the clay creatures of Jewish legend.
As with the Santana ecosystem, the strange life of the Soom sea mainly inhabited the upper waters. There are no bottom-dwelling creatures among the fossils, which suggests that there was something uncomfortable about the seafloor. Sure enough, when Sarah performed a geochemical analysis on the sediment, she found that it was oxygen-deficient-stagnant. Oxygen-deficient seafloors are quite common. The Appalachians and the hills of Wales and Scotland are full of strata formed on such stagnant, ancient seafloors, but preserved soft parts have hardly ever been found in these, despite intense study. Just what was so special about the Soom seafloor?
One line of evidence is the missing hard parts, which were presumably dissolved away before fossilisation could begin. The likeliest culprit is hydrogen sulphide acid that seeped from stinking black muds and was formed when bacteria decomposed the plentiful algae. Crucially, the acid was not neutralised by incoming sediment from rivers-the ancient landscape seems to have been remarkably deficient in lime. As well as dissolving the hard parts of animals, the extreme acid conditions seem to have inhibited bacterial decay and stopped the Santana phosphatisation mechanism. This left the door open for clays to come in on the act.
Clays are ubiquitous at the Earth鈥檚 surface. Under the most powerful microscopes, they look like complex multi-storey buildings made of Meccano, with struts and girders of silicon, oxygen and aluminium ions. The flakes of clay materialising out of the acid, stagnant waters of the Soom lagoon were probably kaolinite, better known as china clay (it鈥檚 what makes the surface of 麻豆传媒 pages smooth and glossy). This kaolinite seems to have copied the cellular and subcellular structure of soft tissue. But how?
This is harder than it sounds. Electrical charges are unevenly distributed in such tiny flakes of matter, making the particles either electrically 鈥渟ticky鈥� if surface charges are opposite, or electrically repulsive if they are the same. Both the kaolinite flakes and the cell walls of the soft tissue would have had negative charges balanced by positive charge in their interiors. So, all things being equal, they should have repelled each other.
All things must not have been equal. Positively charged ions of sodium, potassium, calcium and other metals would have been present in seawater, and Sarah speculates that some could have escaped from the soft tissue as cell walls ruptured. These positive ions could have acted as chemical ambassadors, allowing clay flakes to get close enough to the tissues for short-range forces to kick in, snapping the clay flakes tightly onto the cell walls even as they began to decay.
Remaining queries
Sarah published her clay replication theory last year, but it can鈥檛 be the whole story. If the process was so straightforward, why isn鈥檛 it more common? So far the full details have only been worked out for preservation in the Soom Shale, though Patrick Orr and colleagues at the University of Bristol believe that it may have helped preserve that other famous fossil treasure trove, the Burgess Shale of Canada. Were the acid conditions in these sites much more extreme than those that normally develop in seas and lagoons? The dissolving of apatite bone and shell material would suggest so for the Soom at least. Or were the lagoon waters cold, further inhibiting bacterial action? There is good evidence that ice sheets ground over part of South Africa immediately before the Soom lagoon formed. And why is the muscle tissue replicated by clay in three dimensions, fibre by fibre? A simple surface coating would, seemingly, have been a much easier trick. All of these questions remain to be answered.
But there鈥檚 another twist to the story. In both Santana and Soom, strange chemistry and extreme conditions prevailed. Sounds familiar? We may be re-creating similar conditions right now, as acid lakes spread across high latitudes, and lagoons, rivers and shallow seas become increasingly polluted and stagnant. The vast, chemical 鈥渆xperiments鈥� we are conducting round the globe may be creating the conditions in which ultra-fossilisation could occur. We might not be able to preserve all the Earth鈥檚 species, but perhaps tiny, wonderfully replicated scraps will be left behind, the result of our very own Golden Age of Fossilisation.
