In The Blink Of An Eye (12 page)

Researchers have devised what is said to be the closest possible likeness of the historical Jesus, producing an image far removed from centuries of convention. The skull of a Jewish man from a first-century burial and the latest forensic techniques were combined to create a virtual image that challenges the stereotype in use in art since the Renaissance.

Before the second century Judaic tradition upheld a ban on the pictorial representation of God. Thus only the symbolic representation of Jesus could be depicted, bestowing the form of a fish or a lamb. St John's Gospel includes the statement ‘I am the good shepherd', and in the earliest figurative representations of Jesus he was portrayed as the Good
Shepherd. Later, when Christianity replaced the Roman Empire, Jesus was boldly illustrated as the King of Heaven, and gained the features of the stereotypical Roman aristocrat - he appeared older, more authoritative and beardless. But the Byzantine Church always preferred the bearded Jesus, and so this image became the standard everywhere. Hence a pillar of credibility was constructed that, like those of palaeontology, has proved difficult to topple. Giotto and Raphael among others continued with the bearded tradition that has remained popular up to the present day.

In 2000, road construction workers unearthed a group of skeletons in Jerusalem. Israeli archaeologists studied the alignment of the graves and the artefacts in the surrounding earth to conclude that the burial site was first-century Jewish. All of the skulls found were quite distinct from others of different ages and of different regional tribes. One skull was selected as being a good representative of the group, and typical of the kind of person that would have lived in Jerusalem in the first century AD.

Skulls determine the shape of a face, including the eyebrows, nose and jawline. To bring the Jerusalem skull to virtual life, it was handed over to a forensic expert in England, at Manchester University. Strips of clay were layered upon a plaster cast of the skull, in the proportions known from human postmortems. (This method was employed successfully to identify the remains of a King's Cross fire victim in London in 1987, and can generally boast a 70 per cent success rate for similar identifications. In that case the head still had skin, but the skin colour, and colour and style of the hair, remained in question.)

Fossils of plant life found in Jerusalem from the time of Christ were used to back up ancient texts on climate history. The climate was resolved with precision and the model of Jesus was given dark olive skin, appropriately. This contrasts with the pale, delicate complexion of previous depictions. But still the hair and beard of Jesus remained in question, and the fashion of Jesus' times became important to their reconstructions. The only useful pigments to have been preserved were not contained in 2,000-year-old hair samples, unfortunately, but those in first- and third-century frescoes of synagogues in northern Iraq. These depicted Jesus with short curly hair and a trimmed beard, a style which would be accommodated in the new reconstruction. We have to assume the hair was dark brown.

This is not the true face of Jesus but is probably the most accurate interpretation ever created. Here, archaeological, palaeontological and anatomical science have been united to replace artistic licence. But can we employ multidisciplinary science to bring fossils to virtual life? Or specifically, can we use the living nautilus to breathe life into its extinct relatives, the ammonoids? This could really help solve another mystery - why most ammonoids became fossilised near the interface of sea and land.

For about 500 million years the cephalopod molluscs, including the octopus, squid, cuttlefish, nautilus and ammonoid, have been among the most successful of marine animals. Today squid alone are numerous enough to sustain the world's population of their major predators - sperm whales. Squid possess internal shells. The nautilus, on the other hand, has an external, coiled shell that approximates a logarithmic spiral. Its squid-like tentacles, eyes and jet-propelling siphon protrude from the open end of its shell, snail-style. The shells of the nautilus and ammonoids are similar, and are divided internally into several chambers, each separated by chamber walls. The fossil record of ammonoids is extensive, and the chamber walls tend to preserve well. So what was the function of the shell chambers?

The most obvious reason why ammonite shells have chambers is that their dividing walls provide strength for the shell. Sir Eric Denton, of the Marine Biological Association of the UK in Plymouth, conducted many famous studies on the lifestyle of the living nautilus, and came up with evidence that denied the shell-strength proposition. One study demonstrated that a nautilus shell remains fully intact as the pressure of its surroundings is increased. That is until a critical pressure is reached where, without any warning suggested by cracking, the entire shell shatters. This characteristic was linked to the natural environment of the species - it lives from shallow seas down to waters just prior to a depth at which existence would be perilous, the environment that accommodates its critical pressure. The margin of safety is slight. Examination of shell fragments indicated that shattering at critical pressure was a characteristic of the shell-wall construction, and the chamber walls did not make a difference to the overall pressure tolerance. So the living relatives of
ammonoids indicated that the purpose of the shell chambers was not linked to strength.

The living part of the nautilus occupies the first and largest chamber, which is open-ended. Each chamber thereafter has an additional character - a thin tube running through its centre and through the chamber walls, terminating in the last chamber and taking on the spiral shape of the shell. Similar tubes are evident in ammonites, and in the nautilus this tube is known to contain living tissue. In terms of the body volume, the tube tissue is a minor part of the animal. But in terms of the animal's behaviour, it constitutes a major organ. The role of the tube tissue is to transport water into and out of the otherwise air-filled chambers, and so regulate buoyancy. This means that the nautilus can move vertically in the water with apparent ease.

Studies on the internal tubes of ammonites revealed similar properties to those of the nautilus - that water could have permeated the tube walls through gaps along its length. This led to a lifestyle reconstruction for the extinct ammonoids. They were portrayed as poor swimmers going forwards and backwards, but highly adapted for moving up and down. And then further gaps in our biological knowledge were filled - the image of an ammonoid moving vertically in the water column at speed was linked to its food.

Figure 2.2
Diagrammatic cross-section of a living nautilus (eye not shown) and photograph of a fossil ammonite (part of tube preserved near centre of shell).

Much is known about the characteristic jaws of the living relatives of ammonites such as the squid. I was first drawn to this type of mouth while trying to identify the culprits of a particular form of vandalism. Many electrical cables have been laid on the sea floor, sometimes at great depths. Recently a fault was reported in one such cable, a few centimetres thick, which lay off the Australian coast. It was eventually discovered that the cable had been completely severed. Marine scientists were presented with a section of the cable close to the fracture, and the cause of the fracture immediately became evident - vandalism of some kind and vandalism by something with a beak. The scrape marks in the half-centimetre thick, black plastic casing did not match any bite marks of fish, jawed worms, dolphins or any other animal - except the squid. Squids and their relatives possess hard beaks, similar to those of parrots. Eventually a museum specimen of a squid beak was found which fitted exactly the scrape-like bite marks. We had found our culprit.

It is well known that certain designs of beak are adapted to suit specific food items, in the style of birds of prey or the famous ‘beak of the finch' that Darwin found enlightening. The shape of the ammonoid jaw, which is occasionally found as a fossil, indicates they fed on small prey, probably planktonic. So there would have been a need to make regular vertical migrations to follow the plankton - plankton regularly make vertical migrations today. But most ammonoids became fossilised at the water's edge, which led to the construction of virtual ammonoid environments with characteristically modest depths. Was this a true depiction, or was the evidence just too circumstantial? Sometimes circumstantial evidence can be compelling, but in this case more clues were required to substantiate the ammonoid environment, and evidence was found in the internal tubes of their shells.

It was discovered that the nature and properties of the internal tubes provide a simpler indicator of strength of an ammonoid shell than do the outer shell walls and chamber walls, which are often complex in shape. In turn, the strength data gave rise to depth data, based on the critical pressure principle. Eric Denton's work on a living nautilus provided justification for this projection. And the conclusion drawn for ammonoids? Many species inhabited waters down to at least 600 metres in depth. But this only intensified the problem as to why most
ammonoids became fossilised near to the shore. And it was the death of an ammonoid that held the final solution.

A nautilus shell will, if its living tissues are removed, fill with water, become negatively buoyant and sink to the sea floor. This is how the deceased nautilus had traditionally been considered. But such a fate has proved to be unrealistic. This postmortem sinking was found not to be true for an animal that died with its soft tissues in place. In such a case, gases derive from the process of decomposition of the carcass, which soon expel water from the body chamber and inflate the decaying soft parts. Then, within a few hours, the dead animal will float to the surface. At this point the water and gas levels in the chambers other than the body chamber have remained unaltered since death. But after a couple of days, the decaying body and shell part company and go their separate ways. For the shell, the remaining water in the chambers leaks out via the internal tube. Then it is free to float on the ocean surface, like a coconut, until it encounters land. There it comes to rest, and there it may become a fossil. Here is the solution to the shoreline-fossil problem, and also the reason for such an extensive fossil record of the once common ammonoids. Indeed, if they did begin to sink after death, with natural levels of gas in their chambers, they would reach only as far as their critical depths before imploding. In which case there would be no discernible fossil for such an abundant species. The nautilus story was concluded some thirty years ago, but recently the case was re-opened. A new biological study has revealed a twist, and what emerged to be a crucial adaptation for the ammonoids.

I have already referred in this chapter to the idea of mass extinction. Every so often, the history of life on Earth is punctuated by mass extinction events. There have been several cases of mass extinctions, the most famous happening sixty-five million years ago, which saw the demise of the dinosaurs. But the greatest mass extinction event of all, present predicament excluded, was the momentous Permian extinction.

The Permian, like the Cambrian, is a period in geological time with boundaries defined by events recorded in the fossil record. At the end of the Permian, 250 million years ago, around 90 per cent of the species on Earth disappeared. And again, the rocks can be employed to provide
an answer to the cause of this event, and Doug Erwin of the Smithsonian Institution has pieced the evidence together.

The pore counts in leaves inform us that carbon dioxide levels and global temperatures were high 250 million years ago, following a cooler spell. A sudden drop in sea level at the end of the Permian destroyed near-shore habitats and destabilised the climate. With the death of the abundant flora and fauna that once inhabited the coast came decomposition on a grand scale. Decomposition results in carbon dioxide production, and, as the leaves predict, the carbon dioxide entered the atmosphere in significant amounts. This contributed to global warming and a depletion of oxygen that could dissolve in water. Unfortunately for Permian life, another disaster struck simultaneously - immense volcanoes erupted relentlessly for a few million years. To begin with, the eruptions cooled the Earth, but in the long term they led to global warming and ozone depletion. The effect of all of this on the oceans was that the water had become extremely anoxic - dissolved oxygen was scarce. It is therefore not surprising that most marine species became extinct; they probably suffocated. The filter-feeders were particularly hard hit, and the last of the trilobites disappeared for ever. Although many species of ammonoids also vanished, the ammonoids in general were among the few lucky ones - they made it through the Permian-Triassic boundary. How did they do it? This is where the new biological work on nautilus enters the story.

Recent studies have revealed a further adaptation in a nautilus living in deep water - its shell can behave like a Scuba tank. In the deep, oxygen levels can be low. It is well known that nautilus can counter this by lowering its chemical activity - it simply slows down. But it appears it also employs the oxygen in its buoyancy chambers to eke out the external oxygen supplies even further - it uses it to breathe. And the palaeontological story of ammonoids requires some adjustment because the Scuba scenario has been applied to the ammonoid shell. It is emerging that their Scuba tanks probably carried the ammonoids past the great Permian frontier. The ammonoids were highly adaptable when it came to levels of dissolved gases, and this probably accounted for their dominance throughout a prolonged period of history. The fact that nautilus continued with the Scuba system until today is good evidence that
it indeed provides a competitive edge. So all in all the ammonoids were the master plankton fishermen - they could follow plankton everywhere, within a depth range that no fish today could hope to emulate.