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 y
ears 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.
This story illustrates the importance of understanding ancient environmental conditions before reconstructing ancient animals themselves. Suddenly all the fossil evidence of past climates and gaseous conditions is becoming relevant. But it is worthwhile also considering a different type of fossil evidence, one that can have equally important implications for fossil reconstructions. Ammonoids spent their life suspended in the water column. While alive they never set foot on the ground, their ground being the sea floor. Fortunately for palaeontologists, many animals did move on ground, and they left signs of movement in their wakes.
Trace fossils
Sherlock Holmes, and indeed his creator, Sir Arthur Conan Doyle, had a keen eye for footprints. Holmes used the size and type of print as an identification tool, the orientation of the prints to deduce entry or exit, and the spacing of the prints to determine the impetuosity of the crime. Palaeontologists, it seems, have converged on this practice.
Dinosaurs left their footprints in mud that became hard-baked and preserved through time. Today the prints are known as trace fossils - not parts of the ancient animals themselves, but impressions made by their movements. Footprints have revealed many secrets of ancient movement, feeding and lifestyles, such as group behaviour. This is all old hat. Now the study of dinosaur footprints has advanced a stage further, following the recent discovery of 200-million-year-old, three-dimensionally preserved tracks in Greenland.
In 1998 an American scientific team set out to explore the tree-barren fields of east Greenland. The team, which included Stephen Gatesy, Kevin Middleton, Farish Jenkins Jr and Neil Shubin, had been lured by the Triassic (over 200-million-year-old) exposures and the prospect of discovering early mammals. But the bones and teeth of various ancient vertebrates were temporarily cast aside as the team’s attention became drawn to strange trackways of indistinct footprints.
Figure 2.3 The footprints discovered in Greenland, made in both firm ground and sloppy mud.
There is a law among footprint workers: a trackway is not simply a record of anatomy. Rather it is a record of how a foot behaves under a particular pattern of movement as it makes contact with a particular type of ground. The varying conditions of ground can have a substantial effect on the features of the footprint - contrast a human print left in firm soil with one in wet mud. The Greenland tracks ranged from clear imprints to virtually indistinct traces, but they were made by the same species of theropod (carnivorous) dinosaur in, importantly, different types of ground. The ground varied from firm to sloppy, the range we find on a beach when we walk towards a fluctuating waterline. The prints made in the firm ground were run-of-the-mill, two-dimensional types as known from all corners of the prehistoric globe. As usual, they provided useful information about the owner of the prints and the precise form of the foot. It was the prints in the sloppy mud, however, that led to a breakthrough.
The sloppiness of the mud had preserved a three-dimensional footprint. It preserved the entry and exit ‘wounds’ made by the foot. And following comparisons with living animals, it transpired that the deeper you sink, the more of the movement that usually takes place above ground can take place below it instead. This was an important finding. It indicated that the three-dimensionally preserved footprints, regardless of their futile patterns at the surface, could potentially provide data on the movement of dinosaur feet through the air. Quite amazing when you think about it. And the only way of extracting the informative data was to examine the footprints in cross section.
The American team cross-sectioned plaster casts of the fossil prints in abundance. Eventually they assembled complete three-dimensional images of the footprints on their computers. But at this stage the three-dimensional prints appeared just as puzzling as the surface patterns. To make sense of them, the team turned to biology and studied living guineafowl and turkey. Live birds were run through increasingly sloppy mud and it became apparent that they left very similar, three-dimensional footprints. But it was the way they made the prints that was interesting and led to a theory of how dinosaurs moved their feet in the air as well as on ground.
Live guineafowl and turkey placed their feet into the mud with toes apart. But as they pulled their feet out of the mud their toes were brought together. When the birds walked on hard ground rather than soft, the same series of events took place, although this time they happened in the air. The same was concluded for dinosaurs - they opened their toes as their feet were placed on the ground, and closed them as their feet were lifted. Previously it was believed that some dinosaurs walked on the soles of their feet. But the sloppiness of the sediment revealed that in this theropod dinosaur the heel was carried the lowest, just a bit lower than in birds today. This in turn provided evidence that, compared to birds, the theropod stride was more strongly powered by the
femur, while the lower leg and foot provided more of the power thrust.
The entire three-dimensional movement of a theropod foot through mud was modelled on the computer. This involved grafting the anatomy of a typical theropod foot on to the footfall pattern of a live bird. The images, and consequently the surface patterns made by the theropod, were self-explanatory (see Plate 7). It was nice to demonstrate that theropods walked in a similar fashion to birds, because the evidence from two-dimensional footprints and the bones themselves had been hinting at quite major differences in foot skeletons between the two groups. And of course this continued to feed the debate as to whether or not dinosaurs were ‘birds’. Now it could be demonstrated that locomotion and limb function could have evolved gradually from theropods to birds, in common with many other features.
Adding further flesh to the bones
The precise relationship between dinosaurs and birds is a highly controversial issue. Signs of early feathers on a newly discovered Chinese dinosaur have been rejected by many, who prefer the interpretation that the downy outlines of the fossils are simply fibres from the skin that can fray when reptile skin surface is damaged. Ironically the specimen in question, a 120-million-year-old Sinosauropteryx, a theropod, has been brought to virtual life only to deliver a blow to its excavators, who sit within the ‘dinosaurs-are-birds’ camp.
The fine silt from an ancient lake had preserved the soft structures of Sinosauropteryx, including a clear silhouette of the lungs. John Ruben, a respiratory expert from Oregon State University, took one look at the ‘lungs’ and knew what he was dealing with. He had seen this lung arrangement before - in crocodiles. Immediately he constructed his virtual, living dinosaur, with the same compartmentalisation of lungs, liver and intestines that one would find in a crocodile, and not in a bird. This virtual dinosaur was incapable of the high rates of gas exchange needed for warm-bloodedness. So it contained cold blood, like the crocodile. Also, its bellows-like lungs could not have conceivably evolved into the high-performance lungs of modern birds. But still this evidence, that birds were not descendants of dinosaurs, is far from conclusive. As new fossils are unearthed and analysed with the lives of modern animals in mind, the building of a virtual dinosaur continues.
In The Blink Of An Eye Page 9
