In this chapter, a variety of lines of investigation are explored to reinforce the message that a multiplicity of approaches must be used if we are to comprehend the lives of fossil animals.
Some aspects of dinosaur research have an almost sleuth-like quality to them, perhaps none more so than ichnology - the study of footprints.
There is no branch of detective science which is so important and so much neglected as the art of tracing footsteps.
(Conan Doyle, The Study in Scarlet, 1891)
The study of dinosaur footprints has a surprisingly long history. Some of the first to be collected and exhibited were found in 1802 in Massachusetts by the young Pliny Moody while ploughing a field. These and other large three-toed prints were eventually illustrated and described by Edward Hitchcock in 1836 as the tracks left by gigantic birds; some can still be seen in the Pratt Museum of Amherst College. From the mid-19th century onwards, tracks were discovered at fairly regular intervals in various parts of the world. With the development of an understanding of the anatomy of dinosaurs, and most particularly the shape of their feet, it was realized that the large 'bird-like' three-toed prints that were found in Mesozoic rocks belonged to dinosaurs rather than giant birds. Such tracks, though of local interest, were rarely regarded as of great scientific value. However, in recent years, largely prompted by the work of Martin Lockley of the University of Colorado at Denver, it has begun to be appreciated more widely that tracks may provide a great deal of information.
First, and most obviously, preserved tracks record the activities of living dinosaurs. Individual prints also record the overall shape of the foot and the number of toes, which can often help to narrow down the likely trackmaker, especially if dinosaur skeletons have been discovered in similarly aged rocks nearby. While individual prints may be intrinsically interesting, a series of tracks provides a record of how the creature was actually moving. They reveal the M orientation of the feet as they contact the ground, the length of the | stride, the width of the track (how closely the right and left feet £ were spaced); from this evidence, it is possible to reconstruct how the legs moved in a mechanical sense. Furthermore, taking observations using data from a wide range of living animals it has also proved possible to calculate the speeds at which animals leaving tracks were moving. These estimates are arrived at by simply measuring the size of the prints and length of each stride and making an estimate of the length of the leg. Although the latter might seem at first sight difficult to estimate with great accuracy, the actual size of the footprints has proved to be a remarkably good guide (judging by living animals), and in some instances foot and leg bones or skeletons of dinosaurs that lived at the time the tracks were made are known.
The shape of individual tracks may also reveal information relevant to deducing how such animals were moving: relatively flat, broad prints indicate that the whole foot was in contact with the ground for quite a long time, suggesting that it was moving relatively slowly; in other instances, the tracks may just show the tips of the
toes making contact with the ground - suggesting that the animal was quite literally sprinting on the tips of its toes.
Another interesting aspect of dinosaur tracks relates to the circumstances that led to them being preserved at all. Tracks will not be preserved on hard ground, instead it needs to be relatively soft and usually moist, and ideally of a muddy consistency. Once the prints have been made, it is then important that they are not greatly disturbed before they solidify; this can happen if the prints are buried quickly beneath another layer of mud, because the surface becomes baked hard in the sun, or through the rapid precipitation of minerals that form a kind of cement within the footprint layer. Very frequently, it is possible to deduce from details of the sediment in which the tracks were made exactly what the conditions were like when the dinosaur left its tracks. This can range from the degree to which the mud was disturbed by the feet of the animal and how deeply the feet sank into the sediment, to how the sediment seems to have responded to the movements of the foot. Sometimes it can be seen that a creature was moving up or down slopes simply from the way sediment is scuffed up in front of, or behind, the main footprint. Tracks left by dinosaurs can therefore offer a great deal of information about not only how dinosaurs moved, but the types of environments that they moved in.
M The study of tracks can also reveal information about dinosaur | behaviour. On rare occasions, multiple tracks of dinosaurs have £ been discovered. One famous example, recorded in the Paluxy River at Glen Rose in Texas, was revealed by a famous dinosaur footprint explorer named Roland T. Bird. Two parallel tracks were found at this site, one made by a huge brontosaur and the other by a large carnivorous dinosaur. The tracks seemed to show the big carnivore tracks converging on the brontosaur. At the intersection of the tracks, one print is missing, and Bird suspected that this indicated the point of attack. However, Lockley was able to show from maps of the track site that the brontosaurs (there were several) continued walking beyond the supposed point of attack; and, even though the large theropod was following the brontosaur (some of its prints overlap those of the brontosaur), there is no sign of a 'scuffle'. Very probably this predator was simply tracking potential prey animals by following at a safe distance. More convincing were some tracks observed by Bird at Davenport Ranch, also in Texas. Here he was able to log the tracks of 23 brontosaur-like sauropods walking in the same direction at the same time (Figure 35). This suggested very strongly that some dinosaurs moved around in herds. Herding or gregarious behaviour is impossible to deduce from skeletons, but tracks provide direct evidence.
Increased interest in dinosaur tracks in recent years has brought to light a number of potentially interesting avenues of research. Dinosaur tracks have sometimes been found in areas that have not yielded skeletal remains of dinosaurs, so tracks can help to fill in particular gaps in the known fossil record of dinosaurs. Interesting geological concepts have also emerged from a consideration of dinosaur track properties. Some of the large sauropodomorph dinosaurs (the brontosaurs referred to above) may have weighed as much as 20-40 tonnes in life. These animals would have exerted enormous forces on the ground when they walked. On soft substrate, the pressure from the feet of such dinosaurs would have distorted the earth at a depth of a metre or more beneath the surface - creating a series of 'underprints' formed as echoes of the g original footprint on the surface. The spectre of 'underprints' means jjj that some dinosaur tracks might be considerably over-represented = in the fossil record if a single print can be replicated through ¡5
numerous 'underprints'. h
If herds of such enormous creatures trampled over areas, as they certainly did at Davenport Ranch, then they also had the capacity to greatly disturb the earth beneath - pounding it up and destroying its normal sedimentary structure. This relatively recently recognized phenomenon has been named 'dinoturbation'. 'Dinoturbation' might be a geological phenomenon, but it hints at another distinctly biological effect linked to dinosaur activities that may or may not be measurable over time. That is the potential evolutionary and ecological impact of dinosaurs on terrestrial communities at large. Great herds of multitonne dinosaurs moving across a landscape had the potential to utterly devastate the local ecology. We are aware that elephants today are capable of causing considerable damage to the African savannah because of the way that they can tear up and knock down mature trees. What might a herd of 40-tonne brontosaurs have done? And did this type of destructive activity have an effect upon the other animals and plants living at the time; can we identify or measure such impacts in the long term, and were they important in the evolutionary history of the Mesozoic?
Another slightly less romantic branch of palaeobiological investigation focuses on the dung of animals such as dinosaurs. This material is refered to as coprolites (copros means dung, lithos means stone), and their study has a surprisingly long and relatively illustrious history. The recognition of the importance of preserved dung dates back to the work of William Buckland of Oxford University (the man who described the first dinosaur, Megalosaurus). A pioneering geologist from the first half of the 19th century, Buckland spent considerable time collecting and M studying rocks and fossils from his native area around Lyme Regis | in Dorset, including fossil marine reptiles. Alongside these, £ Buckland noted large numbers of distinctive pebbles that often had a faint spiral shape. On closer inspection, breaking them open and looking at polished sections, Buckland was able to identify shiny fish scales, bones, and the sharp hooks of belemnite (a cephalopod mollusc) tentacles in great concentrations. He concluded that these stones were most probably the lithified excreta of the predatory reptiles found in the same rocks. Clearly, though at first sight somewhat distasteful, the study of coprolites had the potential to reveal evidence concerning the diet of the once-living creature that would not otherwise be obtainable.
As was the case with footprints, the question 'who did this?', though obviously amusing, can present significant problems. Occasionally, coprolites, or indeed gut contents, have been preserved inside the bodies of some fossil vertebrates (notably fish); however, it has been difficult to connect coprolite fossils to specific dinosaurs or even groups of dinosaurs. Karen Chin of the US Geological Survey has devoted herself to the study of coprolites and has had singular difficulty in reliably identifying dinosaur coprolites - until quite recently.
In 1998, Chin and colleagues were able to report the discovery of what they referred to in the title of their article as 'A king-sized theropod coprolite'. The specimen in question was discovered in Maastrichtian (latest Cretaceous) sediments in Saskatchewan and comprised a rather nobbly lump of material, over 40 centimetres long, that had a volume of approximately 2.5 litres. Immediately around and inside the specimen were broken fragments of bone, and a finer, sand-like powder of bone material was present throughout the mass. Chemical analysis of the specimen confirmed that it had very high levels of calcium and phosphorous, confirming a high concentration of bone material. Histological thin sections of the fragments further confirmed the cellular structure of bone and that the most likely prey items that had been digested were o dinosaurian;as suspected, this specimen was most likely a large jjj carnivore's coprolite. Surveying the fauna known from the rocks in = this area, the only creature that was large enough to have been able g a to pass a coprolite of these dimensions was the large theropod h
Tyrannosaurus rex ('king' of the dinosaurs). Examination of the bone fragments preserved in the coprolite showed that this animal had been able to pulverize the bones of its prey in its mouth, and that the most likely prey was a juvenile ceratopian ornithischian (from the structure of the bone in the histological sections). The fact that not all the bone had been digested in this coprolite indicated that the material had moved through the gut with considerable speed, which could be used by some as evidence that T. rex was perhaps a hungry endotherm.
The confirmation of a diet of meat in T. rex is clearly not entirely unexpected, given the overall anatomy of such theropods. However, an interesting pathological consequence of a diet rich in red meat has also been detected in the skeleton of Tyrannosaurus.
'Sue', the large skeleton of Tyrannosaurus rex now on display at the Field Museum in Chicago, is of interest because of the presence of various pathological features. One of its finger bones (metacarpals) exhibits some characteristic, smoothly rounded pits at the joint with its first finger bone; these were subjected to detailed examination by modern-day pathologists as well as palaeontologists. The palaeontologists discovered that other tyrannosaurs also exhibit such lesions, but that these are quite rare in museum collections. The pathologist was able to confirm, following detailed comparison with pathologies from living reptiles and birds, that the lesions were the result of gout. This illness, also known in humans, generally affects the feet and hands, and is extremely painful, causing swelling and inflammation of the areas involved. It is caused by the deposition of urate crystals around the joints. Although gout can be a result of dehydration or of kidney failure, a factor in humans is diet: ingesting food rich in purine, a chemical found in red meat. So,
M Tyrannosaurus not only looked like a meat-eater, its faeces prove it, ii and so does one of the diseases it suffered from.
'Sue' also displays a large number of more conventional pathologies. These are the tell-tale remains of past injuries. When bones are broken during life, they have the capacity to heal themselves. Although modern surgical techniques enable repair of broken bones with considerable precision, in Nature the broken ends of the bone do not usually align themselves precisely, and a callous forms around the area where the ends of bone meet. Such imperfections in the repair process leave marks on the skeleton that can be detected after death. It is clear that 'Sue' suffered a number of injuries during 'her' life. On one occasion, 'she' experienced a major trauma to the chest, which exhibits several clearly broken and repaired ribs. In addition, 'her' spine and tail show a number of breakages that, again, healed during life.
The surprising aspect of these observations is that an animal such as T. rex was clearly able to survive periods of injury and sickness. It might be predicted that a large predator such as T. rex would become extremely vulnerable and therefore potential prey itself once it was injured. That this did not happen (at least in the instance of 'Sue') suggests either that such animals were extraordinarily durable and therefore not unduly affected by quite serious trauma, or that these dinosaurs may have lived in socially cohesive groups that might have acted cooperatively on occasion to assist an injured individual.
Other pathologies have also been noted in various dinosaurs. These range from destructive bone lesions resulting from periodontal abscesses (in the case of jaw bones), or septic arthritis and chronic osteomyelitis in other parts of the skull or skeleton. One particularly unpleasant example of long-term infection of a leg wound was recorded in a small ornithopod. The partial skeleton of this animal was discovered in Early Cretaceous sediments in south-eastern Australia. The hindlimbs and pelvis were well g preserved, but the lower part of the left leg was grossly distorted jjj and shortened (Figure 36). Although the original cause of the =
subsequent infection could not be proved, it was suspected that the ¡5
a animal may have received a severe bite on the shin close to the knee h of its left leg. As a result, the fossilized bones of the shin (tibia and fibula) were severely overgrown by a huge, irregular, callous-like mass of bone.
Examination and X-radiography of the fossil bone revealed that the site of the original injury must have become infected, but that rather than remaining localized the infection spread down the marrow cavity of the shin bone, partially destroying the bone as it went. As the infection spread, extra bony tissue was added to the exterior of the bone as if the body was trying to create its own 'splint' or support. It is clear that the animal's immune system was unable to prevent the continued spread of infection, and large abscesses formed beneath the outer bony sheath; the pus from these must have leaked through from the leg bones and may have run out on to the surface of the skin as a sore. Judging by the amount of bone growth around the site of infection, it seems likely that the animal
lived for as much as a year, while suffering from this horribly crippling injury, before it finally succumbed. The preserved skeleton shows no other sign of pathological infection, and there is no indication of tooth marks or other scavenging activity because its bones were not scattered.
Tumours have only rarely been recognized in dinosaur bones. The most obvious drawback with trying to study the frequency of cancers in dinosaurs has been the need to destroy dinosaurian bone in order to make histological sections - obviously something that has little appeal to museum curators. Recently, Bruce Rothschild has developed a technique for scanning dinosaur bones using X-rays and fluoroscopy. The technique is limited to bones less than 28 centimetres in diameter, and for this reason he surveyed large numbers (over 10,000) of dinosaur vertebrae. The vertebrae came from representatives of all the major dinosaur groups from a large number of museum collections. He discovered that cancers were not only very rare (<0.2% to 3%) but also limited exclusively to hadrosaurs.
Quite why tumours should be so restricted is puzzling. Rothschild was moved to wonder whether the diets of hadrosaurs may have had a bearing on this epidemiology. Rare discoveries of 'mummified' carcasses of hadrosaurs show accumulations of material in the gut that include considerable quantities of conifer tissue; these plants contain high concentrations of tumour-inducing chemicals. Whether this provides evidence either for a genetic predisposition to cancer among hadrosaurs, or for environmental induction (a mutagenic diet), is entirely speculative at present.
Another branch of science known as geochemistry has been using radioactive isotopes of oxygen, particularly oxygen-16 and oxygen-18, and their proportions in chemicals (carbonates) found in the shells of microscopic marine organisms, to estimate the temperature of ancient oceans, and therefore larger-scale climatic conditions. Basically, the understanding is that the higher the proportion of oxygen-18 (compared to oxygen-16) locked into the chemicals of the shells of these organisms, the colder the temperature of the ocean in which the organisms originally lived.
In the early 1990s, a palaeontologist, Reese Barrick, and a geochemist, William Showers, joined forces to see if it might be possible to do the same for the chemicals in bones - particularly the oxygen that forms part of the phosphate molecule in bone minerals. They first applied this approach to some known vertebrates (cows and lizards) by taking samples of bones from different parts of the body (ribs, legs, and tail) and measured the oxygen isotope proportions. Their results showed that for the endothermic mammal (cow) there was very little difference in the body M temperature between the bones of the legs and ribs; as might be | expected, the animal had a constant body temperature. In the £ lizard, however, the tail was between 2 and 9°C lower than its
ribs; the ectotherm did not have such an even distribution of body heat, with the peripheral parts on average cooler than the body core.
Barrick and Showers then performed a similar analysis on various bones from a well-preserved T. rex skeleton collected in Montana. Drilled samples from ribs, leg, toe, and tail bones revealed a rather mammal-like result: the oxygen isotope ratios differed very little, indicating that the body had a fairly even temperature throughout. This was used to promote further the idea that dinosaurs were not only homeothermic but also that they were endothermic. More recent work by these authors seems to confirm their basic finding, and has extended this observation to a range of other dinosaurs, including hadrosaurs.
As is often the case, these results generated a lively discussion. There were concerns that the bones may have been chemically altered during fossilization, which would render the isotopic signals meaningless, and physiologically minded palaeobiologists were far from convinced about what the result meant: a homeothermic signal is consistent with the idea that most dinosaurs were large-bodied mass-homeotherms (Chapter 6) and gives no conclusive evidence of endo- or ectothermy.
This is clearly an interesting line of inquiry; the results are not yet conclusive but provide the grounds for future research.
Dinosaur research: the scanning revolution
The steady improvement in technological resources, as well as their potential to be used to answer palaeobiological questions, has manifested in a number of distinct areas in recent years. A few of these will be examined in the following section; they are not without their limitations and pitfalls, but in some instances questions may now be asked that could not have been dreamt of 10 years ago.
One of the most anguished dilemmas faced by palaeobiologists is the desire to explore as much of any new fossil as possible, but at the same time to minimize the damage caused to the specimen by such action. The discovery of the potential for X-rays to create images on photographic film of the interior of the body has been of enormous importance to medical science. The more recent revolution in medical imaging through the development of CT (computed tomography) and MRI (magnetic resonance imaging) techniques that are linked directly to powerful data-processing computers has resulted in the ability to create three-dimensional images that allow researchers to see inside objects such as the human body or other complex structures that would only normally be possible after major exploratory surgery.
The potential to use CT scanning to see inside fossils was rapidly appreciated. One of the leaders in the field is Tim Rowe, with his team based at the University of Texas in Austin. He has managed to set up one of the finest fossil-dedicated, high-resolution CT scanning systems and, as we shall see below, has put it to some extremely interesting uses.
One obvious use of CT scanning can be demonstrated by referring to the extravagant range of crests seen on some hadrosaurian ornithopods. These dinosaurs were very abundant in Late Cretaceous times and have remarkably similarly shaped bodies; they only really differ in the shape of their headgear, but the reason for this difference has been a long-standing puzzle. When the first 'hooded' dinosaur was described in 1914, it was considered likely that these were simply interesting decorative features. However, in 1920 it was discovered that these 'hoods', or crests, were composed M of thin sheaths of bone that enclosed tubular cavities or chambers of ii considerable complexity.
Theories to explain the purpose of these crests abounded from the 1920s onwards. The very earliest claimed that the crest provided an attachment area for ligaments running from the shoulders to the neck that supported the large and heavy head. From then on, ideas ranged from their use as weapons; that they carried highly developed organs of smell; that they were sexually specific (males had crests and females did not); and, the most far-sighted, that the chambers might have served as resonators, as seen in modern birds. During the 1940s, there was a preference for aquatic theories: that they formed an air-lock to prevent water flooding the lungs when these animals fed on underwater weeds.
Most of the more outlandish suggestions have been abandoned, either because physically impossible or they do not accord with the known anatomy. What has emerged is that the crests probably performed a number of interrelated functions of a mainly social/ sexual type. They probably provided a visual social recognition system for individual species; and, in addition, some elaboration of the crests undoubtedly served a sexual display purpose. A small number of hadrosaur crests were sufficiently robust to have been used either in flank or head-butting activities as part of pre-mating rituals or male-male rivalry competitions. Finally, the chambers and tubular areas associated with the crests or facial structure are thought to have functioned as resonators. Again, this presumed vocal ability (found today in birds and crocodiles) can be linked to aspects of social behaviour in these dinosaurs.
One of the greatest problems associated with the resonator theory was gaining direct access to skull material that would allow detailed reconstruction of the air passages within the crest, without breaking open prized and carefully excavated specimens. CT techniques made such internal investigations feasible. For example, some new material of the very distinctively crested hadrosaur g
Parasaurolophus tubicen was collected from Late Cretaceous jjj sediments in New Mexico. The skull was reasonably complete, =
well preserved, and included a long, curved crest. It was CT ¡5
a scanned along the length of the crest, then the scans were digitally h processed so that the space inside the crest, rather than the crest itself, could be imaged. The rendered version of the interior cavity revealed an extraordinary degree of complexity. Several parallel, narrow tubes looped tightly within the crest, creating the equivalent of a cluster of trombones! There is now little doubt that the crest cavities in animals like Parasaurolophus were capable of acting as resonators as part of their vocal system.
Soft tissues: hearts of stone?
In the late 1990s, a new partial skeleton of a medium-sized ornithopod was discovered in Late Cretaceous sandstones in South Dakota. Part of the skeleton was eroded away, but what remained was extraordinarily well preserved, with evidence of some of the soft tissues, such as cartilage, which are normally lost during fossilization, still visible. During initial preparation of the specimen, a large ferruginous (iron-rich) nodule was discovered in the centre of the chest. Intrigued by this structure, the researchers obtained permission to CT scan a major part of the skeleton using a large veterinary hospital scanner. The results from these scans were intriguing.
The ferruginous nodule appeared to have distinctive anatomical features, and there appeared to be associated nearby structures. The researchers interpreted these as indicating that the heart and some associated blood vessels had been preserved within the nodule. The nodule appeared to show two chambers (interpreted by the researchers as representing the original ventricles of the heart); a little above these was a curved, tube-like structure that they interpret as an aorta (one of the main arteries leaving the heart). On this basis, they went on to suggest that this showed that dinosaurs of this type had a very bird-like, fully divided heart, which M supported the increasing conviction that dinosaurs were generally ii highly active, aerobic animals (see Chapter 6).
As early as 1842, and the extraordinarily prophetic speculations of Richard Owen, it had been supposed that dinosaurs, crocodiles, and birds had a relatively efficient four-chambered (i.e. fully divided) heart. On that basis, this discovery is not so startling. What is astonishing is the thought that the general shape of the soft tissues of the heart of this particular dinosaur might have been preserved through some freak circumstance of fossilization.
Soft tissue preservation is known to occur under some exceptional conditions in the fossil record; these generally comprise a mixture of very fine sediments (muds and clays) that are capable of preserving the impressions of soft tissues. Also, soft tissues, or rather their chemically replaced remnants, can be preserved by chemical precipitation, usually in the absence of oxygen. Neither of these conditions apply to the ornithopod skeleton described above. The specimen was found in coarse sandstone, and under conditions that would have been oxygen-rich, so from a simple geochemical perspective, conditions would appear to be very unlikely to preserve soft tissues of any type.
Not surprisingly, the observations made by the researchers have been challenged. Ironstone nodules are commonly reported in these deposits and are frequently found associated with dinosaur bones. The sedimentary conditions, the chemical environment in which the structures might have been preserved, and the interpretation of all the supposedly heart-like features have been contested. At present, the status of this specimen is therefore uncertain, but whatever else is claimed, if these features are simply those of an ironstone nodule, then it is extraordinary that they are so heart-like.
Fake 'dinobirds': forensic paleobiology
In 1999 an article appeared in the National Geographic magazine o that highlighted the similarities between dinosaurs and birds that jjj had been revealed by the new discoveries made in Liaoning =
Province, China. It brought to light another new and exciting ¡5
a specimen that was namedArchaeoraptor, and was represented by h a nearly complete skeleton that seemed as good an intermediate 'dinobird' as one could imagine. The animal had very bird-like wings and chest bones, yet retained rather theropod-like head, legs, and the long stiffened tail.
The specimen was initially feted by National Geographic through public events. However, the specimen soon became dogged by controversy. It had been bought by a museum based in Utah at a fossil fair in Tucson, Arizona, even though it evidently came from China. This is very unusual because the Chinese government regards all fossils of scientific value as the property of China.
The specimen came to be regarded with suspicion by the scientific community: the front half of the body was almost too bird-like compared to the theropod-like legs and tail. The surface of limestone upon which this specimen was preserved was also
A. X-Ray image of the fossil
Map Key Bones associated bird bones unverifiable 'attached' bones
Associated pieces associated pieces w lying in natural position on its slab of rock
Model Key Relative density bone slab materials air
37. The fake 'Archaeoraptor
B. Map of slab face
, . 'Right' and 'left' tibia/fibula, 9a-" '— (piece and counter-piece) 10
(piece and counter-piece)
7a-b C bone fragment pieces
13a-b a-hh shims dromaeosaur tail pieces unusual, it consisted of a crazy-paving-like series of small slabs held together by a lot of filler (see Figure 37). Within a relatively short period of time, it was declared to be a probable fake -possibly manufactured to order from assorted spare parts collected in Liaoning. Amid the general air of concern, the curator of the Utah museum contacted two palaeontologists who had worked on these Chinese forms, Philip Currie of the Royal Tyrell Museum, Alberta, and Xu Xing of Beijing, China; and Tim Rowe was contacted at Texas to see if he could CT scan and verify the nature of this fossil.
By an amazing coincidence, Xu, on returning to China, located a piece of rock from Liaoning containing most of a dromaeosaur theropod. After studying this specimen, he became convinced that the tail of this fossil was the matching counterpart to the one he had recently seen on Archaeoraptor. Returning to Washington, and the M office of National Geographic, Xu was able to place his recently | discovered fossil against the Archaeoraptor specimen and £ demonstrate that the original Archaeoraptor block was without doubt a composite consisting of at least two different animals (the front half being part of a genuine bird, the back half being that of a dromaeosaur theropod).
Alerted to this, Rowe was able to study the CT scans that he made of the original Archaeoraptor slab in detail. CT cannot distinguish genuine from fraudulent fossils. However, the accuracy of the three-dimensional images of each portion of the slab allowed precise comparison of each piece of the specimen. It became clear that a partial bird fossil formed the main part of the slab, to this had been added the leg bones and feet of a theropod dinosaur. Rowe and his colleagues were able to show that only one leg bone and foot had been used. In this instance, the part and counterpart had been split apart to make a pair of legs and feet! Finally, the tail of the theropod had been added; and to complete the 'picture', additional pieces of paving and filler were added to create a more visually pleasing rectangular ensemble.
These dramatic revelations have had no effect whatever on the debate concerning dinosaur-bird relationships. What they do point to are some unfortunate facts. In China, where poorly paid labourers have helped to excavate some truly wondrous fossils, they have clearly developed a good knowledge of anatomy and an understanding of the sorts of creatures that scientists are looking for. These workers also realize that there is a thriving market in such fossils, which will bring them far better financial rewards if they can sell them to dealers outside China.
Dinosaur mechanics: how Allosaurus fed
Computed tomography has clearly proved to be a very valuable aid to palaeobiological investigations because it has this ability to see inside objects in an almost magical way. Some technologically innovative ways of using CT imaging have been developed by Emily g Rayfield and colleagues, at the University of Cambridge. Using CT jjj images, sophisticated computer software, and a great deal of =
biological and palaeobiological information, it has proved possible ¡5
a to investigate how dinosaurs may have functioned as living h creatures.
As with the case of Tyrannosaurus, we know in very general terms that Allosaurus (Figure 31) was a predatory creature and probably fed on a range of prey living in Late Jurassic times. Sometimes tooth marks or scratches may be found on fossil bones and these can be quite literally lined up against the teeth in the jaws of an allosaur as a form of 'proof of the guilty party. But what does such evidence tell us? The answer is: not as much as we might like. We cannot be sure if the tooth marks were left by a scavenger feeding off an already dead animal, or whether the animal that left the tooth marks was the real killer; equally, we cannot tell what style of predator an allosaur might have been: did it run down its prey after a long chase, or did it lurk and pounce? Did it have a devastating bone-crushing bite, or was it more of a cut and slasher?
I 38. Finite-element modelled image of an Allosaurus skull derived from jj a CT scan c
I 38. Finite-element modelled image of an Allosaurus skull derived from jj a CT scan c
Rayfield was able to obtain CT scan data created from an exceptionally well-preserved skull of the Late Jurassic theropod Allosaurus. High-resolution scans of the skull were used to create a very detailed three-dimensional image of the entire skull. However, rather than simply creating a beautiful hologram-like representation of the skull, Rayfield converted the image data into a three-dimensional 'mesh'. The mesh consisted of a series of point coordinates (rather like the coordinates on a topographic map), each point was linked to its immediate neighbours by short 'elements'. This created what in engineering terms is known as a finite element map of the entire skull (Figure 38): nothing quite as complicated as this had ever been attempted before.
The remarkable property of this type of model is that with the appropriate computer and software it is possible to record, on the finite element map, the material properties of the skull bones, for example the strength of skull bone, of tooth enamel, or of cartilage on the joints between bones. In this way, each 'element' can be prompted to behave as though it were a piece of real skull, and each element is linked to its neighbours as an integrated unit, as it would be in life.
Having mapped the virtual skull of this dinosaur, it was then necessary to work out how powerful its jaw muscles were in life. Using clay, Rayfield was able to quite literally model the jaw muscles of this dinosaur. Once she had done this, she was able to calculate from their dimensions - their length, girth, and angle of attachment to the jaw bones - the amount of force that they could generate. To ensure that these calculations were as realistic as possible, two sets of force estimates were generated: one based on the view that dinosaurs like this one had a rather crocodile-like (ectotherm) physiology, the other assumed an avian/mammalian (endotherm) physiology. d
Using these sets of data, it was then possible to superimpose these = forces on the finite element model of the Allosaurus skull and ¡5
a quite literally 'test' how the skull would respond to maximum bite h forces, and how these would be distributed within the skull. The experiments were intended to probe the construction and shape of the skull, and the way it responded to stresses associated with feeding.
What emerged was fascinating. The skull was extraordinarily strong (despite all the large holes over its surface that might be thought to have weakened it significantly). In fact, the holes proved to be an important part of the strength of the skull. When the virtual skull was tested until it began to 'yield' (that is to say, it was subjected to forces that were beginning to fracture its bones), it was found to be capable of withstanding up to 24 times the force that the jaw muscles could exert when they were biting as hard as 'allosaurianly' possible.
What became obvious from this experimentation was that the allosaur skull was hugely over-engineered. Natural selection usually provides a 'safety factor' in the design of most skeletal features: a sort of trade-off between the amount of energy and materials needed to build that part of the skeleton and its overall strength under normal conditions of life. That 'safety factor' varies, but is generally in the range of 2-5 times the forces normally experienced during normal life activities. To have the skull of Allosaurus built with a 'safety factor' of 24 seemed ludicrous. Re-examination of the skull, and a rethink about its potential methods of feeding, led to the following realization: the lower jaw was actually quite 'weak' in the way it was constructed, so the animal probably did have a genuinely weak bite, compared to its overall skull strength. This suggested that the skull was constructed to withstand very large forces (in excess of 5 tonnes) for other reasons. The most obvious was that the skull may have been used as the principal attack weapon - as a chopper. These animals may well have lunged at their M prey with the jaws opened very wide, and then slammed their head | downward against their prey in a devastating, slashing blow. With £ the weight of the body behind this movement, and the resistance of the prey animal, the skull would need to be capable of withstanding short-term, but extremely high, loads.
Once the prey had been subdued following the first attack, the jaws could then be used to bite off pieces of flesh in the conventional way, but this might reasonably have been aided by using the legs and body to assist with tugs at resistant pieces of meat, again loading the skull quite highly through forces generated by the neck, back, and leg muscles.
In this particular analysis, it has been possible to gain an idea of how feeding may have been achieved in allosaurs in ways that until a few years ago would have been unimaginable. Yet again, the interplay between new technologies and different branches of science (in this instance engineering design) can be used to probe palaeobiological problems and generate new and interesting observations.
I cannot finish this chapter without mentioning the Jurassic Park scenario: discovering dinosaur DNA, using modern biotechnology to reconstitute that DNA, and using this to bring the dinosaur back to life.
There have been sporadic reports of finding fragments of dinosaur DNA in the scientific literature over the past decade, and then using PCR (polymerase chain reaction) biotechnology to amplify the fragments so that they can be studied more easily. Unfortunately, for those who wish to believe in the Hollywood-style scenario, absolutely none of these reports have been verified, and in truth it is exceedingly unlikely that any genuine dinosaur DNA will ever be isolated from dinosaur bone. It is simply the case that DNA is a long and complex biomolecule which degrades over time in the absence g of the metabolic machinery that will maintain and repair it, as jjj occurs in living cells. The chances of any such material surviving = unaltered for over 65 million years while it is buried in the ground ¡5
(and subject there to all the contamination risks presented by h micro-organisms and other biological and chemical sources, and ground water) are effectively zero.
All reports of dino-DNA to date have proved to be records of contaminants. In fact the only reliable fossil DNA that has been identified is far more recent, and even these discoveries have been made possible because of unusual preservational conditions. For example, brown bear fossils whose remains are dated back to about 60,000 years have yielded short strings of mitochondrial DNA - but these fossils had been frozen in permafrost since the animals died, providing the best chance of reducing the rate of degradation of these molecules. Dinosaur remains are of course 1,000 times more ancient than those of arctic brown bears. Although it might be possible to identify some dinosaur-like genes in the DNA of living birds, regenerating a dinosaur is beyond the bounds of science.
One final, but extremely interesting, set of observations concerns the analysis of the appearance and chemical composition of the interior of some tyrannosaur bones from Montana. Mary Schweitzer and colleagues from North Carolina State University were given access to some remarkably well preserved T. rex bones collected by Jack Horner (the real-life model for 'Dr Alan Grant' in the film Jurassic Park). Detailed examination of the skeletal remains suggested that there had been minimal alteration of the internal structure of the long bones; indeed, so unaltered were they that the individual bones of the tyrannosaur had a density that was consistent with that of modern bones that had simply been left to dry.
Schweitzer was looking for ancient biomolecules, or at least the remnant chemical signals that they might have left behind. Having extracted material from the interior of the bones, this was powdered and subjected to a broad range of physical, chemical, and biological M analyses. The idea behind this approach was not only to have the | best chance of 'catching' some trace, but also to have a range of £ semi-independent support for the signal, if it emerged. The burden really is upon the researcher to find some positive proof of the presence of such biomolecules; the time elapsed since death and burial, and the overwhelming probability that any remnant of such molecules has been completely destroyed or flushed away, seem to be overwhelming. Nuclear magnetic resonance and electron spin resonance revealed the presence of molecular residues resembling haemoglobin (the primary chemical constituent of red blood cells); spectroscopic analysis and HPLC (high performance liquid chromatography) generated data that was also consistent with the presence of remnants of the haeme structure. Finally, the dinosaur bone tissues were flushed with solvents to extract any remaining protein fragments; this extract was then injected into laboratory rats to see if it would raise an immune response - and it did! The antiserum created by the rats reacted positively with purified avian and mammalian haemoglobins. From this set of analyses, it seems very probable that chemical remnants of dinosaurian haemoglobin compounds were preserved in these T. rex tissues.
Even more tantalizingly, when thin sections of portions of bone were examined microscopically, small, rounded microstructures could be identified in the vascular channels (blood vessels) within the bone. These microstructures were analysed and found to be notably iron-rich compared to the surrounding tissues (iron being a principal constituent of the haeme molecule). Also the size and general appearance was remarkably reminiscent of avian nucleated blood cells. Although these structures are not actual blood cells, they certainly seem to be the chemically altered 'ghosts' of the originals. Quite how these structures have survived in this state for 65 Ma is a considerable puzzle.
Schweitzer and her co-workers have also been able to identify (using immunological techniques similar to the one mentioned above) biomolecular remnants of the 'tough' proteins known as collagen (a major constituent of natural bone, as well as ligaments g and tendons) and keratin (the material that forms scales, feathers, jjj hair, and claws). =
Although these results have been treated with considerable h scepticism by the research community at large - and rightly so, for the reasons elaborated above - nevertheless, the range of scientific methodologies employed to support their conclusions, and the exemplary caution with which these observations were announced, represent a model of clarity and application of scientific methodologies in this field of palaeobiology.
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