In this topic, 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.
Dinosaur ichnology
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. )
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 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.
35. Parallel rows of tracks made by a group of sauropod dinosaurs as they travelled across a moist lowland plain
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.
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 original footprint on the surface. The spectre of ‘underprints’ means that some dinosaur tracks might be considerably over-represented in the fossil record if a single print can be replicated through numerous ‘underprints’.
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?
Coprolites
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 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 dinosaurian;as suspected, this specimen was most likely a large carnivore’s coprolite. Surveying the fauna known from the rocks in this area, the only creature that was large enough to have been able to pass a coprolite of these dimensions was the large theropod 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.
Dinosaur pathologies
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, Tyrannosaurus not only looked like a meat-eater, its faeces prove it, 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 preserved, but the lower part of the left leg was grossly distorted and shortened (Figure 36). Although the original cause of the subsequent infection could not be proved, it was suspected that the animal may have received a severe bite on the shin close to the knee 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.
36. Septic fossilized dinosaur shin bones have become grossly distorted
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.
Isotopes
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 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 (topic 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.
