The real difficulty, however, is that every tooth surface is covered with pits and scratches, and identifying which ones were made by food items, and which ones by damage, can be a challenge. Especially with fossil teeth, it can be next-to-impossible to be confident that the marks were not made on teeth as they rolled along a river bed, or were otherwise beaten about. It is more reliable to take a micron-scale surface scan of the grinding surface of the tooth using a computerized microscope. The scan is then analysed automatically to classify all surface markings into sets, and the software is ‘trained’ so that, over time, it sorts out and discards random biffs and scrapes acquired during fossilization processes, and concentrates on saving information on plausible feeding marks.
Such studies of dinosaur tooth wear are in their infancy, but a really neat piece of work by Pam Gill, Emily Rayfield and their team discriminated diets of two of the earliest mammals, Morganucodon and Kuehneotherium, which scuttled and squeaked around the feet of the (probably oblivious) dinosaurs of their day. These two mammals are known from teeth and jaws, and some skeletal parts, from the Early Jurassic of South Wales, and their spiky little teeth suggested that they were insect-eaters. By mapping the key characters of modern bats, and their known diets, into a morphospace, the investigators determined that Morganucodon ate hard-cuticled insects such as beetles, whereas Kuehneotherium ate softer insects. This discovery was confirmed by FEA evidence of the jaw function, which showed that Morganucodon had a more powerful bite than Kuehneotherium.
Morphospace showing tooth-wear characteristics among modern bats, and their different insect diets. By analysing fossils taxa such as Morganucodon and Kuehneotherium, their diets can be calibrated from the modern data.
There have been some studies on dinosaurian tooth wear, but they have been disputed. For example, the orientations of sets of scratches on the teeth in hadrosaurs were used to confirm the principal motions of jaw action, but older papers that sought to identify the precise diet from such scratches are now generally disavowed. A key challenge will be to identify appropriate modern analogues.
Dinosaur food webs
All species are interlocked in complex relations between predators and prey, and between species that compete for other resources. The food web, an example of which we saw for Wealden dinosaurs in Chapter 2, is a standard device used by ecologists to document these interactions, and to calculate the movement of energy from the sun as it is captured by green plants, which are then eaten by herbivores, which in turn are eaten by carnivores; and then all eventually die and rot away, releasing energy and carbon through detritus-eating insects and microbes. Food webs are usually shown as a sort of spider diagram that links predators and prey: fox eats rabbit eats grass. The top, or apex, predator (lion, killer whale, T. rex) is at the top of the diagram, and all arrows lead back to it. Dinosaur food webs can be very different from anything we see today.
In the example shown here, representing a particularly well-studied dinosaur fauna from the Late Cretaceous Adamantina Formation of Argentina, the top predators (number 1) are large theropods, including Carnotaurus, an abelisaurid with large head and short arms, carcharodontosaurids, and Megaraptor, a great name meaning ‘big hunter’. In places there are three, four, or five steps from the bottom of the diagram up to these giant predators. Bottom of the pile are a bunch of fishes, frogs, a turtle, and beetles. The beetles are eaten by mammals, birds, lizards, snakes, and the weird armoured crocodile Armadillosuchus. The fishes are eaten by another crocodile, Barreirosuchus.
The strangest thing about this food web is the dominance by crocodiles (all shown in black in the diagram). Some were like modern crocodiles, presumably lurking in and around the rivers, and feeding on fishes and land animals they could snatch from the river banks. But others were much more adapted to life on land, with long legs, moving upright like a dog or hyena, and with short snouts and variable teeth, reflecting their different dietary adaptations. Some were active predators, even hunting dinosaurs, although probably only snatching at juveniles and weaker, old specimens. Bizarrely, some of the Adamantina crocodilians were insect-eaters, and one or two even specialized on a diet of plants. When these plant-eaters were first identified, people doubted that such weird crocodilians would have been possible – but the teeth do not lie. There were nearly twenty species of crocodiles in the Adamantina, and this emphasizes how different the faunas of the past were – indeed, crocodilians had a much wider role than they now do, and in South America, many survived the end-Cretaceous mass extinction (see Chapter 9) and continued as important predators until flesh-eating mammals outcompeted them.
Food web of the Adamantina Formation, showing how crocodiles of all shapes and sizes (shown in black) dominated the scene.
In the Adamantina fauna, there were nine dinosaurs (three large theropods, one slender theropod, and five sauropods), but their diets cannot be differentiated, so they are bunched together in the diagram. In the future, careful studies of coprolites, tooth marks, or other evidence, such as isotopes, might help differentiate their diets. Isotopes of oxygen and nitrogen in the bones of ancient vertebrates can give evidence about diet, such as whether they were feeding on fish or tetrapods such as lizards or mammals, but such work is subtle – the isotopes are determined also by climatic conditions, and they can be altered by fossilization processes.
The beetle shown in this food web is a dung beetle, and these helpful creatures have been noted before in association with other dinosaur faunas. For example, Karen Chin, introduced earlier as the finder of the giant T. rex coprolite, described burrows made by dung beetles in dinosaur faeces from the Two Medicine Formation, in the Late Cretaceous of Montana. The burrows show that the dinosaurs had been eating conifer leaves, and that the dung beetles in turn burrowed through the excrement, feeding on remaining nutritious materials, and then buried it, just as their cousins do today. Chin noted at the end of her 1996 article, ‘This find also reveals a pathway through which fecal resources were recycled and suggests that scarabs evolved coprophagy through association with dinosaurs.’ Coprophagy is the eating of excrement. Her words proved prophetic, when genomic evidence was presented in 2016 that scarab beetles had indeed evolved by the Early Cretaceous, and they presumably had the time of their lives, delving in the tonnes of poop dropped every day by the burgeoning herbivorous dinosaurs.
Collapsing food webs
The dinosaur food web is a human construct, relying on our knowledge of how food webs work today, and rare observations from the fossils (coprolites, tooth marks, etc). However, once constructed, the food webs can be used in advanced computational studies. For example, there are mathematical methods now to determine the stability of an ecosystem by knocking out individual species and predicting how the food web would recover. Estimates are made of who eats whom, and the approximate biomass of each species (that is, the body mass of the individual multiplied by relative abundance). If an ecosystem is stable, you can knock out several species, and it will continue to function. If it is unstable, say after a major environmental crisis, the removal of a couple of species can cause the whole system to collapse.
As ecosystems became more and more complex through the Cretaceous, with the addition of flowering plants, new insect groups, and new groups of lizards, birds, and mammals to the existing dinosaur communities, they paradoxically became both more robust and more vulnerable. The close interactions between plants and herbivores and prey and predators gave added robustness to sections of the ecosystem, where species could be clipped out and others would evolve to fill their places without the whole system breaking down. However, overall, with a highly complex ecosystem, as exists today, and existed in the Cretaceous, a major environmental crisis could cause the whole structure to collapse like a house of cards.
New mathematical modelling tools allow ecologists to explore risk and vulnerability in modern natural systems, with an eye on human threats and conservation of course. Likewise, these methods can be applied to fossil examp
les, both before and after mass extinctions. An initial study by Peter Roopnarine and colleagues has shown how the very last dinosaur-dominated communities of the latest Cretaceous of North America had a so-called ‘lower collapse threshold’ than those that had existed before, meaning it took less of an environmental shove to cause them to collapse. This seems to have been related to two changes in the last 10 million years of dinosaur ecosystems, with an increase in local forms (those found only in a single locality or small region), and the loss of a number of larger herbivores, which sat at the centre of many connections in the food web. Without them, the latest Cretaceous ecosystems were more vulnerable than those that went before.
Niche division and specialization in feeding
A problem with the Adamantina food web was to discriminate among the five species of sauropods – did they all eat the same kinds of plant food, or did they somehow partition resources? This question has not yet been answered for the Adamantina ecosystem, but such studies have shed light on the diversity of herbivores in the Morrison Formation, as we shall see.
In modern ecosystems, animals typically specialize on parts of the available food resources, perhaps feeding on grass, leaves, fruits, or nuts, or perhaps feeding at different levels – close to the ground, at mid-height, or at tree level. Such specialization has advantages, so that each species evolves specific teeth or digestive systems to best cope with their particular food, and they avoid competition. Just as competition between individuals and between species is core to evolution and to ecology, avoiding competition is a normal response.
An interesting dinosaurian conundrum has been to understand the unusually high diversity of sauropods in the Morrison Formation of the American Midwest, home of the top predators Allosaurus and Ceratosaurus, and the plated herbivore Stegosaurus. Here, ten sauropods (Amphicoelias, Apatosaurus, Barosaurus, Brachiosaurus, Camarasaurus (see overleaf), Diplodocus (see pp. 210–11), Haplocanthosaurus, Kaateodocus, Supersaurus and Suuwassea) have been found and, although the Morrison Formation rocks span a period of about 10 million years, up to five of these sauropods shared the landscape at any one time. How are we to understand that they could live side by side? The assumption is that there was some division of labour or specialization, and neck length was noted as a clue. For example, Brachiosaurus had a very long neck and long forelimbs, so it likely could raise its head, giraffe-like, to feed at great heights, whereas the equally long-necked Diplodocus was more horizontally organized, and so probably fed on leaves at lower levels.
Genus:
Stegosaurus
Species:
stenops
Named by:
Othniel Marsh, 1887
Age:
Late Jurassic, 157–152 million years ago
Fossil location:
United States, Tanzania, Portugal
Classification:
Dinosauria: Ornithischia: Thyreophora: Stegosauria
Length:
9 m (30 ft)
Weight:
4.7 tonnes (10,364 lbs)
Little-known fact:
The tail spikes were used in defence, proved by a puncture in an Allosaurus vertebra that fits the spike perfectly.
Genus:
Camarasaurus
Species:
supremus
Named by:
Edward Cope, 1877
Age:
Late Jurassic, 157–152 million years ago
Fossil location:
United States, Tanzania
Classification:
Dinosauria: Saurischia: Sauropodomorpha: Camarasauridae
Length:
15 m (49 ft)
Weight:
18 tonnes (36,683 lbs)
Little-known fact:
One specimen of a pelvis of Camarasaurus shows evidence of gouging by the teeth of top predator Allosaurus.
In a functional study of this problem, David Button, one of Emily Rayfield’s team, explored jaw function in the two sauropods Camarasaurus and Diplodocus. He carried out FEA of their jaws and skulls, and found that Camarasaurus could exert and accommodate greater bite forces than Diplodocus, suggesting that it ate harder food items. This is supported by his analysis of stresses in the skull, in which Camarasaurus showed lower stress values than Diplodocus, confirming that the skull of the first was able to withstand larger forces (see pl. ix). Extending his study to a wider sample of thirty-five sauropod species, he found that he could classify each species into one or other of these functional categories according to their estimated bite force, robustness of the skull, and how the teeth interacted with each other. In his morphospace diagram, the two functional classes are spaced a long way apart, and the Camarasaurus species and their functional analogues occupy a remarkably tight area, indicating considerable similarities.
Genus:
Diplodocus
Species:
carnegii
Named by:
John Hatcher, 1901
Age:
Late Jurassic, 157–152 million years ago
Fossil location:
United States, Tanzania
Classification:
Dinosauria: Saurischia: Sauropodomorpha: Diplodocidae
Length:
25 m (82 ft)
Weight:
16 tonnes (35,274 lbs)
Little-known fact:
The species is named after multimillionaire Andrew Carnegie, who funded the excavations that found the first specimen.
Morphospace plot of different sauropods, reflecting the feeding habits of branch strippers (left) and chompers (right).
Button concluded that Camarasaurus was probably a generalized browser that fed on hard and even woody material, while Diplodocus specialized on softer, but abrasive, plant materials such as horsetails and ferns. Diplodocus has a goofy expression, with a cluster of forward-pointing pencil-shaped teeth concentrated at the front of its jaws, and the researchers posited that it might have been a branch-stripper, gripping a leafy branch in its mouth, and pulling back with the teeth clenched tightly shut, swallowing the leaves as they were stripped. Diplodocus had muscles at the back of the head and along the neck that might have enabled this strong backwards pull and twist. Camarasaurus had heavier, shorter teeth along the whole length of its jaws, and so it probably used a more normal approach to acquiring plant material by snipping and chomping bunches of branches and leaves, without separating leaves and woody material.
We have come a long way in our understanding of dinosaur feeding in the past twenty years. Emily Rayfield summarizes her take on this:
When I started my research, we had some information from the fossils, such as tooth shape, tooth marks, stomach stones, and coprolites. A few experts in biomechanics had suggested ways to model dinosaur jaws like levers, so you could make some basic calculations, but we now have integrated computational methods that allow much more complex – or realistic – questions to be asked.
In a 2018 TV programme about ichthyosaurs, the dolphin-shaped marine reptiles, David Attenborough, the host, asked Emily her opinion: ‘So this was the king of the Jurassic sea?’ ‘Or queen,’ came back Emily in a flash.
The new engineering approaches are all testable, so palaeontologists are no longer speculating about feeding in extinct animals. Smart new approaches in ecology, especially using food webs, are also beginning to help, but there is so much more to do. We can expect to see an integration of both approaches soon, with dinosaur food webs modelled with accurate data on feeding mechanics and diets on the one hand, and understanding of how robust these ecosystems are to outside environmental pressures on the other. Emily Rayfield’s main intention now is to work hard with her team in another direction, to integrate our understanding of feeding modes and styles with evolution, so we can document why some dinosaur groups were more successful than others, and how different adaptations for feeding might have fared through the Mesozoic.
Chapter 8
How Did They Move and Run?
&n
bsp; The study of dinosaur locomotion is a perfect example of how palaeobiology has shifted from speculation to science. Two pioneers drove this revolution, one an inspirational English professor with a long beard, the other an American professor who settled in England, but so far without sprouting a beard.
The first professor is the legendary late biomechanics expert, R. McNeill Alexander (1934–2016) from the University of Leeds. He led the field, as the author of papers on the biomechanics of everything from fishes to mammals, as well as numerous standard textbooks such as his Animal Mechanics, Functional Design in Fishes, and Locomotion of Animals, published through the 1960s to 2000s. His lectures were famous: with his extensive knowledge of the function of all animals from fleas to elephants, and with his lanky frame and flowing beard, he would mimic animals hopping, jumping, and flying. Alexander’s lilting Ulster accent, retained from his birth and schooling in Northern Ireland, added engaging colour to his speech. From time to time, Alexander made a foray into the world of dinosaurian palaeobiology, with suggestions about how to estimate their body mass using plastic models or how to calculate their running speed. His insights on dinosaur running speed changed the field; overnight, in 1976, palaeontologists were provided with a reliable formula that told them the speed, not just a guess.
Dinosaurs Rediscovered Page 18