Fossil location:
United States, Tanzania, Portugal
Classification:
Dinosauria: Saurischia: Theropoda: Allosauridae
Length:
8.5 m (28 ft)
Weight:
2.5 tonnes (5,513 lbs)
Little-known fact:
One Allosaurus individual from Wyoming had nineteen broken bones, some of which had begun to heal, but others were infected – he suffered for six months before he died.
Emily Rayfield was motivated by the desire to develop new methods to shift the study of dinosaur feeding from speculation to testable science. As she says, ‘The basics of how dinosaurs fed had been known for 200 years ever since the first discoveries of dinosaurs, and not much had changed since then.’ For example, Megalosaurus from the Middle Jurassic of England, named in 1824, showed the same sharp, scimitar-like teeth as in Allosaurus, and from the start it had been identified as a meat-eater, from a comparison with modern crocodiles. On the other hand, Iguanodon from the Early Cretaceous of England, the second dinosaur ever to be named, had huge, blunt-ended teeth, which were compared with those of the modern iguana, a plant-eating lizard. In coming to their conclusions about diet, the early dinosaur scientists were using the classic principle of ‘analogy with living forms’ to make their functional interpretations. In other words, they were applying a common-sense assumption that conditions in the past were uniform and concordant with those today. We assume that teeth in the Jurassic and Cretaceous share functional characteristics with those today, even if we are looking at animals, such as dinosaurs, without obvious modern relatives that shared their lifestyles.
Emily Rayfield, the great innovator in engineering approaches to dinosaur feeding function, in her laboratory.
Emily Rayfield had this basic information, but she wanted to know more. Her solution was to apply an engineering method called finite element analysis (FEA) to her dinosaur question. The FEA method had been developed in the 1940s as a tool for engineers and architects to improve the efficiency of their structures. Instead of following medieval practice, and making buildings super-massive so they wouldn’t fall down, FEA was a way for architects to stress-test accurate models digitally before construction began. The trick is to make the 3D model and then to apply the correct material properties, such as how bendy the material is under tension and compression, how elastic it is, how it deforms under opposing forces, and its density.
During the next three years, Rayfield had to sweat to apply the FEA method to the skull of Allosaurus. ‘If I had failed to find a way to make the FEA software read in the details of the detailed structure of the dinosaur skull, I wouldn’t have had a thesis to present to my doctoral committee,’ she recalls. Nobody had done this before, and she was not a trained software engineer. Nonetheless, after a great deal of effort, forcing different software programs to talk to each other, it worked. She had figured out how to make a reliable engineering method tell us how dinosaur jaws worked.
Even getting the CT scans was a challenge. The skull of Allosaurus was in the Museum of the Rockies in Montana, and this 1-tonne (2,205-pound) specimen had to be moved by truck to the Deaconess Hospital in Bozeman, 3.2 kilometres (2 miles) away, where it was scanned. Then, rendering the 3D digital model and undistorting it to make an anatomically accurate model took more than a year. The image had to be passed from software package to software package, with the risk at every step of computer meltdown, or at least a string of error messages. Hardest of all was to read the model into the standard FEA software to complete the functional analysis. The 3D digital model of the skull had to be divided into geometric shapes, converting the skull into a kind of wire mesh model composed of numerous pyramids, called elements. Then each element was given material properties measured from the bone of modern animals, often pigs or cows because of the similarities in the internal structure of dinosaur and large mammal bone. Rayfield recalls:
This was a stressful time, but the method worked. After that, I concentrated on making it more efficient, and advances in computing mean it goes much faster, and it’s all a lot less risky. We combine our engineering studies with close observation of the fossils to find other clues about dinosaur function and behaviour and confirm our fossil models with studies on living animal bone function.
The methods she applied have led to remarkable new discoveries and astonishing precision in results, as we shall see.
Digital modelling and dinosaur bite force
We know FEA works because bridges and skyscrapers normally do not collapse, and aeroplanes on the whole manage to survive storms without falling to bits. So, the method presumably also works for dinosaurs. This is perhaps the nearest we can get to really testing aspects of the palaeobiology of dinosaurs. We are no longer guessing about how the bones functioned, but actually putting them to the test, using a digital model that is as near to the real thing as we can construct, and establishing hypotheses about function that are open to criticism and testing.
So, what was the maximum bite force of Allosaurus? Bite force is measured in newtons, with one newton equivalent to the force of holding a pencil. Tapping your boiled egg would take a force of about 35 newtons. Human bite forces are between 200 and 700 newtons, a lion is capable of 4,000 newtons, and the strongest bite force of all living animals is by the Great White shark, which can clamp down with a force of 18,000 newtons – thirty-six times the force humans can exert. Newtons of force can be converted to an equivalent mass, at roughly 9,800 newtons per tonne, so the Great White shark bite force is the same as applying a weight of about 1.8 tonnes. Rayfield’s study showed that Allosaurus had a bite force of 35,000 newtons, much greater than any living predator.
Tyrannosaurus rex was even larger than Allosaurus, so it may have had an even more deadly bite. In a smart piece of independent confirmation, palaeontologists have homed in on this question from two directions. The first was through mechanical force tests in the laboratory. A piece of Triceratops bone was found with a deep gash on the surface. The researchers made a cast of the tooth puncture, and found it was identical to the tip of a T. rex tooth that had been driven in for 3 centimetres (1¼ inches) of its overall length of four times that. (Note that 12 centimetres is the length of the crown, the exposed part of the tooth – the whole tooth, including its root, was the length and shape of a regular banana, but distinctly sharpened at the business end.) The researchers then made a model T. rex tooth, mounted it on a pressure indenting rig, and drove it into pieces of cow bone. To match the 3-centimetre tooth pit took a force of 13,400 newtons, equivalent to 1.4 tonnes of weight applied. Was this the maximum possible bite force that T. rex was capable of?
Emily Rayfield tested the range of forces T. rex could have exerted within the limits of the strength of its skull and jaws, and came up with a maximum value of 31,000 newtons for the sum of values for each tooth, similar to the bite force of Allosaurus. In a further study, using a different biomechanical computing approach called multi-body dynamic modelling, Karl Bates and Peter Falkingham estimated a range of bite force values from 35,000 to 57,000 newtons, or equivalent to the application of 3.6 to 5.8 tonnes of weight. This is the strongest bite force ever demonstrated in any animal living or extinct, far higher than that of the living Great White shark, and far higher than the value calculated by the puncture experiment with the Triceratops; but then the T. rex who made that bite clearly wasn’t really trying very hard. Importantly, all these different approaches give similar values, which suggests there’s a good chance they might be correct. Further, it allows us to answer the classic dino-geek question: could T. rex have bitten a car in half? The answer is a resounding ‘yes’.
In her comparison of three meat-eating theropods – the modestly sized Coelophysis from the Late Triassic, Allosaurus from the Late Jurassic, and Tyrannosaurus from the Late Cretaceous – Emily Rayfield showed they all employed different feeding methods (see pl. x). In Tyrannosaurus, peak stresses are in the snout, whereas in the
other two, peak stresses are located further back, above the eye socket. This shows that T. rex used a puncture-pull means of killing and feeding, snapping with the front of its jaws to kill its prey, and then pulling back to tear the flesh from the carcass held down by its great foot. The other two theropods were capable of a more powerful bite along a greater stretch of the jaws, and so were perhaps juggling the prey in their mouths and chomping it into bits before swallowing.
In a later study of the strange, long-snouted theropod Spinosaurus from the Early Cretaceous of North Africa, Rayfield and colleagues found that it functioned more like the skull of the modern gharial, or gavial (a slender-snouted fish-eating crocodile that patrols the River Ganges in northern India), than a crocodile or alligator, with their broad snouts. The researchers constructed 3D models of the snouts of all these animals, and then applied FEA. Crocodiles and alligators today employ twist-feeding, in which they sink their jaws into their prey, say a wildebeest snatched from the side of the river, drag it under the water, and then throw their body into great contortions. The rotation of the crocodile’s body is transmitted to the side of its hapless prey, and a great chunk of flesh is twisted off. Gharials feed on fish by snapping their long delicate jaws shut and catching them in a cage of slender teeth. This is what spinosaurs did, and their bony palates acted to prevent their long snout from bending up and down, whereas in crocodiles and alligators the palate acts to strengthen the snout against twisting from side to side.
Establishing the function of the snout of the spinosaurid Baryonyx (D) with crocodile (A), gharial (B) and Allosaurus (C).
Fossil evidence for dinosaur diets
Tooth shape tells us whether a dinosaur was a herbivore or a carnivore. There are other kinds of fossil clues to diet, however. For example, palaeontologists look with sharp eyes for bite marks on bones, and dozens of examples have been published. Some of the biters can be identified rather precisely. If a T. rex bit into the bones of another dinosaur, it sometimes left scratch marks in parallel rows, and their spacing can be measured to test whether it matches the typical tooth spacing along a T. rex jaw.
There are also reports of stomach contents and even fossil excrement. The evidence of stomach contents can be contentious, as the discoverer has to convince others that the bones or stones or twigs inside the dinosaur rib cage really came from its stomach, and weren’t simply washed in later by sedimentary processes after the animal died. On reconsideration of the evidence, it turns out that it was rare for the giant sauropods to swallow such stones, but small theropods related to birds frequently did. Modern birds, especially those that feed on tough plants, often swallow masses of small stones (gastroliths), which then reside in the crop, an expansion of the gastric system located above the stomach, and essentially take over the role of teeth in reducing the food to digestible bits. Mammals, of course, chew their food, and so few need gastroliths.
The metre-long coprolite of T. rex.
And then we come to coprolites, fossil faeces. The theologian and palaeontologist William Buckland, being English and privately educated, was fascinated by poop of all sorts. He was the first to describe coprolites formally, and he pictured a Jurassic ocean full of all the wonderful new marine reptiles that had been found by the famed fossil collector Mary Anning around 1820 on the Dorset coast – with each one shedding droppings into the ocean. Now, there are hundreds of reports of dinosaur coprolites, most of them probably accurate, but it can be hard to identify the culprit. One spectacular specimen – the granddaddy of all coprolites – was reported by Karen Chin in 1998. It’s a 44-centimetre-long (17-inch) behemoth containing numerous bones of unidentified dinosaurs. As Chin said at the time, ‘We’re pretty sure this was dropped by Tyrannosaurus, but it can be tough to identify the poopetrator.’
Looking at teeth and coprolites is one thing, but such specimen-based studies do not help us to understand how dinosaurs actually fed. There are many more questions than simply, did they eat plants or flesh? For example, it would be good to understand whether particular herbivores fed on high or low plants, and whether they snipped or tore the food off, crushed or shredded it, and how much they ate every day. For carnivores, it would be good to know if they hunted actively or scavenged from dead carcasses, and whether they snapped at the flesh or sank their jaws in and twisted, as crocodiles do. Emily Rayfield’s FEA work can resolve some of these questions, but the fundamental construction of the teeth themselves can be important.
Tooth engineering and plant-eating
So far, we have assumed that teeth are just teeth, but in fact they can be highly complex and exquisitely engineered tools. In our teeth, we have the classic three-part construction, a thin crystalline layer of protective enamel on the outside, and dentine, which forms most of the tooth and contains fine canals that connect to the nerves and blood vessels inside the pulp cavity. When the enamel is dissolved by bad food, as most of us can attest, our teeth become very sensitive and must be capped or repaired. If too much of the enamel and dentine is damaged, the whole tooth has to be removed, and this is not a trivial operation in humans because our teeth sit in deep sockets and they are held tight by cement.
Also, we generally want to hang onto our teeth because we only ever have two sets, our baby teeth and our adult teeth. In mammals, including humans, the teeth are replaced just once, when we are young, and we never grow any more. In fish and reptiles, on the other hand, the teeth are replaced dozens of times throughout their lives. If mammals had not reduced their tooth replacement to once only, we could eat what we liked, and there would be no need for brushing or flossing – or dentists, for that matter.
Dinosaur teeth can be enormously common as fossils. I remember vividly, as do many other palaeontologists, walking along the edges of the Sahara Desert, and picking up the large teeth of two predatory dinosaurs, Spinosaurus and Carcharodontosaurus, each about the length of the palm of your hand, and wondering at the sheer numbers of teeth those animals shed. This is like modern sharks, which are casting their worn teeth all the time, and predatory dinosaurs probably shed hundreds of teeth, getting through twenty or thirty full replacements in their lifetimes. In the dinosaur jaw, teeth line up below the socket, and new ones are always pushing the old ones out, sometimes even before they are worn away. This, though, is a part of the risk for an active predator, like a theropod or a shark – when Allosaurus was twisting and turning to subdue its prey, teeth got bent and torn from the mouth. Because new teeth grow into place sporadically, the jawline of a shark or dinosaur may be quite snaggly, with half the teeth more or less erupted from the gums, and others just poking through randomly along the jaws.
The most successful dinosaurs in terms of sheer numbers of individuals were the plant-eating hadrosaurs, commonly called duck-billed dinosaurs, because their long, horse-like skulls expanded at the front into a broad toothless structure, like a duck’s bill. Hadrosaurs have been called the ‘sheep of the Cretaceous’, and in places, especially in North America and Mongolia, collectors often find hundreds of specimens together. Hadrosaurs had a remarkably standard skeleton and skull, but showed great diversity in their extraordinary head crests, different in each species. It’s their teeth, however, that seem to have made them so successful. The huge success of hadrosaurs is hard to understand because stomach contents and coprolites show they were mainly eating conifers, and the tough twigs and needles of conifers seem pretty uncompromising as a food choice.
In a detailed study in 2012, dinosaur palaeobiologist Greg Erickson showed that hadrosaur teeth comprised six different tissues that worked together to make them remarkably effective and durable (see pl. xviii). Hadrosaurs were already famous for their huge numbers of teeth – as many as 2,000 altogether. Most of these were replacement teeth, lining up on the inside of the jawbone beneath the twenty to thirty functional teeth on each jaw margin. Their teeth were arranged in a straight line along each jaw, and tooth-to-tooth grinding kept them sharp. But it was the numerous different hard tissues
that came as a surprise – just as in modern bison and elephants, the dental tissues were folded intricately so the harder enamel had maximum effect in tearing tough vegetation. Other tissues include forms of dentine and tooth cement, and giant filled tubules branching from the pulp cavity.
As the teeth ground each other down, while chopping tough conifer leaves, the six tooth tissues formed vertical structures that ground and chattered across each other, like a carpenter’s rasp file. Erickson performed hardness and wear experiments on the fossil teeth and found the fossilized hard tissues responded as if they were fresh, providing values comparable with the most efficient grinding-toothed mammals today.
Hadrosaurs were successful even though – or maybe because – they tackled tough kinds of vegetation that, perhaps, other plant-eating dinosaurs could not manage. In effect they had bionic teeth, built like a steel rasp, and endless tooth replacement, so they could afford to let their teeth wear down quickly and then shed them. With such levels of wear, it would surely be useful if there were a smart way to divine the diet of dinosaurs by inspecting microscopic tooth wear patterns.
The unique teeth of hadrosaurs, showing how many teeth these dinosaurs had (B, C), and their multiple folds like those of a bison or horse.
Microwear on teeth can reveal diet
Palaeoanthropologists were the first to interpret microscopic pits and grooves on the teeth of early humans as clues to their diets, whether soft or tough plant materials, meat, or a mixture of both. In one of the textbooks from the 1970s, the instructor says that comparisons should be made between the fossil teeth and tooth wear in living animals. Indeed, experiments are described in which monkeys and apes are fed varied diets – maybe one eating cabbage, another eating grains, and another eating fruit – for a few weeks, and then their teeth are examined. In these enlightened times, we do not kill the animals, of course, but grip them between our knees, hold the mouth open, and insert some moulding compound to snatch an impression from their molar. In the textbook photographs, the monkey, held as in a vice between the anthropologist’s knees, looks furious, and I wonder how exact the measurements could have been.
Dinosaurs Rediscovered Page 17
