Dinosaurs Rediscovered
Page 16
Posture shift (opposite) and growth curve (above) in Psittacosaurus.
Another interesting discovery about Psittacosaurus was that it underwent a posture shift at about the age of three, from being quadrupedal as an infant to being bipedal as an adult. On measuring the lengths of key limb bones from a series of specimens, Qi matched these with their estimated ages. This analysis showed that the animal grew fast in its first four years, doubling in length each year, and then reached adult size by about the age of seven or eight. This modest-sized dinosaur was ready to breed by the age of six or seven. Like all dinosaurs, and most modern reptiles, it continued to grow more or less forever – this accounts for travellers’ tales of rare, absolutely huge, ancient crocodiles or snakes. Most animals die younger. By contrast, mammals and birds cease growing in height or length soon after sexual maturity.
In his studies of different dinosaurs, Martin Sander of the University of Bonn found the greatest age in the sauropod Janenschia, which took about fifty years to reach its full 20-tonne (44,100-pound) size. Despite earlier guesses, no bone histological evidence suggests any dinosaur reached an age of 100 years, and they were all breeding much earlier than that. This makes biological sense in terms of evolution and the need to breed as soon as reasonably possible.
So far, we have established that dinosaurs shared four unique features – bird-like respiration and efficient use of oxygen, warm-bloodedness because of their large size, a mixed quantity-quality reproductive mode (laying lots of eggs; small eggs; scant parental care), and relatively fast growth to adult size. These four characteristics seem to be smart ways for dinosaurs to save energy, but can they work together to explain in some way how dinosaurs could have achieved the truly colossal sizes that they did?
How could dinosaurs be so huge?
I am often asked ‘what is the point of palaeontology?’ I usually mumble something about origins and evolution, and wider cultural understanding of the history of life and Earth’s environments. One key reason, though, is that some ancient organisms broke all the rules. Biologists say that elephants are about as big as a land animal can be without collapsing under the weight of too much flesh, or starving to death when the climate changes. So, the dinosaurs, and sauropods in particular, are a fine example of the impossible being real – we can’t say that gravity was lower in the Jurassic, or that they spent their entire lives under water (even though some crackpots make these claims). So, these huge sauropods really existed and they were really too big to be true. How on Earth can we explain that?
The question has been solved thanks to the combined efforts of many palaeobiologists, focused by Martin Sander. He had a big idea, and he raised 5 million euros for a long-running research project from 2004–2015, and one that every school child might dream of – his project was titled ‘Biology of the sauropod dinosaurs: the evolution of gigantism’. Sander recruited twenty or more researchers, not just palaeontologists, but also experts on nutrition, botanists, and zoo keepers. He wanted to resolve once and for all just why the sauropods were so huge.
He had in mind the largest dinosaur of all time, Brachiosaurus (see overleaf) from the Late Jurassic of Tanzania, in east Africa, and the midwestern United States. Its skeleton was a staggering 26 metres (85 feet) long, equivalent to two regular coaches parked nose to tail, and it raised its head to as much as 9 metres (30 feet) above the ground, the height of a three-storey building. Unlike other sauropods, Brachiosaurus had extra-long front legs, which raised the front half of its body, somewhat like a giraffe, and the vertebrae of the neck show that the natural position of the neck was at about a 45-degree slope. In other sauropods such as Diplodocus and Camarasaurus, the neck was held more horizontal. So, the point of Sander’s study was to work out how these 40- to 50-tonne monsters could function.
I went to one of the consortium meetings in Bonn in 2011 and was fascinated to hear about experiments on human physiology in which American professors had recruited students to survive on a range of bizarre diets, such as nothing but burgers or cabbage for a month (experiments that probably would not be permitted today), and the zoo keepers who measure the inputs and outputs of elephants and other beasts in their charge. The zoo keepers reported that an elephant must eat up to 270 kilograms (600 pounds) of forage each day. As Sander noted, if sauropods had the same physiology as a modern elephant, they would require ten times as much, in other words 2.7 tonnes (6,000 pounds). That’s a pile of leaves as big as a passenger coach, each day. Further, noted the zoo keepers, rolling their eyes to the heavens, their elephants turn those 270 kilograms of plant food into 70 kilograms (over 150 pounds) of dung each day – that’s several dozen wheelbarrows-full.
Flow chart showing how the characters of sauropod dinosaurs allowed them to be so much larger than any mammal.
Sander wanted to know what food plants were available to sauropods in the Mesozoic, and how sauropod physiology differed from that of elephants. Of course, their bone histology indicates warm-bloodedness, but they were so huge, at 50 tonnes or so, that if they fed at the rate of an elephant, they could not have packed away enough plant food through their tiny heads and long necks. So, he wove together what we know about dinosaurs in general, and sauropods in particular, to develop an overview of their secret – here is how the sauropods, the largest animals of all time, apparently did the impossible.
It was a combination of many small offspring and small eggs but no parental care; small head and no chewing; and bird-like lungs, which processed oxygen pick-up more efficiently than reptile and mammal lungs. These characteristics allowed sauropods to achieve huge size for minimal food intake – probably as much as an elephant, or even less, for a body that was ten times as large. They achieved steady body temperature by being huge, not by eating lots and having complex inner furnaces, as elephants and humans do. They laid small eggs and abandoned them, unlike elephants and humans, who spend a huge amount of time and energy caring for one or two babies, which can exhaust the mother’s reserves of food. Martin Sander’s spider diagram explains it all in a very convincing way – this is how sauropods broke free from the constraints that limit elephant, and mammal, size.
Genus:
Brachiosaurus
Species:
altithorax
Named by:
Elmer Riggs, 1903
Age:
Late Jurassic, 157–152 million years ago
Fossil location:
United States, Tanzania
Classification:
Dinosauria: Saurischia: Sauropodomorpha: Brachiosauridae
Length:
26 m (85 ft)
Weight:
58 tonnes (127,868 lbs)
Little-known fact:
The skeleton of Brachiosaurus from Tanzania on show in Berlin’s Humboldt Museum is the largest dinosaur on show anywhere, standing 9 m (30 ft) tall.
Were there ever dwarf dinosaurs?
Having achieved huge sizes, why would dinosaurs become small? One lineage, the maniraptoran theropods, became smaller and smaller, and sprouted long arms to improve their adaptations to tree-dwelling and eventually flight (see Chapters 4 and 8). Their new mode of life, hopping about in the trees, can explain the move to small size. Here and there, some dinosaurs became small because they lived on islands. The most famous are the dwarf dinosaurs of Transylvania – which sounds a bit like a movie title, but is not. These dwarf dinosaurs did indeed live in the corner of Romania classically called Transylvania, and they were reported first by Baron Franz Nopcsa, who was an impoverished nobleman from what was then (at the end of the nineteenth century) part of the Austro-Hungarian empire.
I first did research in Romania in 1993, four years after that country had overthrown the Soviet-oriented government in a violent uprising – I was shown bullet holes on some of the buildings of Bucharest University. The Discovery Channel were keen to film a programme about Nopcsa, mainly because of his colourful life – he was not only a nobleman, but was also gay, and travelled Europ
e with his faithful secretary and lover, Bajazid Doda. Nopcsa was multilingual and spoke about his dinosaur work at conferences in England, France, and Germany, and had to sell fossil collections to remain financially solvent. He spied for both sides during the First World War, the Austro-Hungarian empire and Britain, worked with the Albanian partisans, and offered himself to be King of Albania. Eventually, in poverty and despair, he shot Doda and himself in 1933. This was deemed to be about enough for a thirty-minute film, but I kept insisting we needed some science, and that the dwarf dinosaurs were really something biologically important.
Nopcsa was the first to note that his Transylvanian dinosaurs were dwarfed, at a meeting in Vienna in 1912. He observed that the Transylvanian dinosaurs rarely exceeded 4 metres (13 feet) in length and the largest one, a sauropod later named Magyarosaurus dacus (see overleaf), was a puny 6 metres (20 feet) long compared to 15–20 metres (49–66 feet) for its closest relatives elsewhere. During the discussion following his paper, Othenio Abel, a great Austrian palaeobiologist, agreed and said the phenomenon was just like the dwarfing of elephants, hippopotamus, and deer on Mediterranean islands during the Ice Ages.
Between them, Nopcsa and Abel had nailed it. There have been many evolutionary explanations for the phenomenon, but it is surely mostly to do with the fact that islands support fewer species and have simpler ecosystems than comparable sections of the mainland. So, with fewer species, less food, and smaller range sizes, animals can adapt their sizes, diets, and habits, and large forms have to become smaller. The pygmy elephants of the last million years of the Mediterranean, on islands such as Malta, Sicily, and Sardinia, were only 50 centimetres to 1 metre tall (1½–3 feet) at the shoulder, compared to 4–5 metres (13–16½ feet) for an adult elephant today. Evidently, elephants, hippos, and other African mammals got across to these islands at times when Mediterranean sea levels were much lower than they are today because water was locked up in huge northern ice caps.
Baron Franz Nopcsa in the costume of an Albanian freedom-fighter.
The Transylvanian dwarf dinosaurs lived on Haţeg island, which measured 100–200 kilometres (62–124 miles) across, and was one of several large islands in the Late Cretaceous, when sea levels were very high and flooded most of the southern parts of Europe. Studies of the bone histology of three of the dwarf dinosaurs, the sauropod Magyarosaurus and the ornithopods Telmatosaurus and Zalmoxes, show that these were adults, not juveniles. They were all from one-third to one-half of the body length of their nearest relatives from mainland areas in Europe and North America.
Genus:
Magyarosaurus
Species:
dacus
Named by:
Friedrich von Huene, 1932
Age:
Late Cretaceous, 72–66 million years ago
Fossil location:
Romania
Classification:
Dinosauria: Saurischia: Sauropodomorpha: Titanosauridae
Length:
6 m (20 ft)
Weight:
0.75 tonnes (1,654 lbs)
Little-known fact:
Magyarosaurus is an ‘island dwarf’, much smaller than related sauropods, because it lived on the Haţeg island of Transylvania.
Palaeogeography of Europe in the Late Cretaceous, showing how southern and eastern Europe were divided into many islands because of high sea levels at the time (Haţeg island is marked by a black star).
Not only had they shrunk in size, but they seemed somehow to be ‘primitive’, dinosaurs whose closest relatives were known from mainland spots 20–30 million years older. Presumably their ancestors had become established on the coastal area and then cut off as sea levels rose. Then, while their nearest relatives on the mainland continued to evolve and change, the island forms carried on as they were, in less complex ecosystems, and perhaps not subject to the same competitive pressures.
So, mostly dinosaurs grew larger and larger through evolutionary time, except in the case of these rare island forms. It’s interesting to know that dinosaurs showed just the same adaptive capabilities as mammals, and they could become small when it was advantageous in evolutionary terms.
How the dinosaurs achieved giant size is one of the key conundrums in palaeontology. In fact, it has become almost a philosophical question to consider what was special about dinosaurs in general, and the giant sauropods in particular, that enabled them to be ten times as big as an elephant. The philosophical issue is that the dinosaurs stretch our understanding of what is possible in physiology and in evolution. The question also reminds us to keep a firm control on speculation and not to suggest, for example, that the Jurassic monsters lived underwater or that gravity was lower in the past.
Determining growth rates from close study of dinosaurian bone structure and growth rings has been a major advance. In a 2017 paper, Greg Erickson and colleagues applied growth-ring analysis to the teeth of embryo dinosaurs, and they found evidence that development in the egg was slow, lasting perhaps two to six months, at rates akin to modern reptiles rather than the much faster rates of modern birds, whose embryos develop from fertilization to hatching in eleven days to three months. Each of these steps in research knowledge is slow, and the work often painstaking. The trickier the work, the harsher the criticisms, but that is a manifestation of the self-correcting property of science.
What’s still to be discovered in this field? Greg Erickson reflects on what might come next:
The new generation of palaeobiologists have made great headway in establishing methods to study the life history of dinosaurs and bring these animals back to life. Reconstructing age-mass growth curves was pivotal. This allowed quantification for how these animals developed, leading the field from speculation into the realm of science. Growth curves enabled standardized direct comparisons of dinosaur growth to those of living animals, and with one another. This provided inroads to understanding myriad aspects of dinosaurian biology including links between growth and evolution, physiology, reproduction, population biology, and even the evolution of the modern bird characters. You name it! There are many more species to be studied and we need more cross-checks from those that have been studied to add statistical power. The advent of modern high-resolution imaging techniques, including synchrotron X-ray imaging, hold great promise for allowing rapid, non-destructive analyses of dinosaur growth. These will speed up our accumulation of knowledge and allow access to rare specimens that curators are still reluctant to provide for destructive sampling.
1The scan files have been made available, so anyone can laser-print a 3D Massospondylus skull if they wish: https://3dprint.com/200131/dinosaur-fossil-3d-scanning/.
Chapter 7
How Did Dinosaurs Eat?
Employing the software used to design skyscrapers to determine how dinosaur jaws worked might seem far-fetched. Yet our understanding of dinosaur feeding has been revolutionized by the application of an engineering design tool developed in the 1940s. The pioneer was Emily Rayfield, now a professor at the University of Bristol. She is a no-nonsense daughter of a Yorkshire pig-farmer, and brings that practical background to bear on her work: ‘Once I had to borrow a couple of pig skulls from my father for a research project testing our computer models of bone strength,’ she recalls. Rayfield is at the centre of a growing team of successful students she has mentored over the years in their engineering-based studies of dinosaur function.
When she began her doctorate in 1997 at the University of Cambridge, Rayfield was set the task of determining the feeding mechanics of Allosaurus (see overleaf), a large predator from the Late Jurassic Morrison Formation of North America. Allosaurus was known from many skeletons and skulls, and it was the top predator of its day, feeding on two-legged plant-eaters, as well as the iconic Stegosaurus, with its tiny head held low, the great arched back lined with a double row of bony plates along the midline, and its long tail that ended with four vertical spines.
The lower jaw of Megalosaurus, the first dinosaur to be
named, showing the sharp, knife-like teeth of a predator.
Allosaurus was a biped, some 8.5 metres (28 feet) long, and with massive hind legs for powerful trotting and short, but strong, arms with which it could manipulate its prey. It shared the role of top predator in the Morrison Formation with Ceratosaurus, a 5.7-metre-long (18¾-foot) biped that was remarkable for the heavy bony shelves and projections on top of its skull. Allosaurus had a high skull, with many openings for sensory organs and other structures, and strong struts between. Each jaw was lined with between fourteen and seventeen scimitar-like teeth, each 6 centimetres (2½ inches) long, pointed and curved, and with jagged edges front and back. This is a classic design in predatory dinosaurs, with the teeth curved back to hold the prey animal firm and make sure that if it struggles it is forced backwards down the throat.
The teeth of Iguanodon, the second dinosaur to be named, showing the blunt-edged, ridged teeth of a herbivore.
Genus:
Allosaurus
Species:
fragilis
Named by:
Othniel Marsh, 1877
Age:
Late Jurassic, 157–152 million years ago