The company used the sample from Celia’s ear, and they took adult body cells and fused them with egg cells from goats that had had their nuclei removed. The ibex–goat embryos were then transferred into domestic goats, which acted as surrogate mothers. In 2009, after several years of failed attempts, the first two Pyrenean ibex kids were born to their goat mothers – but sadly they died soon after birth. Further cloning attempts were made in 2014, and if future cloned young survive, then an extinct species will have been resurrected.
There are many other cloning projects working with the genetic material of recently extinct mammals, a new science called de-extinction or resurrection science. Targets include not only the Pyrenean ibex, but also the thylacine or Tasmanian tiger, the aurochs, a European wild cow, the quagga, an extinct species of zebra, and the passenger pigeon. More daringly, several teams are talking about cloning and de-extincting the mammoth, using a female Asian elephant as the surrogate mother. So far, though, it’s all talk and no results.
The extinct Pyrenean ibex, drawn when the species had not yet died out.
How to clone a mammoth.
If it is so hard to succeed with multi-species surrogacy when there are really close living relatives (such as the Pyrenean ibex and goat), how much more technically difficult it is going to be to cross species boundaries from Asian elephant to mammoth? And, as for dinosaurs, which species would serve best as their living surrogate? Of course, they laid eggs, like birds and crocodiles, so the mother would not have to carry the bizarre dinosaur embryo inside, but, as in Jurassic Park, the biotechnologists would have to engineer the dinosaur embryo and induce it to develop inside the egg of a living species. We’re a very long way away from seeing this happen.
Can we say anything about the dinosaurian genome?
The genome is all the genetic code contained in our cells. Molecular biologists talk about two aspects of the genome – its overall size (in other words, the total number of genes, those portions of the genome that have specific functions) and how it is organized in the chromosomes, the X- and Y-shaped structures inside the nucleus.
In terms of overall genome size, it seems that birds, theropods, and sauropodomorphs had small genomes, whereas ornithischian dinosaurs had much larger genomes. Nobody has ever seen the genome of a dinosaur, but the size of the genome has been shown to relate to the average size of cells. By measuring the size of cells within fossil bones, Chris Organ and colleagues were able to speculate about overall genome size. They suggested that the small genome size of those dinosaurs, and birds, was related to their warm-bloodedness, and especially the origin of flight in theropods and birds.
The chromosome organization in dinosaurs has also been reconstructed according to a 2018 paper. In this paper, molecular biologist Rebecca O’Connor and her team from the University of Kent mapped the DNA from different chromosomes of modern birds and reptiles, and looked for shared components. By comparing the complete genetic sequence of birds and turtles, they could be sure they were including all relatives of turtles and birds, and that includes dinosaurs.
The team used fluorescent labels, or ‘DNA probes’, to identify shared portions of the genetic sequences between turtles and birds, and then they could assume these had been present in the common ancestor of both turtles and birds – which would have been a reptile that lived over 300 million years ago, and well before the dinosaurs originated. But those shared features of the DNA of turtles and birds were almost certainly present in dinosaurs.
They concluded that most elements of the modern bird genetic code and its arrangement in 40 chromosome pairs were present in the ancestral form, and so this reorganization had occurred before the origin of dinosaurs, and likely was shared with them all. This is a remarkable discovery because it implies that some special features of the genetic code of birds had arisen much earlier than expected. For example, birds have the high number of 40 chromosome pairs (compared to 33 pairs in turtles and 23 pairs in humans), and the new evidence is that the multiplication of chromosomes happened early, and that dinosaurs all shared that change in genetic architecture.
Identifying ancestral gene sequences shared by all reptiles and birds provides a template of the minimal composition of the dinosaurian genome. The authors wisely say nothing, however, about whether this could ever be a basis for cloning dinosaurs. They do go so far as to say ‘that the overall genome organisation and evolution of dinosaur chromosomes…might have been a major contributing factor to their morphological disparity, physiology, high rates of morphological change and ultimate survival’. They also note that the apparently very early acquisition of bird-like chromosomes might correlate with the discovery of the early acquisition of many supposedly unique bird characters (including feathers, hollow bones, and the wishbone) by theropod dinosaurs.
The news about bringing dinosaurs back to life is not promising. The methods can all be identified, but DNA does not survive for long, and so there is currently no prospect of obtaining any dinosaurian DNA. Without the genetic code in the DNA, the whole Jurassic Park scenario collapses. Even if we could clone extinct animals – which we have so far failed to do – we still need that genetic code.
The pursuit, however, has not been fruitless. Some have perhaps allowed their enthusiasm to outpace the quality of their data in making announcements of DNA, blood cells, and other miraculous examples of preservation from the age of the dinosaurs, but these are tricky areas at present. Others are equally damning of our case for melanin and melanosomes. The glory of these fields of palaeobiology is that they are perfectly interdisciplinary, drawing in the expertise of molecular biologists, geneticists, and organic chemists. Long may the hunt for extraordinary fossils continue!
Chapter 6
From Baby to Giant
Dinosaurs were often huge – which is a conundrum in itself – but they started life from relatively small eggs, so they either had to grow up very fast or live for a very long time. The subjects of growth and size go to the heart of some key questions about dinosaurian palaeobiology, and they are topics that have given rise to quite wild speculation over the years.
Even if the dream of Jurassic Park may never be realized, we still know a great deal about the development of dinosaurs from egg to adult. Skeletons of dinosaurs show all growth stages, from embryos in the egg, through hatchlings and juveniles, to adults. This is what drew palaeontologist Greg Erickson, now a professor at Florida State University, into the subject. In his words:
When I began we didn’t know much about the basic biology of dinosaurs, such as how long they lived, how fast they grew, aspects of their physiology, reproduction, etc. My career goal was to develop methods to glean such information. I began by figuring out how fast they grew, a proxy for metabolic rates. At the time it was debated whether they grew slowly like cold-blooded reptiles, perhaps taking over a hundred years to mature, which frankly seemed unlikely to me, or much more rapidly like warm-blooded birds and mammals.
Although Erickson liked palaeontology and geology as a kid, he did not entertain a career in those sciences when he first entered college at the University of Washington in Seattle. In fact, he graduated still unsure of what he wanted to do. He ended up getting a degree in geology, and a palaeontologist invited him to participate in some expeditions, and encouraged him to pursue an academic career. After graduating he worked in a miserable job as a construction worker, which provided ample time for him to mull over what the professor had said. ‘Learning what I didn’t want to do showed me the way to what I did want to do – palaeontology.’
As a young professor, he recalls, ‘I was at the Field Museum in Chicago and noticed growth lines in the bones of Sue, the largest known T. rex, which had been purchased for 8.36 million dollars. This led me to ask, “Can I cut up your seemingly priceless dinosaur?” Fortunately, after much debate among the museum higher-ups, I got the approval, and that springboarded the work.’ Growth rings in the bone are key to understanding the age of a dinosaur skeleton, and
that enabled Erickson and others to draw up growth curves for dinosaurs, as we shall see.
Dinosaurs all hatched from eggs. Birds and crocodiles lay eggs with hard shells made from calcium carbonate, and it is no surprise that dinosaurs laid eggs as well. Fossil dinosaur eggs were first reported, not from North America or Mongolia, but from the Cretaceous of the south of France in 1859.
The egg of the sauropod Hypselosaurus found in France; the eggshell is shattered but repaired.
Jean-Jacques Pouech, a Roman Catholic priest, and head of Pamiers Seminary, was exploring the rocks in the foothills of the Pyrenees, and he found great plated fragments of shell covered with pustular, or regularly roughened, surfaces. He reported:
The most remarkable are eggshell fragments of very great dimensions. At first, I thought that they could be integumentary plates of reptiles, but their constant thickness between two perfectly parallel surfaces, their fibrous structure, normal to the surfaces, and especially their regular curvature, definitely suggest that they are enormous eggshells, at least four times the volume of ostrich eggs.
Pouech identified the French fossil eggs as those of giant birds.
Much more famous were the discoveries of dinosaur eggs and nests from the Cretaceous of Mongolia, in the 1920s. Roy Chapman Andrews, the explorer hired by the American Museum of Natural History (AMNH), led several large-scale expeditions to Mongolia, working from his base in war-torn Beijing, and driving northwards towards Ulaanbaatar in a cavalcade of black model-T Fords, and then off into the remotest deserts with hundreds of gallons of water, stacks of food, and rifles. His expeditions were really about going into the unknown, as Beijing was at that time over-run by warlords struggling to take power in China. This was the far-from-peaceful base that Chapman Andrews used to prepare equipment and supplies before driving 1,000 kilometres (620 miles) into the Gobi Desert. Despite the risks, on the first expedition, the team excavated dozens of dinosaur skeletons, as well as nests.
The most famous exhibit at the AMNH was of several small ceratopsian (horn-faced) dinosaurs, Protoceratops, clustered round their nests, and with the predatory dinosaur Oviraptor, which means ‘egg thief’, scouting around and threatening them. The nests were about 1 metre (3 feet) across and each contained twenty to twenty-five eggs, arranged in concentric circles. The eggs are long and cylindrical, and they were laid with their pointed end inwards, so making it easy for them to roll naturally into a circular array. When the first specimens were put on show in New York they attracted crowds, and the public loved the narrative of the humble herbivore Protoceratops trying to defend its nest from the wicked egg thief. But was this true?
Dinosaurs started so small, and yet some grew to be truly huge. This poses a number of interesting conundrums. By scaling to their adult size, dinosaur eggs should have been much larger than they were; and of course the largest dinosaurs, ten times the size of an elephant, just defy understanding – how could they be so huge when nothing on land today even approaches their dimensions? New studies suggest how dinosaurs achieved the impossible.
Genus:
Protoceratops
Species:
andrewsi:
Named by:
Walter Granger and William Gregory, 1923
Age:
Late Cretaceous, 84–72 million years ago
Fossil location:
Mongolia
Classification:
Dinosauria: Ornithischia: Ceratopsia: Protoceratopsidae
Length:
1.8 m (6 ft)
Weight:
83 kg (183 lbs)
Little-known fact:
Skulls of Protoceratops might be the first fossils ever seen, when ancient Greeks wandering over the Gobi Desert thought their skulls were the remains of dragons.
Why were dinosaur eggs and babies so small?
Were dinosaur eggs and babies unusually small? Certainly, by scaling to modern birds, the largest dinosaur eggs should have been about the size of a Smart car, maybe 2 metres (6½ feet) long, and yet the largest dinosaur eggs were only 60 centimetres (23½ inches) long and about 20 centimetres (8 inches) in diameter, so quite long and sausage-shaped. Even these eggs, reported from China in 2017, were comparatively large by dinosaur standards; dinosaur eggs rarely exceed the size of a rugby ball or American football, about 30 centimetres long. These giant eggs from China contained tiny bones, which identified the embryo inside as a relative of Oviraptor, the so-called ‘egg thief ’ of Mongolia. These were from a much larger relative, maybe 2 tonnes (4,410 pounds) in weight when fully grown, but only a couple of kilograms at birth.
The scaling is all wrong when you compare these dinosaurian examples with birds. The relationship isn’t exactly simple – it’s an exponential curve, and the proportion of egg mass to adult female body mass also changes. Small birds such as hummingbirds and tits have relatively enormous eggs, making up 20 per cent of the female body mass, whereas in larger birds such as gulls and ostriches, the proportion is 5 per cent or less. Even allowing for continuing relative decline in the proportion as adult body size increases, a 10-tonne dinosaur should lay an egg of, say, 2 per cent of adult body mass, so 200 kg; and a 50-tonne sauropod might have produced an egg of 500 kg (say 1 per cent of 50,000 kg). So, their eggs, perhaps weighing at most 2–3 kg (5–7 pounds), were far too small. But why?
It’s a combination of basic mechanics and energy saving. In terms of mechanics, the thickness of an eggshell is proportional to the volume of the egg – after all, the mineralized shell must be robust enough to prevent the egg from collapsing. A hen’s eggshell is a fraction of a millimetre thick, whereas an ostrich eggshell is 2–3 millimetres thick, and a hypothetical 500-kilogram (1,100-pound) dinosaur egg would have to have an eggshell several centimetres thick. This would be catastrophic for two reasons. Oxygen could not percolate in, nor carbon dioxide out, through such a thick crystalline structure, so the embryo would die; but also, when it came to hatching, the poor baby, as large as a pony, would struggle to bash its way out.
The energy-saving aspect of laying small eggs is part of the overall life strategy of dinosaurs. Ecologists often characterize animals according to whether they emphasize quantity or quality of their offspring. Those that focus on quantity simply produce as many eggs as they can, but do not invest much effort in them. A classic example is the cod fish, which produces more than a million eggs at a single sitting, seemingly an excellent way to fill the world’s oceans with cod. In fact, the eggs and babies are useful food for many predators, and only two or three survive to adulthood, but that is enough to keep the cod species alive (barring over-fishing). The quantity-focus life strategy seems wasteful in resources – all those eggs and babies are produced, but 99.9999 per cent end up as food for others.
By contrast, mammals, including humans, invest heavily in the care of their young, the quality strategy, and so they tend to produce fewer young at a time, and seek to ensure that they survive. However, this strategy is also wasteful in resources because the parents, or just the mother, devote a large portion of their lives to child care rather than their own survival.
Dinosaurs did not produce a million eggs at a time, but more typically three to five for some species, and fifteen to twenty for others, such as Oviraptor. This compares with modern birds, which have clutch sizes ranging from a single egg to eighteen, but with an average of three, as shown in a recent analysis of more than 5,000 species of living birds. The single-egg birds are mainly large sea birds such as albatrosses, shearwaters, and petrels, that struggle to feed their chick, and so could not rear more than one or two simultaneously. Large clutches, typically ranging from seven to eighteen eggs, are produced by temperate-climate species such as ducks, pheasants, and partridges, and they rear their young at specific times of the year when seasonal food supplies are rich.
So, which of these two parenting strategies did dinosaurs adopt? On the one hand, crocodiles and other living reptiles mainly adopt the quantity life strategy – lots of eggs tha
t they abandon after laying. Birds, on the other hand, are quality strategists, like mammals, laying usually modest numbers of eggs and caring for the young after they hatch. Dinosaurs seemingly did a bit of both, but tended towards the quantity strategy, laying a reasonable number of eggs, and then abandoning them to their fates. Importantly, the small size of the eggs contributed hugely to saving energy devoted to reproduction. This retention of reptilian behaviour is a major plank in theories for how dinosaurs could reach such large sizes, as we shall see.
What do we know about dinosaur embryos?
When palaeontologists first began collecting dinosaur eggs, the specimens were often simply broken pieces of eggshell. Even when palaeontologists found complete eggs, they did not think to look inside, even though the fact the egg was complete indicated it had not hatched, and so might very well contain an embryo (unless, of course, like our breakfast eggs, it had never been fertilized).
Later, some damaged dinosaur eggs showed hints of tiny bones inside, but these had to be exposed by cleaning away sandstone by laborious efforts under the microscope. But this careful work with a needle could still cause damage to the tiny, delicate bones, and it seemed the study of embryo skeletons was doomed.
Scanning has changed all that. As we saw earlier, CT scanners can produce remarkably detailed information about even tiny fossils buried in the rock, but not every lab has a CT scanner. A combination of classical preparation methods and CT scanning has revealed a lot about one group of dinosaur embryos. In 1976, James Kitching, world-famous fossil collector in South Africa, excavated a clutch of six dinosaur eggs and brought them back to the Bernard Price Palaeontological Institute in Johannesberg. There they sat for some time, before an international team began the research.
Dinosaurs Rediscovered Page 14