Diane Scott, fossil preparator at the University of Toronto in Canada, did some delicate work with the needle to clear the rock, grain by grain, from one of the South African embryos. The foetus was confirmed as the young of Massospondylus (see overleaf), the most abundant plant-eater of its day, and 5 metres (16½ feet) long as an adult. The embryo is curled up inside the egg, with the head and body clearly visible, arms and legs tucked neatly below, and the tail curling round the back. The bones of the skull are disarticulated, which is not unexpected because bones are not fused together in such young individuals. In fact, this is true of all babies, and parents know that their children have a fontanelle, or gap, between the frontal and parietal bones on top of the head when they are very young. In the case of the South African Massospondylus embryo, the dinosaur head is 1 centimetre long, making the baby only 15 centimetres (6 inches) long in all, representing 3 per cent of adult length at birth. By contrast, human babies are 25–30 per cent of adult length at birth, and 50 per cent of adult height at two years, the famous measuring point most parents know about (see pls xvi, xvii).
The Massospondylus embryo highlights some interesting facts about growth. Just as with humans, the head is relatively large, the eyes are also large (this makes babies look cute to their parents), and the neck and tail are unusually short. As Massospondylus grew, its head and eyes grew larger more slowly than the rest of the body. On the other hand, the neck and tail sprouted in length much faster than the growth in length of the torso. Finally, the arms and legs were already strongly built in the embryo – it looks as if these little dinosaurs might have been ready to run the minute they were born, just like a baby deer or a calf today. Such offspring are up and ready to run within minutes of birth, whereas human babies have short and weak arms and legs, and they cannot support their weight for many months.
The embryo of Massospondylus curled up neatly inside its egg.
Genus:
Massospondylus
Species:
carinatus
Named by:
Richard Owen, 1854
Age:
Early Jurassic, 201–191 million years ago
Fossil location:
South Africa, Zimbabwe
Classification:
Dinosauria: Saurischia: Sauropodomorpha: Massospondylidae
Length:
4 m (13 ft)
Weight:
490 kg (1,080 lbs)
Little-known fact:
A close relative, Sarahsaurus, is known from Arizona in the United States.
The solution to the dilemma of delicate bones and needle damage was to scan the embryos. The team took one of the Massospondylus embryos to the European Synchrotron Radiation Facility (ESRF) for CT scanning. The buildings of the ESRF are on the banks of the Rivers Drac and Isère, on the edge of the city of Grenoble, dominated by a ring-shaped structure, 844 metres (2,769 feet) across, and housing a linear accelerator gun that produces the most powerful X-rays in the world. The beam is tapped off through forty-four beamlines and used for thousands of experiments in all branches of science each year. The team of palaeontologists made ultra-high-resolution scans of the tiny embryo.
The scan (see pl. xvi) shows a slightly flattened skull, but with all the component bones present, and highlighted in different, bright colours after processing. The scan confirms that this embryo had a full set of teeth, with rather long, sharp incisors at the front, and broader cheek teeth behind. This fully developed set of teeth suggests that the embryo was ready to feed the minute it hatched out – no evidence of parental care here, but a well-developed baby that hatched out and staggered towards the nearest plant food it could find on day one. This confirms the evidence from the stocky little limbs – this dinosaur could look after itself as soon as it hatched.
In 2018, two scientists from the Bernard Price Institute in Johannesburg published a full description of the CT-scanned skull1 of an adult Massospondylus (see pl. xvii), and this time they did not have to fly to Grenoble as the University of Witwatersrand had purchased its own CT scanner. The skull shows how the eye socket, though huge, is relatively smaller than in the baby, but the snout is about the same length (babies often have short snouts). We now have all the detail of baby and mother.
What about dinosaur nests and parental care?
Did dinosaurs care for their young? The fact that they laid lots of eggs and that their young could fend for themselves as soon as they hatched suggests they did not look after their offspring. The mother cod does not do much for her young, simply dumping the eggs and leaving, but other fish do protect their eggs. Some brood the young in their mouths, and the male seahorse famously has a brood pouch in which up to 2,000 miniature seahorses hatch and grow up. Crocodiles and turtles lay their eggs in safe spots on the beach or on the banks of rivers. We’ve all seen film of the mother sea turtle hauling herself laboriously up the beach, digging out a deep pit, laying her eggs, and covering them. Then she (and of course the father) are nowhere to be seen when the young hatch and scuttle down the beach.
Crocodiles and alligators construct a bowl-like nest in the mud on the shore of their river, lay between ten and forty-five eggs, and cover them with soil and leaves. They generally don’t disappear, however, and naturalists were amazed in the 1960s to confirm that crocodiles and alligators offer some parental care, belying their Victorian image as brutal creatures that were best observed in the form of handbags. The mother and father defend their riverbank territory from predators, including (in Florida) raccoons, which will gladly eat all the eggs in a nest. As they come near hatching, the young alligators chirrup to each other and to their parents.
Genus:
Maiasaura
Species:
carinatus
Named by:
Jack Horner and Robert Makela, 1979
Age:
Late Cretaceous, 80–75 million years ago
Fossil location:
Montana, USA
Classification:
Dinosauria: Ornithischia: Hadrosauridae
Length:
9 m (30 ft)
Weight:
4–5 tonnes (8,800–11,000 lbs)
Little-known fact:
Most focus is on the female Maiasaura, but males may have head-butted when seeking mates, using spiky crests in front of their eyes.
The young bash their way out of the egg using a special egg-tooth on the snout (as do birds, and so almost certainly dinosaurs had the egg-tooth too) and the parents will stick around to defend their young from predators. The mother often carries her hatchlings down to the water in her mouth, where they quickly learn to catch small prey such as snails, insects, tadpoles, minnows, and crayfish. She will protect them for two years, at which point she chases them off and prepares to produce a new egg clutch. When early naturalists saw crocodiles and alligators carrying their young to the water, they declared (of course) that these beastly reptiles were eating their own young.
Determining whether dinosaurs cared for their young or not is almost a political thing. The Victorian view would have been ‘not’. Dinosaurs were brute beasts to them, like the evil modern crocodiles, and so it was taken for granted that the ancient animals had no parental instincts at all. Then entered a revisionist view in the 1970s, linked with new views about dinosaurs as warm-blooded and active, and dinosaurs suddenly became loving and gentle. The heroine was Maiasaura, meaning ‘good mother reptile’, named from abundant remains in the Late Cretaceous of Montana that had been found in association with numerous nests and eggs. At the time, it was argued that whole gangs of Maiasaura mothers congregated to lay their eggs in nest mounds, and they sat there, chirping to each other, and spaced out enough to be sociable, but not so close that they would interfere with each other. The nesting sites showed stacks of nests, accumulated over many years, so the dinosaurs seemed to show nest-site fidelity, coming back year after year. Most importantly, these dinosaurian Earth mothers were said to have cared for their young, collec
ting succulent planty morsels for them, and hanging around the nest site as the babies took their first steps (see pl. xiv).
The original authors have stepped back from some of the more extreme interpretations. Levels of proof must be maintained, however attractive the idea of loving and laid-back dinosaurs, and some of the assumptions were hard to demonstrate with fossil evidence. Much more convincing is evidence from those so-called Protoceratops nests from Mongolia first reported by Roy Chapman Andrews in the 1920s. It turns out that the palaeontologists back in the 1920s had got it all wrong.
In 1993, in a second series of AMNH expeditions to Mongolia, Mark Norell and colleagues found further nests like those reported in the 1920s, but this time they examined the eggs in more detail. The palaeontologists found to their surprise that tiny bones inside some of the eggs belonged to a flesh-eating theropod dinosaur, not Protoceratops at all. It turns out that the ‘egg thief ’ Oviraptor had been sorely misjudged; it was hanging around the nests because they were its own nests.
Even more remarkable, Norell found a complete skeleton of an Oviraptor parent, apparently incubating the eggs. The presumed mother had her legs tucked underneath her body, running between two half circles of eggs, and her arms stretched round and back at her sides. In life, she was covered in feathers, and so she was clearly incubating her eggs in the ground nest, just as an ostrich does today. Presumably, she stepped into the middle of the nest, being careful not to crush any of her eggs, and then folded her legs while shuffling eggs out of the way, and then settled down, and flopped her feathered arms out over the egg rows on either side.
This is a great story in which further research has corrected an earlier misconception – not so much about whose eggs they really were, but more that the parents incubated their eggs. This is very bird-like behaviour, and it could not have been assumed in dinosaurs – they might very well have laid their eggs and covered them with soil and a compost of leaves, as crocodiles do, and then more or less abandoned them. Now we know that at least the small and medium-sized theropod dinosaurs, those that happen to be most closely related to birds, did incubate their eggs in a bird-like way.
The famous skeleton of a mother Oviraptor incubating her eggs.
Did they engage in further parental care? The well-developed teeth of Massospondylus suggested that the babies were self-sufficient on hatching, so speaking against parental care in those dinosaurs; and such well-developed, even slightly worn, teeth have been noted in other dinosaur embryos. But then, sometimes whole clusters of dinosaurs are found together, most famously in the case of a horn-faced plant-eater, the ceratopsian Psittacosaurus (see overleaf and pl. xv). Hundreds of clusters of juvenile Psittacosaurus have been found in the Early Cretaceous rocks of north China – but this may simply represent groups of young hanging out together for safety, and it’s not clear whether their parents were there to supervise the crèche, except in the case of one disputed specimen where an adult Psittacosaurus skull appears to have been attached to a cluster of twenty or so babies.
Genus:
Oviraptor
Species:
philoceratops
Named by:
Henry Osborn, 1924
Age:
Late Cretaceous, 76–72 million years ago
Fossil location:
Mongolia
Classification:
Dinosauria: Saurischia: Theropoda: Oviraptoridae
Length:
2 m (6½ ft)
Weight:
20–30 kg (44–66 lbs)
Little-known fact:
Oviraptor had a crest on its short snout, possibly coloured in life and used in display.
Genus:
Psittacosaurus
Species:
mongoliensis
Named by:
Henry Osborn, 1923
Age:
Early Cretaceous, 125–100 million years ago
Fossil location:
Mongolia, China
Classification:
Dinosauria: Ornithischia: Ceratopsia: Psittacosauridae
Length:
2 m (6½ ft)
Weight:
40 kg (88 lbs)
Little-known fact:
This is the most abundant of all dinosaurs, with thousands of specimens already found in northern China.
How fast did dinosaurs grow up?
If you start small and end up huge you must either grow very fast or live for a very long time. This was the conundrum that Greg Erickson decided to tackle early in his career. He has shown that dinosaurs generally grew to adult size really fast, and this is another aspect of how they could be so huge.
The evidence comes from growth rates in the bone. As we saw in Chapter 4, dinosaurian bone represents a midway stage between the bone of most modern reptiles and most modern mammals. In thin section, dinosaur bone can show all the details seen in a thin section of modern bone, so it’s possible to make direct comparisons and without any fear that details have been crushed or remodelled. In fact, dinosaur bone sections can be very mammal-like in showing high-energy open-weave bone structures with evidence of secondary remodelling as minerals were recycled. But, at the same time, in certain bones of the skeleton, very clear growth rings are seen. These, as in modern reptiles, and indeed in trees, are light and spacious when growth is rapid (usually summer), and tight and dark when growth is slow (usually winter), when life conditions are poor.
In a series of papers, Greg Erickson has explored the growth rates of many dinosaurs using observations of their growth rings. In a classic study of T. rex and its relatives, Erickson counted growth rings in bones from animals of all sizes. In one example, he counted to nineteen growth rings, and he was confident he had reached the endpoint because the outside of the bone was finished with some tightly packed bone layers, called the external fundamental system (EFS). (The true age at which the animal had died was twenty-eight years, when growth rings effaced by remodelling were accounted for.) Getting access to the bones took a long time. ‘Curators initially weren’t too enthusiastic about my requests to cut their treasured dinosaurs into pieces,’ he notes. ‘But things have loosened up a bit. Successes in ageing specimens by my research team and by others have made access much easier today than it was in the past.’ Over years, he accumulated enough examples to be able to plot the growth curves for T. rex and its close relatives.
Erickson found that all the tyrannosaur growth curves were S-shaped, with slow rates of growth for the first five years, then rapid increase in size till the ages of fourteen to eighteen, and a levelling off at that point. The final point of adulthood varies by species, from age thirteen to fifteen in the smaller tyrannosaurs Albertosaurus, Gorgosaurus, and Daspletosaurus to twenty to twenty-five years in Tyrannosaurus itself – this is when growth slows down. This suggests that T. rex, for example, grew from a 1- to 2-kilogram baby to a 6-tonne adult very fast, and it put on most weight between ages fourteen and eighteen, so it added about half a tonne per year during that period of sustained growth.
Graph showing the estimated growth curves of different specimens.
This is a contentious subject, and some critics argue that the growth rings are not always annual. For example, if food supplies are abundant and the winter is mild, perhaps, they argue, you would not get the dense, winter ring when growth normally slows. Or, on the other hand, a time of freak weather conditions when plants disappeared or the weather became stormy in summer might add an extra dense, slow-growth ring. Greg Erickson responds that:
…all of this might be true, but modern reptiles show pretty good matching of growth rings and age regardless of environmental conditions. Furthermore, reptiles raised under constant conditions or from non-seasonal environments still lay down single annual lines. This suggests the rings are primarily the result of annual developmental fluctuations in growth, rather than being caused by sporadic climatic fluctuations. We probably get it right for dinosaurs, most of the time.
He also notes th
at ‘in some cases we were able to cross-check multiple specimens of dinosaurs of the same size, and their ages estimated from the numbers of growth rings were always in the same ball park’.
Other researchers have applied the growth-ring method. For example, Qi Zhao, then my doctoral student, explored bone growth in his favourite dinosaur, Psittacosaurus. This is a plant-eating dinosaur, about 2 metres (6½ feet) long, that is very commonly found in the Early Cretaceous rocks of northern China, and many other parts of Asia. It has a short, blocky head, with a blunt nose and well-developed beak for chopping tough plants, and was bipedal as an adult, with strong arms and legs. The juveniles, on the other hand, were tiny, maybe 10–20 centimetres (4–8 inches) long, and quadrupedal. Qi used bone rings to work out the ages of each little baby in one of the juvenile crèche specimens (see pl. xv). Much to his surprise, he found that five of the six little chaps were only two years old, whereas a sixth was three years old – we usually assumed that such clusters of young would all have been the same age. So, these little dinosaurs, perhaps siblings, clustered together for protection from predators, and then, in this case, were swamped by ash fall, in the famous Chinese locality of Lujiatun in Liaoning province, sometimes called the ‘Chinese Pompeii’.
Dinosaurs Rediscovered Page 15