Dinosaur lifecycles: from go to woe

From birth to growth and death, the fossil record preserves fascinating hints about the lifecycle of a dinosaur.

Reproduction

Dinosaur sex is a lively topic of debate and wonderful images spring to mind. But, unfortunately, working out how they did it remains guesswork. Dinosaur passion cannot be preserved in stone.

Male or female?

It is difficult to determine the sex of a dinosaur skeleton. A variety of suggestions has been made based on differences between fossils of the same species. However, the evidence is ambiguous and there is much debate among scientists.

It is often suggested that the crests of hadrosaurs like Parasaurolophus show consistent size differences and that those with more prominent crests could be males. The fact that physical differences between sexes occur with many living animals suggests that this may be a possibility. Unfortunately, the fossil evidence is difficult to interpret.

Life begins

Dinosaur life started out in a hard-shelled egg. These came in a variety of shapes and sizes depending on the species, but all had shells with internal structures more similar to modern bird eggs than to those of any living reptile. Most were laid in nests of mud or hollows scooped in sand, although some sauropods appeared to lay their eggs in simple lines. Large nesting colonies have also been found, suggesting breeding was social for some species – just like many birds today.

Where have eggs been found?

Dinosaur eggs have been found at over 200 sites around the world, with important sites in the USA, China, France, Argentina and India. No eggs have yet been found in Australia.

Egg size

Aepyornis maximus is an extinct bird that laid some of the largest eggs of any known animal. Its egg is close to the maximum size for an egg. Bigger eggs require thicker shells so they don’t collapse. However, if the shell is too thick the hatchling can’t escape and if the egg is too large the embryo suffocates as not enough oxygen and carbon dioxide pass across the membrane.

Whose egg?

The vast majority of dinosaur eggs found are fossilised empty eggshells. This is not much to work with if you want to know what dinosaurs laid them. However, we can distinguish broad egg groups based on size, shape and shell characteristics. Shell characteristics found on fossils are used to identify egg groups. The egg's thickness and surface texture can be quite clear, but crystal structure and air-hole patterns can only be seen under a microscope. If more evidence is later recovered, these groups can be attributed to a family or species. Occasionally the task is made easier. Some eggs hold the remains of an embryo that can be identified to a particular species. A few extremely rare fossils even preserve an adult after it died on its nest.

Some examples of identified egg fossils include:

• Hadrosaur eggs from China
• Saltasaurus, a large titanosaur sauropod, is one of the few species whose eggs have been positively identified from well-preserved embryos.
• Gobipteryx was a primitive bird (and therefore a dinosaur) that lived alongside other Cretaceous dinosaurs. Fossilised embryos were discovered in Mongolia, allowing the eggs to be identified.
• An adult Oviraptor philoceratops found in Mongolia in 1995. It appeared to have died while incubating or protecting its eggs.

Nature or nurture?

Did dinosaurs adopt the ‘lay and leave’ strategy of many modern reptiles or were they like most birds in raising young? It appears that dinosaurs cared for their eggs and hatchlings in different ways, just like living animals. Some species probably left the young to fend for themselves while others used their bodies to incubate or protect their eggs. Many nested in rookeries and a few even practised communal creching with hatchlings – typical bird behaviours.

Maiasaura, whose name means ‘good mother lizard’, has one of the most complete records regarding eggs, embryos, nests and juveniles of any dinosaur. Evidence shows they used communal nesting grounds and tended their newly hatched young. The young also grew quickly, reaching about three metres in length after one year.

Growing up

Did dinosaurs grow quickly like most warm-blooded mammals and birds or slowly like cold-blooded reptiles?
Many dinosaurs, particularly large theropods, hadrosaurs and sauropods, probably grew very quickly during the early years of life and slowed as they reached adulthood. However, it appears they had unique growth patterns that were faster than living reptiles but slower than most living mammals or birds.

The evidence of growth

• Some dinosaurs had internal bone structures, such as blood-channels, that are characteristic of living animals that grow fast.
• Cross section of bones can show growth rings. These are layers of bone laid down during periods of interrupted or slowed growth – often due to cold seasons. Some scientists think these rings may reflect a yearly growth stage and that counting rings provides an approximate age for the individual.
• Comparing the same bone from different-sized individuals of the same species helps create an approximate growth series. Combining this with the approximate age determined from other means creates an age-versus-size growth curve. For example, a hadrosaur hatchling can be shown to have grown to over three metres within a year, based on fossil evidence.

Estimating weight

The weights of dinosaurs are estimated in two ways. One method uses the cross-sections of weight-bearing bones, usually limb bones, to work out how much weight they could support. The other measures the volume of liquid displaced when an accurate scale model of the dinosaur is put in water. Both methods are full of uncertainty, which explains why there is a range of weights for any given dinosaur.

Smallest and largest

The smallest:

• Epidexipteryx hui (China): 25 cm long (or 44.5 cm including its long tail feathers) and 0.16 kg
• Parvicursor remotus (Mongolia): 39 cm long and 0.16 kg
• Anchiornis huxleyi (China): 40 cm long and 0.25 kg

The largest: Titanosaurid sauropods

• Argentinosaurus huinculensis (Argentina): 35-40 m long and 80 tonnes
• Puertasaurus reuili (Argentina): 35-40 m long and 80 tonnes
• Maraapunisaurus fragillimus (USA): 35 m long and 70 tonnes
• Paralititan stromeri (Egypt): 32 m long and 65-80 tonnes

The tallest: Brachiosaurid sauropods

• Sauroposeidon proteles (USA): 17-18 m or taller
• Giraffatitan brancai (Tanzania): 12 m tall
• Brachiosaurus altithorax (USA): 12 m tall

The largest meat-eater:

• Mapusaurus rosaea (Argentina): 14 m long and 7–8 tonnes
• Giganotosaurus carolinii (Argentina): 13–14 m long and 7 tonnes
• Spinosaurus aegyptiacus (Egypt): 15 m long and 5-7 tonnes
• Tyrannosaurus rex (USA): 12–13 m long and 7 tonnes

What was the lifespan of dinosaurs?

In living animals, lifespan depends mainly on size and metabolism. For instance, reptiles with slow metabolisms tend to have longer life spans than warm-blooded birds and mammals of the same size. It is possible that sauropods reached 50-100 years, large theropods a bit less and smaller dinosaurs could live to about 10 or 20 years old.