|University of Bristol|
How big and how small?
Huge dinosaurs are often assumed to have laid giant eggs. In films they are
often depicted to be many times bigger than a human. However, there is a
restriction to the maximum size of the egg an animal can lay. As the animal
gets bigger, the eggshell becomes thicker. If an eggshell is too thick, the baby cannot hatch out.
The largest eggs come from sauropods, in particular the titanosaurs. These eggs are up to 18cm long and up to 5mm thick, able to contain 5.5
litres of fluid. A metre-long egg would be several centimetres thick,
making respiration and hatching near impossible. Imagine how thick an egg several times the size of a human would be!
One might ask therefore how a sauropod from an 18 cm diameter egg could grow to a 30 metre long animal weighing about 20 tonnes. There are two possible answers:
Other dinosaur eggs are often smaller. The smallest eggs are perhaps from the prosauropods. Eggs from south Africa are reported to be only 6.5 cm long and 5.5 cm wide. The shell is only 0.2/0.3 mm (0.02 cm!) thick. Theropod eggs are generally 10-15 cm long, and a couple of millimetres thick. Duckbills (hadrosaurs) also have eggs 10-12 cm long and 7.9 cm wide.
- The animal grows throughout its lifecycle, perhaps continuing to grow until
death. Many reptiles do a similar thing.
- The animal undergoes accelerated growth in 5-15 years. Growth patterns from dinosaur bone such as the 'duck-billed' dinosaur Maiasaura, indicate it reached adult size in 5 years. Elephants take about 15 years to reach sexual maturity. Maybe sauropods lied in between.
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