Reading Practice 20

[1] Fossils give us a lot of evidence about the behaviour of extinct animals, reproduction and parental care. To discover dinosaur fossils, it’s important to find rocks of the right age. For example, in ancient rock exposures, we find only fossils of plants or marine organisms. In younger rocks, we find fossils of mammals, such as whales and horses. Palaeontologists can provide information on the exact age of rocks (225 to 65.6 million years old) that contain dinosaur fossils.

 

[2] Important clues of dinosaur behaviour include fossilized eggs and nests. There is evidence that dinosaurs took care of their babies after hatching from the discovery of skeletons of the theropod dinosaur Citipati. These skeletons were found in brooding positions which suggests that these dinosaurs were protecting their eggs. Bones of older babies in nests of the duckbill dinosaur Maiasaura were also discovered, along with a fossil of an adult Psittacosaurus preserved with the skeletons of tiny babies.

 

[3] Discovery of trackways suggests that some dinosaurs lived in family groups. These trackways are the prints of dinosaurs, of the same species, going in the same direction. Founded on these pieces of evidence as well as the behaviour of dinosaur descendants – birds and crocodiles – we know that dinosaurs took care of their young.

 

[4] Based on the studies of dinosaur skeletons, scientists understand that dinosaurs stood with their legs directly underneath their bodies, as horses and elephants do. For example, the sauropod dinosaur walked with huge, straight legs that had not yet adapted to running and jumping. However, Ornithomimid dinosaurs had long, skinny legs and Ostrich-like feet. These feet permitted them to run fast. The trackway evidence shows the actual movement of the dinosaur. We can estimate the speed of the animal and whether or not it used two or four legs. Other marks indicate how the dinosaur sat down.

 

[5] Fossils also give clues of the diet of our ancient predecessors. Scientists examine fossilized stomach contents to see what dinosaurs ate. The teeth and jaws can also give insight into feeding behavior and diets of dinosaurs. Dinosaurs with big muscles and huge jaws probably killed and ate large animals. The teeth vary from thin or sharp to large and thick. Similarities in the shape and type of teeth and jaws reflect a similar diet of a species. We can assume, from this hypothesis, that these dinosaurs were good at biting but their teeth did not allow them to chew very well. Scientists are beginning to understand which dinosaurs were meat-eaters and which ones were plant-eaters, based on their study of microwear (wear marks) on tooth enamel.

 

[6] Fossils of faeces are called coprolites. They consist of digested animal or plant tissues. A close study of these tissues gives information about the diet of dinosaurs. However, it is very difficult to identify the type of dinosaur with the coprolite and to associate the species of a dinosaur with its footprints. Nevertheless, scientists can narrow down the identities of dinosaur trackmakers by contrasting the foot size and shape, movement pattern and evidence of body size revealed by the trackmakers with other information about dinosaur species.

 

[7] For example, the front foot of an ankylosaur has five digits – or fingers – in an arch pattern. The organization of the hand bones and fingers has formed prints discovered in ancient rock. We know from skeletons that this type of hand or palm belonged to this species of dinosaur. Other dinosaurs have fewer ‘fingers’. From further evidence, scientists have also been able to identify some ankylosaur prints as nodosaur prints, common in the Early Cretaceous period.