The information available to help in reconstructing a fossil and inferring its life habits come from three sources: from modern relatives, from modern analogs, or from trace fossils (Fig. 1.10).
Living relatives of a fossil are extremely useful in inferring information about that fossil's ecology. Living Nautilus, for example, uses a system of jet propulsion to power it through the water. Extinct ammonites share a common ancestor with this group and have a similar shell morphology. It therefore seems sensible to suggest that both groups of shelled mollusks use or used a jet propulsion system for making rapid movements.
A different example of the use of homologs comes from the discovery of what appears to be the skull modifications for whiskers in some early mammal-like reptiles. In modern mammals, whiskers are developed as highly modified hairs. If a fossil can be shown to have had whiskers, it also had fur, and fur in modern organisms is used to insulate a warm-blooded organism. Mammal-like reptiles can therefore be inferred to have been warm blooded.
Modern analogs rely on comparisons between similar environments, similar-looking organisms, or on inferences based on physics or engineering that do not change over geological time. A good example of this is the work done on the bone strength of dinosaur legs, in particular the large sauropods like Diplodocus. Recent work has shown that, like eggs, the bones are extremely strong in some directions and that they could easily have borne the weight of the animals as long as most of the legs were weight bearing at any one time. This has led to new reconstructions of sauropods walking on land, but not running, which would involve supporting their weight on only two legs at once.
Trace fossils preserve a moment of activity in the life of an organism, and can show that the animal did something in particular, if they can be related to the thing that made them. Trace fossils such as Cruziana and Rusophycus demonstrate that some species of trilobites walked on the sea bed, and burrowed for soft prey into the sediment, but only some trilobites can be linked directly to the traces they made.
Some information is unavailable for any fossil group, for example true color or seasonal or diurnal variations in activity. Living organisms are capable of a wide range of behaviors, and function in different manners depending on factors such as their stress levels, mating state, level of hunger, and so on.
Reconstructing wings can be done based on exceptionally well-preserved pterosaurs, where the wing membrane can be seen. The shape of the wing can be used to infer the flight characteristics of the animal, as all wings will have predictable aerodynamic properties. Long and relatively large wings like these would have facilitated efficient gliding flight such as is seen in modern seabirds like albatrosses
Exceptionally preserved pterosaurs show that the body was covered in a mat of coarse fibers, analogous to mammalian fur. The primary use of fur is in keeping warm-blooded animals insulated against heat loss. Pterosaurs are therefore inferred to have been warm blooded. This is supported by the observation that flight is a very energy-intensive activity and that most modern fliers are endotherms
Pterosaur trackways show that the animal walked on all-fours, with the front legs held wide and the back legs making narrow tracks. They walked by moving both feet on the same side of the body, one after the other, and before the two feet on the other side, which is an unusual pattern, possibly implying that they evolved from bipedal ancestors
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