Sunday, 27 December 2009

Comparative Studies

By looking at specific features of the fossil skeletons of pterosaurs it is possible to compare them to see how closely individuals are related. The science of comparative anatomy is limited in its application, but it is a very useful tool when dealing with fossil species. For instance - does Pterodaurstro fit more closely with the Pterodactyloids of the Rhamphorhynchoids.By looking at a comparison of wing bone lengths in relation to the wing metacarpal bone, it can be seen that the shape formed on the above graph places Pterodaustro clearly alongside that of a Pterodactyle. In many cases, the relationship of the wing bones to the wing metacarpal can be used to distinguish families of pterosaurs. However, this type of analysis should be used with caution as there are a few exceptions to the rule. Other features like skull anatomy, vertebra structure and pelvic anatomy are also needed to confirm any comparative relationships.

This graph shows the wing profile relationships between two individuals of the same species of pterosaur, found within the same sediment. The slight difference is likely due to natural variation.
Here is a wing profile graph that is generated from two unrelated pterosaurs from different sites at about the same geological age. The pterosaurs look similar in most features, but the analysis suggests significant differences in wing bone development.
When using comparators, patterns will emerge to indicate differences. Scatter graphs will often be a useful way of determining differences. The above plot shows the relationship between wing metacarpal length and first wing phalanx length in a random selection of pterosaurs. The differences between the Rhamphorhynchoid pterosaurs and the Pterodactyloid pterosaurs can be indicated by circling each group.
The final graph shows a calculated plot based on the relationship of measurements for the humerus, ulna and wing metacarpal. In this plot, there is no overlap between the two major pterosaur groups within this sample. The scales displayed are arbitrary units in the plot calculations.

Comparative anatomy is all that we can use for direct comparison of extinct fossil species. It gives a very good indication of relationships and enables structure to be developed in an evolutionary hierarchy. However, whilst this is a useful tool, it is only a guide and must be treated as such. It is often down to individual opinion as to how accurately the selected comparator work. This is why scientists develop slightly different evolutionary trees of the pterosaurs. Each may be valid within the scheme of analysis and each should be respected in that light.

Good science is all about debate and discussion but in the case of fossil analysis the picture is often quite disjointed. differences of opinion are what makes this subject interesting.