In 1986 Roup and Sepkopski published a paper which identified 8 distinct extinction events throughout geological time. The methodology they had used was simple and revealing. By looking at how many families of key animal types there were in each geological age, a graph was produced to show distribution over time. Significant dips in the graph meant fewer families surviving and these were interpreted as extinction events.
This concept can be applied in isolation to the pterosaurs showing the evolutionary changes that occurred over time in the numbers of families. The graphical data can be applied to small scale changes or to large scale changes. Below is a graph to show the large scale changes to pterosaur family groups over time.
Family level survivorship amongst pterosaurs over time.
This graph shows that pterosaur family numbers peak in the Upper Jurassic and decline towards the end of the Cretaceous Era when there were only 4 main pterosaur families extant.
The same information can be applied at the Species Level. With some families being large and others small, looking at the number of different species can give a different slant to the data as shown below.
Survivorship of pterosaurs at the Species level.
At this level the pterosaur species numbers peak in the Lower Cretaceous and then start to go into decline.
When viewing these data, consideration should be allowed for the current state of discoveries. There may be more species to be found and the classification into families may be a little flawed as there is still some debate about the way pterosaurs are classified. However, the general pattern is probably sound and minor changes should not alter the shape of the graphs very much. (Some indeterminate species have been excluded from these data sets.)
The patterns of these graphs do change when smaller timescales
are applied. They become a little less distinct and peaks and troughs can
be seen at key geological events.
Raup D. and Sepkoski J., 1986, Periodic extinction of families and genera. Science 231 (4740): 833–836