Pterosaur Survivorship Curves
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 Early (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
Raup D. and Sepkoski J., 1986, Periodic extinction of families and genera. Science 231 (4740): 833–836
One interpretation for the decline in pterosaurs can be explained using niche theory. In any environment, only one dominant species will exploit one major food resource at the expense of other less able species. Unfit species will decline, being less able to compete for the resources. It is clear that some pterosaurs were able to exploit their large size and specialised soaring abilities that allowed them to travel long distances. This allowed the big pterosaurs to dominate the skies world wide. By the end of the cretaceous, the exploitation of small scale environmental resources by birds meant that the smaller pterosaurs were less able to compete and survive and they are seen to disappear from the fossil record as birds evolve.