This figure has taken me a good deal of time to make. Not really in the actual production, but it’s been a long time gestating since conception.
The genus Diplodon, as determined by the specimens to which that name has been applied, has been around since the Middle to Late Triassic. In the dissertation dataset, this works out to the 220 Ma time slice, or the Carnian stage. This is a map of what the world may have looked like at about that time period*.
Why is this important? In general, it’s important because it shows the geographical relationship among these occurrences as it may have been when these organisms were alive. Many paleogeography or historical biogeography papers ignore what the past geographic relationships may have been and focus on mapping a paleolandscape or biogeographic distribution onto a modern map.
Consider the possibility that these occurrences are not the earliest record of this genus (you would be right). If you were looking for additional material with only these four occurrences on which to base your search, you would look geographically nearby. Looking at a modern map would limit you to southern and eastern North America, but as you can see from the figure here, the paleogeography could support a South American or even African population. (I’ll tell you later why this this probably won’t work out.)
For the dissertation, this map is important because it (and others like it) can help show how far this genus is about to spread, and how long this is going to take. You may remember that I’m more interested in names than evolutionary relationships, so I hope to answer the question: how much time and space does there need to be between occurrences before we throw up our hands and say “this genus can’t possibly have survived that long?” The map series will help define where (and where not) there was a chance for lineage continuity.
*The background map, an achievement in itself that I take no credit for, is a product of Ron Blakey and Colorado Plateau Geosystems Inc. The positions of the continents are supported by Chris Scotese’s plate tectonic reconstructions as part of the Earth System History GIS collection. The positions of the Diplodon occurrences were mathematically rotated to these positions using the PointTracker software, also from Scotese.