March of the Ratites
The job was a chart that would display the fossil history of the ratites. (Ratites are the giant flightless birds that include the ostrich, emu and the like). There was one previous attempt: the diagram of bird fossil histories from Unwin (1993). It’s rather daunting, isn’t it?
The chart shows the geological timescale up the left (from the Maastrichtian in the late Cretaceous through to the Holocene and the present day), and when different groups of birds occur in the fossil record (the vertical lines). The timescale implies the different geological stages are all the same length, which is certainly not the case, but there is no scale in years to make this clear. The bird fossils only actually occupy half the timescale, though the gridlines and boxes continue all the way to the bottom of the page. The legend takes up almost as much room as the data, and actually reading information back off the chart requires constantly referring to the key, making comparison fairly difficult.
How could this be improved? For its size, the chart contains very little information; most of it is furniture. For each group, there are only three data points: first appearance, last appearance, and how much of the fossil record is patchy or continuous. I was interested in showing just the ratites (roughly groups 18 to 29), and some of these have no fossil record to speak of, so there was plenty of opportunity to add more information. I wanted to show actual names, localities, a picture of the birds concerned indicating relative sizes, flying ability, whether they were actually ratites or not, and some indication of how many bones had been discovered from each. So I started sketching.
I realized I could combine the bone information with the outlines, and indicate flying ability by showing the wings of flying ratites. At this point I put together a geological timeline, to scale, in Illustrator. I decided to show the actual boundary dates of geological periods as well as a linear timescale.
I roughly sketched each ratite (in some cases reconstructing them from partial skeletons, in other cases working from Googled photographs). I then photocopied them up to full-page size and traced the outline with magic marker. Each ratite was then scanned into Photoshop, the outlines filled in, and the contrast adjusted so only the black silhouette remained. This was then placed into Illustrator as a template. Illustrator has autotrace tools for converting a scanned image into an outline, but I found it much easier to click and drag my way around the silhouette with the pen tool. The nice thing about working large like this is that when you shrink the graphic back down it looks pretty sharp.
Here’s a version of the final graphic. I simplified the crowd of ratites clustered around the Holocene by noting in the text which groups did not have a significant fossil record. The shades of gray indicate how closely-related each species is to modern ratites.
Where only partial skeletons are known, bones were added to the silhouettes. Where the fossil record becomes continuous, the line for each group becomes solid. I hoped these conventions would be intuitive enough that the chart would pretty much speak for itself.
One side effect is that I now have a bunch of scalable silhouettes I can use to label other diagrams, phylogenetic trees, and the like, so it was worth the effort.
Reference: Unwin, D.M., 1993. Aves. In Benton, M.J. The Fossil Record II. London: Benton & Hall.
Criticisms of this chart should in no way be seen as a reflection on the expertise of Dave Unwin, a splendid fellow, and not just because he let me hold the Berlin Archaeopteryx.