Our goal in reconstructing the baby baleen whale “Buttercup” is to model a complete skull for exhibit purposes. In Part 1 we molded and cast the six preserved fragments, but these only add up to about 20% of the skull. We feel like we can make a reasonable reconstruction of “Buttercup” because we have a nearly complete skull from the same species, “Sinistra“. So to make “Buttercup” we should be able to just scale down “Sinistra”, right? Not so fast!
The problem is that whales exhibit allometric growth, meaning that different parts of the body grow at different rates. This includes different parts of the skull, so that within a single species skull proportions are different in individuals of different ages. So we not only have to adjust the size of “Buttercup”, but we have to make a prediction about how the proportions change with age.
Our first step is to figure out just how big “Buttercup” should be. Excluding the periotic (which has its own allometry issues), the best measurement we could get on both “Buttercup” and “Sinistra” was the width across the occipital condyles (as shown in the image at the top). On this measurement, “Buttercup” was 76.5% the size of “Sinistra”.
Now the arm-waving gets really intense. We don’t have enough specimens of Diorocetus to know much about its changes during growth, but we can use modern baleen whales as a proxy, even though they’re pretty distant relatives. Unfortunately, detailed published measurements of modern whale skulls are also fairly scarce. Cobbling together measurements from various sources and my own photos, it seems that in modern Balaenoptera, if a juvenile whale has width across the condyles that’s about 77% of the adult, then its overall skull length is only about 54% of the adult length (there’s a lot of variation, though). If we use these numbers for Diorocetus, since “Sinistra’s” skull is 142 cm long, that gives us a skull length in “Buttercup” of about 77 cm.
We can use the same method to get dimensions for various subunits of the skull; for example, the rostrum makes up a smaller percentage of the overall skull length in juveniles than in adults. Once we have some base measurements estimated, we can sketch out a life-size estimate of “Buttercup’s” shape:
I have to emphasize that there’s a HUGE amount of uncertainty here. We’re taking a process that already has a lot of individual variation, trying to pin it down to a single set of proportions in modern whales, and then applying those numbers to a whole different family of fossil whales. And it’s all pinned to a single comparative measurement in our fossil specimens. I feel certain that if we recovered a complete juvenile Diorocetus skull that it would look somewhat different. That said, our estimate is probably not too bad, and is certainly good enough to use for producing an exhibit model.
In Part 3, we’ll look at converting the paper drawing into a 3-dimensional model.