In July 2011, when we collected the first fragments from the baby baleen whale “Buttercup“, we of course collected a lot of additional material found in the same pit. One piece that went unnoticed at the time was the small fragment shown above.
This is the anterior tip of a very small baleen whale left dentary (lower jaw), seen in medial view. The fragment is pretty heavily abraded, and the pointed anterior end (on the right) is an artifact of that abrasion.
While there isn’t a whole lot of material here, there is enough to measure the vertical thickness of the dentary; it’s about 44 mm. That’s tiny for a baleen whale, and the fact that it came from the same 0.5-square-meter section of sediment as the cranium of “Buttercup” certainly is interesting. Could this be a piece of “Buttercup’s” jaw?
I decided to see if this was even a possibility. “Buttercup” is the same species as “Sinistra” (Diorocetus hiatus), and “Sinistra” has a complete jaw. “Sinistra’s” jaw is about 87 mm deep near the anterior end, and has a straight-line length of 1360 mm. If we assume the jaw exhibits isometric growth in these dimensions (that is, that these proportions don’t change with age; not necessarily a safe assumption), we can set up a ratio to estimate the total original length of the small jaw (assuming it’s also Diorocetus). That gives us 87:1360 = 44:688, so the complete jaw would have been 688 mm long in a straight line.
So, how does that compare to “Buttercup”? As we only recovered fragments of “Buttercup’s” cranium, we don’t have a definite length for the skull. However, comparing “Buttercup” to “Sinistra” and using modern baleen whales as a growth model we came up with an estimate for “Buttercup’s” skull size and shape, and used that to produce a model of the skull. So today I took a measurement on that model from the front of the glenoid fossa (the lower jaw attachment area) to the tip of the premaxilla, which should be approximately equal to the straight-line length of the lower jaw, and came up with … 690 mm!
Now, I still don’t know for sure if this is “Buttercup’s” dentary, and probably never will. This is all resting on a gigantic mountain of assumptions. But it is interesting and suggestive how closely the estimates match, especially considering that we came up with the skull length estimates before we even knew about the jaw fragment.