I’m back in Martinsville for a couple of weeks, but I’ll be back at Carmel Church soon. One of the reasons I returned is because of the release today of a new paper by Brian Beatty and me on one of the whales from Carmel Church.
The whale discussed in the paper is the same Diorocetus specimen we’ve been preparing since the museum opened in 2007 (search the blog for “Sinistra” for a history of posts about this whale).
Basically, this is a fleshed-out version of the talk we presented at the 2008 SEAVP meeting. The outstanding feature of this whale is the devastating injury that it suffered to it’s lower jaw, in which the left dentary was snapped completely in half:
The whale has other injuries as well. The tip of the left premaxilla was crushed, and also partially healed (from left to right: left dorsal view, right dorsal view, right ventral view, left ventral view):
The left glenoid fossa and postglenoid process of the squamosal (the lower jaw articulation) also appear to be damaged (anteroventral view of the left and right squamosals; these should be mirror images of each other):
All this suggests that the whale suffered an almost head-on collision on the left side. The premaxilla was crunched and the dentary jammed back into the jaw joint. The curved dentary responded to the collision by flexing outward until it snapped. The dentary was actually shortened by several centimeters because of the break.
Brian and I believe this occurred when the whale was engaged in bottom feeding. This is a practice employed by modern gray whales, in which they scoop up a mouthfull of sediment and filter out the invertebrate animals living in it. The whale presumably collided with something hard on the seafloor, causing the injury.
We think that the jaw pieces never rejoined because the whale kept attempting (probably unsuccessfully) to feed. Every time it opened its mouth, the broken ends of the dentary shifted, preventing the injury from completely healing. Likely unable to feed, the whale would have eventually starved, or (more likely) weakened to the point that it was vulnerable to predators such as sharks. The rest of the skeleton indicates that the whale was a mature adult, with no evidence of arthritis or other age-related degenerative diseases, so it seems it was an otherwise healthy whale.
The injury has an additional interesting implication, in that modern gray whales are handed (or, more scientifically, lateralized). Basically, gray whales only feed from one side of the mouth. In right-handed whales, which make up about 80% of the population, the baleen plates are more worn on the right side. The fact that the Carmel Church whale’s injury was on the left side indicates that it was a left-handed whale.
Here’s a reconstruction of the moment the injury occurred (drawing by Michael Morriss):
There is actually an independent line of evidence that this whale was a bottom-feeder. It had remarkably dense, osteosclerotic ribs, in which there is very little cancellous (spongy) bone in the rib’s interior:
This is reminiscent of the ribs of sirenians, which have no cancellous bone in the ribs at all. In sirenians, this has been interpreted as ballast to aid the sea cow is staying on the bottom while feeding on sea grasses.
To document the relatively high rib density in Diorocetus, Brian and I measured rib thicknesses in a variety of whales, and found that the Carmel Church Diorocetus did have remarkably dense ribs compared to its contemporaries. To our surprise, however, we found that very early (Oligocene) baleen whales, as well as Oligocene toothed mysticetes such as Aetiocetus, have ribs as dense as, or more dense than, Diorocetus. If our assumption that high rib density correlates with bottom feeding is correct, it suggests that baleen originally evolved as a mechanism for benthic feeding. The use of baleen to capture swimming animals such as small fish and krill in the water column (employed by almost all living baleen whales) may be a behavior that evolved later.
One final note, not included in the paper. What exactly did this whale collide with to cause such an injury? Our assumption was that it was a rock on the sea floor, buried in the mud where the whale couldn’t see it. Big rocks are certainly not something we typically see in the silts and clays of the Calvert Formation, but there is one place we know of where the Calvert sea floor was littered with large rocks – Carmel Church:
It’s an intriguing thought that this whale may have injured itself trying to feed at Carmel Church. We know that it lived for probably weeks after the injury occurred, which would mean it didn’t leave the immediate area.
The Virginian-Pilot has a nice article about this paper and our Carmel Church activities:
The paper was published in Jeffersoniana, and is available as a free pdf download. The full citation is: