A few months ago a student working on a project measured and photographed all the whale vertebrae we’ve prepared from Carmel Church. More than half of these vertebrae are isolated bones that are not part of an associated skeleton, and most of them have not been studied very closely. As we’ve come to expect with Carmel Church, a few oddities popped up in the sample, including the one shown above.
The issue with this vertebra is that there appear to be two transverse processes on each side, one at the top of the centrum and one further down the side. This is a rather unusual condition for whale vertebrae, but it requires a little background anatomy to understand the problem.
Transverse processes are projections that stick out from each side of a vertebra. In mammals, if the vertebra is a thoracic vertebra (a vertebra that has a rib), the rib articulates with the tip of the transverse process. The position of the transverse process can vary. If it’s located high on the vertebra, on the neural canal or at the top of the centrum, it’s called a diapophysis. If it’s located low on the vertebra, on the side of the centrum, it’s called a parapophysis.
In cetaceans, normally the only vertebrae that have both diapophyses and parapophyses are the third, fourth, and fifth cervical (neck) vertebrae, as can be seen in this image of the third cervical vertebra of Diorocetus:
However, the cervical vertebrae in whales are always shortened front to back; in fact, they’re always shorter than they are wide (except, perhaps, in very primitive whales). The Carmel Church vertebra has the centrum proportions one would expect in a thoracic vertebra.
So what do the transverse processes look like in thoracic vertebrae? In whales, the anterior thoracics have a diapophysis located high on the neural arch. As you move further back in the series, the transverse process gets lower, until it’s at the top of the centrum. Further back still, the process is located on the centrum as a parapophysis. At no point do the thoracic vertebrae have both a diapophysis and a parapophysis. This sequence is visible in the images below, all anterior or posterior views from Squalodon whitmorei. On the left is the 2nd thoracic, the middle is the 8th thoracic, and the right is the 11th thoracic:
Of course, there is an exception, one odd whale that doesn’t fit the pattern. In most respects the modern sperm whale, Physeter macrocephalus, has pretty typical odontocete vertebrae. Anterior sperm whale thoracics have diapophyses while the posterior thoracics have parapophyses. However, in sperm whales it appears that the diapophysis doesn’t move lower as you get further back in the skeleton. Instead, on about the 9th thoracic vertebra a small parapophysis shows up. In the 10th and 11th thoracics the parapophyses get larger while the diapophyses get smaller, so that by the 12th thoracic the diapophysis is completely gone and you have a normal looking posterior thoracic. With this structure the transitional thoracics (9-11) in Physeter appear to have two pairs of transverse processes instead of one.
The earliest record I’ve found of this in the literature is in W. H. Flower’s 1885 textbook on mammalian osteology. Flower figured these vertebrae (below) and pointed out that they raised questions about whether all the things called transverse processes are really homologous.
Is the Carmel Church vertebra from a sperm whale? I don’t know. Among fossil sperm whales the only extensive postcranial material that’s been published (as far as I know) is from Brygmophyseter shigensis from the middle Miocene of Japan. It’s difficult to tell from the published images, but Brygmophyseter doesn’t appear to have the transitional thoracic vertebrae.
There are some features of the Carmel Church vertebra that muddy the waters. While it does seem to have four transverse processes, the development of the parapophyses seems to be very different on each side of the centrum. Looking at the vertebra in dorsal view, the right parapophysis seems to be an incomplete structure that has a huge hole in the middle and is quite thin compared to the left parapophysis:
Perhaps the apparent extra processes is some type of pathological feature in a non-physeterid whale. I tend to doubt this explanation, however, because the vertebra shows no other indications of any pathologies. Moreover, we occasionally find vertebrae at Carmel Church which have had big holes sculpted out of them by some as-yet-unexplained postmortem process; this could have happened to the right parapophysis. So while a pathological origin of this structure isn’t out of the question, the evidence isn’t exactly compelling.
One more mysterious bone to add to our collection of unexplained Carmel Church specimens!