Over the last few weeks I’ve been doing some anatomical work on baleen whales, including the Lake Waccamaw Balaenula skull (above). As part of this work I’ve been trying to identify various features of the skull and compare them to other baleen whales.
At the most basic level, when a biologist or a paleontologist attempts to determine how a group of organisms are related to each other they search for homologous structures. Homologous structures are simply features that have a common developmental origin, which is typically reflected in a commonality of structure. The standard introductory textbook example is the tetrapod forelimb, as shown in the example from Wikipedia:
I’ve been focusing a lot of my attention on the petrosal, or periotic bone, part of the ear structure in whales, and I’d like to walk through part of the process of identifying homologies in this bone. Here is the right petrosal of Balaenula (bottom) next to Eobalaenoptera, in ventral view with anterior to the left (we only have the left petrosal of Eobalaenoptera, so I’ve reversed its image so it appears to be a right):
One of the things I’m interested in is a structure called the anterior process, so I need to be able to identify various features in order to can locate it. There are some easily recognized features that we can start with. The ventral side of the petrosal has a large knob of bone called the promontorium, which contains the cochlea and has lots of openings for various nerves and other structures. The petrosal also articulates with another bone, the tympanic bulla, at two points (the anterior and posterior pedicles). Below, I’ve outlined the promontorium in each specimen in green, and the pedicles in orange:
In each bone there is also a large projection sticking off to the right (posterior). This is called the posterior process (outlined in red below). At the other end is the much smaller anterior process (outlined in blue):
I’ve you look at Eobalaenoptera, the anterior process has bits sticking out on each side. One of these is the lateral projection of the anterior process, and it’s adjacent to the anterior pedicle for the tympanic bulla. Using the pedicle as a landmark, we can find the same structure in Balaenula:
Notice that even though Balaenula‘s lateral projection is not offset by a prominent notch like it is in Eobalaenoptera, it’s a relatively much larger structure (look at the distance between the anterior pedicle and the edge of the bone).
Eobalaenoptera also has a large medial projection of the anterior process, located opposite the lateral projection next to the promontorium. How about Balaenula?
In Balaenula, the medial projection is reduced almost to the point of absence, with only a small bulge in the appropriate area. Since otherwise these two bones are homologous, that gives us two possibilities for the evolution of the medial projection. One is that the common ancestor of Balaenula and Eobalaenoptera did not have a medial projection, and it evolved later in the line that included Eobalaenoptera. The other option is that the ancestor had a medial projection, but that it was lost in Balaenula while being retained in Eobalaenoptera. Determining between the two options would involve looking at potential ancestors as well as seeing how the feature is distributed across other baleen whales.
What if we throw another whale into the mix? Here’s the petrosal from a modern whale, I believe the right whale Eubalaena (although it could be the bowhead whale, Balaena) (from the U. S. National Museum):
At a glance this bone appears to be very different from our two examples above, starting with the odd “<“-shape. Even so, we can identify homologous structures between the three bones. Here are some of the features color-coded in the same way as above (promontorium in green, pedicles in orange, posterior process in red):
Just as we did with Balaenula, we can use the anterior pedicle as a landmark to help us locate the lateral projection. By doing this we can see that, like Balaenula, Eubalaena has an enlarged lateral projection (relative to Eobalaenoptera). However, in Eubalaena it’s hugely enlarged and extends posteriorly alongside the posterior process, resulting in the “<“-shape. If we account for this single unusual feature, Eubalaena‘s petrosal is really not all that different from Balaenula; both include an enlarged lateral projection, a fairly small central part of the anterior process and a practically nonexistent medial projection (among other features I haven’t discussed). In fact, Balaenula and Eubalaena are thought to be fairly closely related, and both are placed in the Family Balaenidae based on a whole suite of shared features found throughout the body. Even bones like the petrosal that seem very different at a glance can turn out to be quite similar when they’re examined in detail and homologies are identified.
As a side note, today marks the fourth anniversary of the first posting on “Updates”.