By Dana E. Korneisel, PhD Candidate at Carleton University
When we land vertebrates evolved from fish, we developed a neck to separate our front limbs from our skull. This allowed for the loss or fusion of many of the building blocks of the skull and vertebrae, making it harder to accurately compare the bones of living vertebrates to one another and to fossils. To figure out how these bones have evolved by comparing fossil and living animals, we must first be able to determine which bones contribute to modern skulls and vertebrae. Are some bones lost, rearranged, or are they just fused to another bone?
Alligators are a perfect study animal for the skull-neck boundary because they have many unfused neck bone components, and it is controversial whether or not they have a bone called the postparietal in their skull. In a recently published paper in The Anatomical Record, I and my co-authors found out that a bone component that is located on the second vertebra is actually part of the first vertebra. We did not find evidence of a postparietal in the alligator skull, in contrast to other research in the area.
We used three methods to visualize developing alligator skulls: micro Computerized Tomography (CT, a series of x-ray images from which we make a 3D model of the skeleton), diaphanization (a method of making soft tissue clear and staining the bones so that you can look through the head and neck to see the bones), and histology (cutting thin sections of tissue and staining them to differentiate different tissue types). We did not identify the mysterious postparietal bone with any of these methods.
Instead, we saw in the micro-CT scans that a large bone at the back of the adult alligator skull first appears in embryonic alligators as at least three centers of bone formation that later fuse together. This large bone is directly behind where the postparietal would be. In a micro-CT scan, when comparing the young embryo (with 3 separate bone centers) to a hatchling-aged alligator skull that closely resembles that of an adult, it is clear that these three bone centers will contribute to one bone. However, the process of clearing and staining can dissolve very thin bone, so we think that, with this method alone, one of these bone centers could be easily mistaken for the postparietal when an embryo is cleared and stained.
Historically, researchers chose not to describe the development of the first two neck bones because of their uniqueness. There is a part of an alligator’s second vertebra that you could pop off the bone with a single finger. This part is called the odontoid bone. With the odontoid removed, the second vertebra looks like any other alligator neck vertebra. The odontoid bone appears to be an addition, but from where? We found out that the odontoid bone is actually part of the first vertebra.
However, this interpretation poses a problem in terms of ribs. The ribs usually connect to a specific part at the front of each vertebra, but the second vertebra’s ribs connect to the odontoid bone. If the odontoid is really part of the first vertebra, it is not the part that should have ribs. If the ribs attach to it, then the second vertebra doesn’t have any ribs at all.
With the thin sectioning we did in this study, we could see that the odontoid bone sits directly on top of the part of the second vertebra where ribs should connect. The rib-bearing part of the second vertebra is sandwiched between the odontoid and the main body of the second vertebra in a way that makes it impossible to see with clearing and staining, but the rib cartilage connects directly to it. When the bone forms, you can no longer distinguish the odontoid from the rib-bearing portion of the second vertebra, causing the confusion. So, the ribs of the first and second vertebra are both attached to the appropriate part of the vertebrae, it is just not visible once the cartilage is replaced by bone.
So, we found out that a portion of the first vertebra ends up stuck to the second vertebra and we did not find a postparietal. It is still possible that the postparietal only appears in some alligator embryos – to find out if this is the case we would have to look at a very large sample of embryos to quantify how often this happens.
By describing the development of the bones at the skull-neck boundary, we make it possible to knowledgably compare them evolutionarily between other extant tetrapods and learn about the evolutionary histories of these bones. The evolution of a neck in the first vertebrates to start to live on land allowed them to evolve diverse bodies. These observations on modern embryos contribute to our understanding of how the many diverse and interesting vertebrates found today and in the fossil record evolved.
Images provided courtesy of Dana Korneisel and Rebekah Vice.