Spinal cords of extinct animals

This is something I’ve become a bit interested in recently while looking at a number of pterosaur vertebrae and thought that I’d discuss it a bit here.

Looking at the nervous system of extinct animals is something that has been fairly common in palaeontology. We are frequently interested in looking at the endocranium (the part of the skull where the brain sits) in order to reconstruct the brain of extinct animals, which can tell us much about how that animal behaved. It’s fairly well documented that the endocranium preserves the shape of different parts of the brain, and can therefore be reconstructed with some kind of accuracy.

But what about the rest of the nervous system? The other major part of the nervous system, of course, is the spinal cord, which transmits all that information that is important to the rest of your body to and from the brain. Plus, there’s a complicated network of nerves as well. While most focus in palaeontology is on the brain, is there anything we can say about the spinal cord?

Spinal columns of an alligator and human showing the relationship between the spinal cord, nerves, plexus location, and vertebrae. From Giffin 1995a.

For this, of course you need to look at the vertebrae. But how much can you say really from the vertebrae? While it’s been sometime since this has been looked at, there is some information you can gain from looking at the vertebrae, and specifically the size of the neural canal. Emily Buchholtz (Giffin) pioneered this field in the 1990’s by looking at the spinal column of several living groups and comparing them to fossils. She found that the neural canal and spinal cord area were well correlated, allowing for the size of one to predict the other, useful for fossils.

From Giffin 1995b

This allows for more accurate reconstruction and visualisation of the spinal cord, and then leads to the next question of how we can better understand some things like behaviour. Buchholtz also suggested that through a standardised measurement of the neural canal area throughout the vertebral column, you can interpret some kind of behaviour or locomotory patterns. By plotting this standardised area throughout the vertebral column, she noticed that the area increased in areas of the vertebral column where the limbs would have been. This is not particularly surprising as you would expect that the spinal cord would need to increase in size in order for nerves to start and run into the limbs, and many nerves would be going into the limbs at this point. What’s interesting though is that she found that different locomotory strategies revealed unique patterns in the neural canal morphology.

Bird standardised spinal cord area. Columba and Turdus are both flying birds, while Struthio is an ostrich (Giffin 1995b)
Lizard standardised spinal cord area (Giffin 1995b)

Birds have different locomotory patterns, but flying birds rely heavily on both their legs and their wings. Their legs are important in take off, while their wings obviously are important in flight. In flying birds, you can see that both the wings (first peak) and legs (second peak) show higher spinal cord area, while the ostrich (Struthio) has low area at the wings, and high at the legs. Ostriches barely use their vestigial wings, and certainly don’t use them to fly, so they do not need heavy innervation, hence the lower amounts. Birds also have higher peaks in general than those seen in lizards (bottom graph). In general, the lizards have higher canal area in the front limbs than the hind limbs, but not by very much. Again, you can see that as both limbs are used heavily in locomotion, both limbs show high peaks in canal area, but there is a slightly higher peak for the front limbs.

Unfortunately, trying to do this in fossils is more difficult. As fossils are so often found incomplete, fragmentary, and poorly preserved, this can be hard to study. Buchholtz did try to do this with some dinosaurs (Allosaurus and Saurornitholestes), but as you can see from the graph below, it’s a lot noisier and not as clear as the modern animals. She also tried it with some fossil crocodilians, which seemed to work a bit better, indicating that Leidyosuchus may have used the back legs substantially.

Dinosaur standardised neural canal area. From Giffin 1995b
Crocodilian standardised neural canal area. From Giffin 1995b

Of course most of this requires some kind of complete or semi-complete vertebral column well enough preserved so you can see the neural canal, and disarticulated so you can do these measurements. With the advent of CT scanning, and the easier way of looking at the internal structures of fossils even if they are in matrix or articulated, this is something that might be more possible now. Maybe there is information we can learn even with just partial vertebral columns? Can we learn anything about an animal’s locomotory capabilities this way?

I’m very interested in people’s thoughts on these methods and approaches. It hasn’t been used or worked on since the 1990’s, and I don’t know if that’s because it fell out of favour for particular reasons, was not received well in the scientific community, or just that no one bothered to look at it more. I’ve seen it referred to in books, and never negatively, but I find it odd that no one has tried to use it again.

References
Giffin, E.B. 1995a. Functional interpretation of spinal anatomy in living and fossil amniotes. In: Thomason, J. (ed.) Functional morphology in vertebrate paleontology. Cambridge University Press. pp. 235-248.
Giffin, E.B. 1995b. Postcranial paleoneurology of the Diapsida. Journal of Zoology 235: 389-410.

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