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.

Hatzegopteryx and friends

Throughout this blog I have alluded to and mentioned giant pterosaurs, but I've never actually described them or discussed them properly. As previously mentioned, pterosaurs included the largest animals to ever take to the air, and these large pterosaurs are not just one-off weird things. Animals with wingspans of 5-8m are fairly common in the Cretaceous with the large ornithocheirid pterosaurs of Brazil, including Ornithocheirus and Tropeognathus, and of course one of the most famous pterosaurs of all, Pteranodon from the Kansas chalk deposited from the Western Interior Seaway.

The true giants, however, are the azhdarchids, the most common (if not only) pterosaurs in the latest Cretaceous. Although some azhdarchids were of smaller size (2.5-3 m in Montanazhdarcho and Eurazhdarcho), they also reach absurd sizes of 10-12m wingspans. In contrast, the largest living flying birds (the wandering albatross) had wingspans of approximately 3m (but as much as 3.5m), while the largest extinct flying bird, Pelagornis [1], had a wingspan of 6-7 m. These giant pterosaurs in comparison would have rivalled airplanes with their wingspans, and reached as high as a giraffe when standing.

Giant azhdarchid Arambourgiania with a giraffe and human for scale. Image copyright Mark Witton.

So how many giant pterosaurs were there? So far, there are 3 described species of giant pterosaur: Hatzegopteryx thambena [2] from Romania, Arambourgiania philadelphia [3] from Jordan, and arguably the most famous of the three Quetzalcoatlus northropi [4] from the USA. Unfortunately, these are all currently known from pretty fragmentary remains, but there's just enough to get an idea of what kind of things these animals were up to, and how big they really were. 

Hatzegopteryx

The proximal humerus fragment of Hatzegopteryx [5].
Scale bar is 10 cm.

The first one I'll talk about is the one I'm most familiar with - Hatzegopteryx thambena. Hatzegopteryx was first described in 2002 by Eric Buffetaut and colleagues, from a portion of the skull, humerus, and femur. The skull and humerus came from the Vălioara locality, and the femur from Tustea of the Haţeg Basin of Romania. These localities are terrestrial deposits, deposited during the Maastrichtian (latest Cretaceous) in an area that was then full of small islands. The skull portions consist of part of the and occipital region. Only the proximal (closest to the body) part of the humerus (upper arm bone) is preserved, and it is massive, measuring over 16 cm in it's widest part, and shows a large unwarped deltopectoral crest, which helps in the identification of the group of pterosaurs it belong to (this means it is not an ornithocherioid). The deltopectoral crest is large, and represents the area where the large flight muscles would attach.

Undescribed giant azhdarchid cervical (neck)
vertebra from Romania [7].

Although only known from a few fragmentary remains, much can be determined about this animal and it's size, by scaling up other more complete azhdarchids such as Montanazhdarcho. Original estimates suggested this animal has a wingspan of 12-15 m, but more conservative recent estimates suggest it had a wingspan of 10-11 m [5]. Even at just 11 m wingspans, this animal would have been terrifying. As mentioned in a previous post, the palaeoecology of this region is interesting. Hatzegopteryx appears to be the largest carnivore and predator of the Late Cretaceous of Romania. This has lead people to suggest that Hatzegopteryx would have preyed on small dinosaurs, termed the 'terrestrial stalking' hypothesis [6]. Islands limit the amount of resources available for animals, and typically result in island dwarfism. Pterosaurs, however, would not have had this restriction as they could have flown from island to island. Additional remains from Romania that are currently being described suggest that there may have been other giant pterosaurs, and further Hatzegopteryx-like material has been found. Unfortunately, the lack of overlapping material between specimens makes it difficult to determine if they are the same species or different ones.

Arambourgiania

Arambourgiania philadelphiae is probably the least well known and recognisable of the giant pterosaurs, although it was described first. First described by French palaeontologist Camille Arambourg as "Titanopteryx" philadelphiae in 1959  [3], it was later redescribed as Arambourgiania philadelphiae by Nesov in 1987, as Titanopteryx was already taken as the name of a beetle. Arambourgiania was originally described from a single partial cervical vertebra, which at the time was described as a metacarpal, but later recognised as a cervical vertebra [8,9]. All of the material comes from the Maastrichtian of Jordan, and additional material including a wing phalanx fragment and a cervical vertebra. Additional specimens exist from other museums that have been undescribed or referred to including the Natural History Museum of London. The best estimate for a wingspan of Arambourgiania philadelphiae is similar to Hatzegopteryx with 10.5m being the estimated greatest wingspan possible [5]. 

Quetzalcoatlus northropi

Giant Q. northropi humerus (b, c), and
smaller Quetzalcoatlus sp. humerus (d)
and cervical vertebra (a) [4]

Quetzalcoatlus northropi, on the other hand, is probably the best known or at least most popular in the media of the giant pterosaurs. From the Maastrichtian of Texas, only a few bones of Q. northropi have been described in the literature. The genus Quetzalcoatlus is known from 2 size morphs: a smaller one generally referred to as Quetzalcoatlus sp., and the giant Q. northropi. First described in 1975 by graduate student Douglas Lawson, Q. northropi is known from a fragmentary wing (humerus, carpals, phalanges), with just the humerus figured in the original description [4]. Approximately 40 km away, a number of much smaller specimens were found and described as the same genus, but a different unnamed species, hence Quetzalcoatlus sp. In total, and at the time of original description in 1975, the material existing for Quetzalcoatlus consisted of four wings, a neck, hind limbs, and the lower jaw. Frustratingly, no more of this material has been described. It represents the best known giant azhdarchid, but it has never been properly described in the literature, although there is hope that this may happen soon. 

The lack of description for Quetzalcoatlus is particularly frustrating as it means that additional material from North America cannot be properly described. For example, there is large azhdarchid material from Alberta that is thought to represent at least Quetzalcoatlus sp., and possibly even Quetzalcoatlus northropi. However, until these specimens are properly described, it's unknown if this is the case. Subsequently, a lot of this material is not properly described and people are just waiting for the other material to be described. Hopefully that will come though!

While only 3 species of giant pterosaur are currently known, they seem to not have been restricted to one area as they are known from Europe, the Middle East, and North America. Hopefully more material will be found and described with ongoing studies in Romania and North America especially, and new species may pop up!

Terrestrially stalking Hatzegopteryx preying on small sauropods in Romania [6]

References:

1. Ksepka, D. 2014. Flight performance of the largest volant bird. PNAS 111: 10624-10629.

2. Burretaut, E., et al. 2002. A new giant pterosaur with a robust skull from the latest Cretaceous of Romania. Naturwissenschaften 89: 180-184. 

3. Arambourg, C. 1959. Titanopteryx philadelphiae nov. gen., nov. sp., ptérosaurien géant . Notes et Mémoires sur le Moyen-Orient 7: 229-234.

4. Lawson, D. A. 1975. Pterosaur from the latest Cretaceous of west Texas: discovery of the largest flying creature. Science 187: 947-948.

5. Witton, M. P., and Habib, M. B. 2010. On the size and flight diversity of giant pterosaurs, the use of birds as pterosaur analogues and comments on pterosaur flightlessness. PLoS ONE 5: e13982.

6. Witton, M. P., and Naish, D. 2008. A reappraisal of azhdarchid pterosaur functional morphology and paleoecology. PLoS ONE 3: e2271.

7. Vremir, M. 2010. New faunal elements from the late Cretaceous (Maastrichtian) continental deposits of Sebes area (Transylvania). Terra Sebus, Acta Musei Sabesiensis 2: 635-684.

8. Martill, D. M., et al. 1998. Discovery of the holotype of the giant pterosaur Titanopteryx philadelphiae Arambourg, 1959 and the status of Arambourgiania and Quetzalcoatlus. Neues Jahrbuch für Geologie und Paläontologie Abhandungen 207: 57-76.

9. Frey, E., and Martill, D. M. 1996. A reappraisal of Arambourgiania (Pterosauria, Pterodactyloidea): one of the world's largest flying animals. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen 199: 221-247.