Spinosaurus – a Hint

So last year, I illustrated a series of vertebrae into a novel arrangement, using a variety of cues to inform my decision. Note that this was done in order peek at a topic I’ve been hashing around in regards to potentially presenting in some formal venue, SVP perhaps.

The first step in this analysis was presented in the first post, in which I approached the unusual dorsal neural spines of Spinosaurus aegyptiacus. Many people have approached this specimen, but only one of them has had the opportunity to do so in person: Ernst von Stromer. Sadly, von Stromer is no longer around to share in his understanding of a specimen lost to us through war, but much data persists, including an extraordinarily detailed descriptive paper, one whose model and layout (of description, exhaustive discussion, and comparison occurs well before the author names the form) I wished most modern taxonomists would use. But, I digress.

Spinosaurus aegyptiacus skeleton, after de Lapparent and Lavocat. Stolen from Wikipedia.

Spinosaurus aegyptiacus skeletal, by Lucas Panzarin, from Theropoda.com (Andrea Cau’s blog).

Spinosauridae skeletals, by Scott Hartman, from dal Sasso et al., 2005.

In each of these skeletals, something is amiss. First, de Lapparent & Lavocat (top) illustrated these bones along with a large array of bones not included in the specimen von Stromer described, borrowing from several sites in Tunisia where numerous additional spinosaurid teeth have also been recovered. S0me of this material may not belong to Spinosaurus aegyptiacus. The second image, drawn by paleo-life artist Lukas Panzarin, illustrates a bison-like silhouette much like the older and more southerly (but also sail-backed — and herbivorous!) Ouranosaurus nigeriensis. While Panzarin illustrates the vertebral spines relatively accurately, their orientation is certainly to be desired, as the swept back final spine is placed into a vertical position, which disagrees with the neural arch body below the spine. Nonetheless, a decent skeletal.

Scott Hartman, who is a friend, borrowed from de Lapparent in illustrating the centra of vertebrae without centra preserved and extending the sacral neural spine too far (although Scott extends it further). Scott also illustrated centra with flat anterior faces, which does not match any of the preserved vertebrae. This condition exists in Baryonyx walkeri and Suchomimus tenerensis, which are related spinosaurids, but only just so: the centra in Baryonyx walkeri, like Spinosaurus aegyptiacus, have large ball-shaped anterior faces and deeply-cupped posterior faces, grading into a amphiplatyan (flat on both sides) condition right up into the sacrals.

All of the above reconstructions place the tallest spine at the final dorsal position, and thus this alters the form of the sail as it may have appeared.

Silhouettes of Spinosaurus aegyptiacus.

In the above image, we see two different silhouettes, although they compare closely, differing almost exclusively in the relative placement of the first two dorsal vertebrae. While these vertebrae may be adjacent, the strong sloping profile and nearly serial quality of each adjacent neural spine implies there was another, intermediate spine. I separated each vertebrae by another in this diagram merely to emphasize each element in particular, and assumed there were only 12 precise dorsals (there may be as many as 14, depending on the cervical series — comparisons to other spinosaurids is hindered due to the lack of a complete articulated neck in either Baryonyx walkeri or Suchomimus tenerensis). As I discussed in the other post, the tall neural spine appears to be a caudal vertebra, and while von Stromer (and everyone else) associates the centrum found near it with it, I argue it is an anterior dorsal centrum, and should not belong to the spine. Thus they are well-separated. This is the most extreme conclusion of this analysis, and illustrated here (not showing another {possibly associated and unrelated} [dinosaurian] caudal vertebra nor the several ribs found with the vertebrae) one gets the sense that the “sail” was far longer and in aspect rather lower than other illustrators have provided.

I have illustrated two silhouettes here, one in grey (small) showing what happens when the first few preserved spines are placed in the anterior dorsal positions, right after the cervicals (placing the first dorsal here above the likely position of the coracoid). This results in a gradual elevation from the neck into the dorsal spine “sail.” A large gap is placed between the first two and the third dorsal in position (a centrum) and the fourth (an articulated, partially fused arch and centrum with nearly complete spine).

The larger illustration, however, shows what happens when the contour of the spines are involved, bring them closer in line with a likely radial array. This rounds the anterior portion of the “sail” a bit, and is purely artistic. Vertebra “3” and “4” (von Stromer’s elements I and F) are and remain in the middle dorsal series, and dorsals “1” and “2” (C and D) are further from the cervicodorsal transition, as they bear no features in common with such vertebrae in Baryonyx walkeri. The swept back caudal position here pulls the “sail” further back, and argues I would think for a more progressive decline into the tail, as in other “sail-backed” dinosaurs. This also suggests the spine was not likely to be the tallest in the column, and that honor may have belonged to a posterior dorsal or sacral, as it is in both Baryonyx walkeri and Suchomimus tenerensis.

This reconstruction is not perfect, and it’s not meant to be. It is meant instead to illustrate what I think are likely to be more accurate positions of individual vertebrae. New discoveries have been made in Morocco and elsewhere in the southern coast of the Mediterranean of norther Africa, from Morocco to Egypt, that may help explain the body form of sail-backed spinosaurids. I doubt that all of these specimens particularly apply to Spinosaurus aegyptiacus, although others have argued for a more inclusive concept of northern African spinosaurs.

A better understanding, then, must await better material.

Update:

I missed a reference and its conclusion regarding the orientation and arrangement of spines in this specimen. In Smith et al. (2006), the authors reconstructed the series as I did, placing element “f” anterior to element “e” and “h” anterior to “g”, followed by “i” as the last dorsal.

Spinosaurus aegyptiacus spines, after Smith et al., 2006 (fig. 4). 1, is von Stromer's original series, and 2, is Smith et al.s proposal.

This reconstruction differs from mine in two things, but one of them was discussed above: placement of element “i” as the last dorsal along with articulation (as von Stromer assumed) of the element “i” centrum, but also the use of 13 dorsals, while mine assumes only 12.

Update 2:

Andrea Cau of Theropoda reminds me (as he did in my last post on Spinosaurus, that I forgot) that he had reconstructed element “i” as a caudal vertebrae, and while both he and Smith et al. reconstruct the caudal with an attached centrum (along with all other vertebrae), this interdates Smith et al. and my reconstruction, firmly giving me two reconstructions I may now have used to influence my analysis. Kudos or Andrea (and Smith et al.). Here it is for easy parsing:

Andrea Cau's Spinosaurus aegyptiacus. Note use of the skull after Irritator challengeri and dal Sasso et al..

Dal Sasso, C., Maganuco, S., Buffetaut, E. & Mendez, M. A. 2005. New information on the skull of the enigmatic theropod Spinosaurus, with remarks on its sizes and affinities. Journal of Vertebrate Paleontology 25(4):888–896.
de Lapparent, A. F. & Lavocat, R. 1955. Dinosauriens [Dinosaurs]. pg.785-962 in Piveteau (ed.) Traité de Paléontologie. Tome V. La Sortie des Eaux. Naissance de la Tétrapodie. L’Exubérance de la Vie Végétative. La Conquête de l’Air Amphibiens. Reptiles. Oiseaux. (Masson et Cie, Paris).
Smith, J. B., Lamanna, M. C., Mayr, H. & Lacovara, K. J. 2006. New information regarding the holotype of Spinosaurus aegyptiacus Stromer, 1915. Journal of Paleontology 80(2):400-406.
von Stromer, E. 1915. Wirbeltier-Reste der baharije-Stufe (unterstes Cenoman). 3. Das Original des Theropoden Spinosaurus aegyptiacus nov. gen. et nov. spec. [Vertebrate remains of the Baharije Formation (lowest Cenomanian). 3. The original of the theropod Spinosaurus aegyptiacus nov. gen. et nov. spec.] Abhandlungen der Königlichen Bayerischen Akademie der Wissenschaften Mathematisch-physikalische Klasse Abhandlung 28(4):1–32.
Taquet, P. & Russell, D. A. 1998. New data on spinosaurid dinosaurs from the Early Cretaceous of the Sahara. Comptes Rendus de l’Académie des Sciences, IIA: Earth & Planetary Sciences 327:347−353.