What big neural spines you have…

While Tyrannosaurus rex may always be the unofficial “ambassador” of dinosaurs, there is scarcely a large theropod more intriguing than Spinosaurus. As a child it was one of my favorites, and it seemed to make at least some appearance in every dinosaur book, but I had never actually seen any fossils of it. If there was some huge sail-backed carnosaur, where are all the fossils? Most of the depictions looked like a Megalosaurus or Allosaurus with a fin tacked on its back, and I was very surprised when I saw it “brought back to life again” in Jurassic Park III; it was even weirder and more wonderful than I could have imagined.

By now the story is relatively well-known, thanks to headlining discoveries made in Egypt over the last decade and some popular books/documentaries like The Lost Dinosaurs of Egypt (which is ok, but it doesn’t hold a candle to Sternberg’s Life of a Fossil Hunter). In 1912 Richard Markgraf, under the direction of German paleontologist Ernst Stromer, collected the first known remains of Spinosaurus aegyptiacus near the Bahariya Oasis in western Egypt. The fossil material that pointed to a new, large theropod unlike any other known at the time was lost, however, as in April 1944 the skeletal elements (a lower jaw, some teeth, and some of the vertebrae with extended neural spines) were destroyed in an Allied bombing raid on Munich.

The Spinosaurus (prior to its destruction) on display at the Palaontologische Staatssammlung Munchen (from Smith, et al., 2006). Note how the vertebrae are likely not in the correct order, but were sorted by neural spine length (Stromer recognized this problem with the mount as well).

Given the strange lower jaw shape of Spinosaurus, I wonder why it was interpreted as essentially a big Allosaurus by some illustrators, toy-makers, etc. Granted, some might not have seen any photos of the remains (simply knowing that it was a big theropod with a sail), but lower jaw that Stromer examined is clearly different from that of any theropod known at the time, causing Stromer to give it it’s own group, the Spinosauridae.

Stromer’s reconstruction of the material shown above from his monograph on Spinosaurus (via Wikipedia: Spinosaurus)

Reconstructions aside, in subsequent years some paleontologists thought they had found more Spinosaurus material, but none seemed to be conclusive. Then, in 1996 remains of another Spinosaurus (attributed to Spinosaurus marocannus, but this is probably a nomum dubium, based upon inadequate fossil material) were found, and in 1998 more material from Spinosaurus aegyptiacus was recovered, giving us a much better look at this long-lost dinosaur. In 2005 even more material from the huge upper jaw of Spinosaurus was described, showing that this animal had one of the longest skulls of all theropod dinosaurs (although certainly not as robust as that of Tyrannosaurus). What is interesting, however, is that we still don’t know much about Spinosaurus. We have a good idea what the skull and vertebrae look like, but the limbs have yet to make any appearance, and even though the Jurassic Park version used its huge claws to break the neck of Tyrannosaurus execution-style, we can only infer that it had long-powerful arms with huge claws from its close relatives like Suchomimus and Baryonyx.

Despite the lack of appendicular skeleton elements, the skull of Spinosaurus has been highly diagnostic and helped determine its relationship to other similar theropods that have been found only within the last few decades. Looking at the skulls of the smaller Baryonyx and Suchomimus, there is a substantial bump where the premaxilla and maxilla meet on the upper jaw. Indeed, the premaxilla has a bit of a convex bump to it, smaller teeth bordering at least two larger teeth, and then the maxilla has a concave shape downwards, with one or two small teeth before the teeth get larger, then becoming smaller as you move backwards in the jaw. In Spinosaurus and Angaturama/Irritator, however, it almost looks as if the snout has been pulled out a bit, the concave/convex features still being apparent, but not as deep as in the earlier forms. The differentiation of tooth size is also apparent, but the teeth in the convex portion of the Spinosaurus premaxilla are much smaller than the relatively huge teeth at the crest of the concave portion of the maxilla. It should also be noted that the nose moved backward in this lineage, in the earlier forms being just behind the concave bump in the maxilla, but in Spinosaurus it is much further back towards the eye. These differences, (among others) are enough to put these spinosaurs into two groups, the Baryonychinae (Baryonyx and Suchomimus) and Spinosaurinae (Spinosaurus, Irritator, and Angaturama).(All these observations are based off of a comparison drawing made in Sasso, et al., 2005, which I do not reproduce here for fear of copyright issues).

But what about the “sail”? Many prehistoric creatures have been known to be fin-backed, including Arizonasaurus, Dimetrodon, Edaphosaurus, Ouranosaurus, and possibly even a sauropod, Rebbachisaurus [and Playhystrix too, thanks Zach!], and there have been just as many different attempts to explain why neural spines of the vertebrae would have become so elongated. In the case of Edaphosaurus, E.D. Cope initially proposed that it used it’s fin to literally “sail,” the enigmatic cross-bars of the extended spines reminding him of aspects of sailing ships, although this idea has not been taken seriously for some time. More reasonable explanations deal with body temperature/heat, either too much or too little. Growing up I remember explanations that sail-backed dinosaurs and other animals needed to “warm-up” in the morning to get the best of their prey, so while other cold-blooded animals were still sluggish they could be active. Later, sexual selection came into play, the sails possibly becoming flushed with color or serving as signals (and this idea does not exclude others). Debate will still go on, but in the case of Spinosaurus and Ouranosaurus, it’s possible that the sails served multiple roles. During the day it would be exceedingly hot, so the sails may have been used to get rid of excess heat (which is hard to do if you’re a large animal). Desert nights are known to be cold, however, so the sails could have collected more heat towards the end of the day before the sun went down. These suggestions actually remind me of some modern termite mounds in Africa, built in a way that they capture heat in the morning and evening, but in midday the run is right above them so there is less surface area, allowing the mound to remain relatively cool.

But what about sexual selection? The membranes between the spines would likely have been filled with blood vessels (especially if they were used in heat gathering/loss), and may have been able to become flushed with color if the capillaries expanded or contracted. Indeed, why should such a huge organ only have one use? Sails did not just pop up out of the backs of fin-backed dinosaurs overnight, however. If we look at Suchomimus (and even the American Acrocanthosaurus) we see the beginnings of neural-spine extension, but certainly not to the same degree as in Spinosaurus. Could sexual selection have played a role here? What function elongated neural spines would have had at such stages, I don’t really know, but could the production of such big sails have resulted from one sex preferring a bigger “hump” or sail on a mate? Such sexual selection would allow for the growth of a sail over evolutionary time relatively quickly, and I certainly think it’s an avenue that should be considered when thinking about the evolution of sails in whatever group we might be talking about (especially if changes in the rest of the animal are relatively minor over time). Once sails got big enough, they could then be co-opted for heat gathering/loss purposes, thus having a double usage for the animals. All this is inference/hypothesis, however, and does not prove anything just because I can think of it. Still, fin-backed dinosaurs are amazing creatures, and fins seem one of those morphological attributes that has shown up over and over again in the history of life. I’m sure as we continue to dig and open up new areas for exploration, more of them will start coming out of the ground.

References:

Sasso, et al. “NEW INFORMATION ON THE SKULL OF THE ENIGMATIC THEROPOD SPINOSAURUS, WITH REMARKS ON ITS SIZE AND AFFINITIES.” Journal of Vertebrate Paleontology 25(4):888–896, December 2005

Sereno, et al. “A Long-Snouted Predatory Dinosaur from Africa and the Evolution of Spinosaurids.” Science, 13 November 1998

Smith, et al. “NEW INFORMATION REGARDING THE HOLOTYPE OF SPINOSAURUS AEGYPTIACUS STROMER, 1915.” J. Paleont., 80(2), 2006, pp. 400–406