After the success of The Lost World: Jurassic Park, the minds behind the franchise were in a bit of a fix. Tyrannosaurus and “Velociraptor”* had the run of the fictional islands for the past two movies; something new was needed to up the ante. The decision was made to make Spinosaurus the new villain in Jurassic Park III, the strange theropod being said to possess an 8-foot-long skull and stretch 60 feet from the tip of its crocodile-like snout to the end of its tail by the creative team. As if this weren’t enough, the writers of the film had Spinosaurus kick the tar of of Tyrannosaurus, trying to drive the point home that Spinosaurus was the biggest, baddest dinosaur that ever lived. But was it really?
* I put Velociraptor in quotes as the film version is really meant to be Deinonychus, although the mistake has never been corrected in the films. This messy attitude towards taxonomy can make things confusing for the public, especially when Jack Horner said in an interview that skulls of Velociraptor have only recently been discovered. This is blatantly false. While the skull of Deinonychus has undergone some changes based upon new discoveries of fragmentary material and recently discovered relatives, the complete skull of Velociraptor has been known since 1924.
With dinosaurs there is often a fascination with the biggest, the fiercest, and the weirdest, and for about 80 years Tyrannosaurus was the carnivorous dinosaur, the living terror of the Cretaceous. Around the turn of the 21st century, however, numerous reports appeared about theropods from Africa and South America that may have been ever bigger, but the question of what “bigger” means is a bit hazy. Are we talking about length? Height? Weight? As with the largest of the sauropod dinosaurs, which dinosaurs is the “biggest” may depend on what sort of information you choose, leading to a sort of “My dinosaur is bigger than yours” contest.
Lets start off with figuring out just how big Spinosaurus was and what evidence is available to figure this out. First described by the German paleontologist Ernst Stromer in 1915, the original material primarily consisting of part of the maxilla, a lower jaw, and a number of vertebrae bearing elongated neural spines, the dinosaur bearing a “sail” on its back. Unfortunately this material was destroyed in 1944 during an Allied bombing raid on Germany, although Spinosaurus lived on in many popular books, often portrayed as a kind of tall-spined Allosaurus.
In 1996, another Spinosaurus specimen turned up, the fossils leading paleontologist Dale Russell to erect a new species (Spinosaurus marocannus), although the 1996 material probably belongs to the species Spinosaurus aegyptiacus established by Stromer. The skull was estimated to be a little more than four and a half feet long, a little bit shorter than the estimate given for the original Spinosaurus material. In 2005, however, a new paper appeared in the Journal of Vertebrate Paleontology (Dal Sasso, et al 2005) describing a much more intact portion of the skull (containing the premaxillae, maxillae, nasals, and what appears to be part of a small crest), the size estimation for the skull of this specimen being about five and 3/4 feet in length.
What does this mean for the Spinosaurus in Jurassic Park III? The film appeared in theaters in 2001, requiring several years of preparation. The largest skull portion, the one described in 2005, was discovered in 1975 but was in a private collection until 2002, so it is doubtful that it provided the basis for the JP III dinosaur. Even if it did, the actual estimated skull length falls more than two feet short of the eight-foot measurement Jack Horner gives in an interview on the special features section of the DVD, and it seems as if the size of Spinosaurus was pumped up to make it seem even more impressive.
Even if the Hollywood version of Spinosaurus was super-sized, it was still among the largest of the theropod dinosaurs, its main competition being Tyrannosaurus, Giganotosaurus, and (although not typically mentioned, probably because it wasn’t a terrifying predator) Therizinosaurus. Determining which was the largest, however, has been a bit of a problem. Tyrannosaurus is known from a number of skeletons that have provided paleontologists with an excellent idea of what the whole animal looked like, even at various times during the life of the animal. With other theropods we aren’t so lucky, specimens being less numerous and more fragmentary (although this will hopefully be subject to change as the years roll on). A study published by Therrien and Henderson in 2007, however, suggests that skull length and body length in theropods is tightly correlated; as theropods become larger their skulls become proportionally longer compared to their body size.
The problem is that not all theropods were feeding in the same ways, and the construction of their skulls could vary. Spinosaurids, for example, probably had proportionally longer skull for their body size due to their fish-eating habits, and this must be taken into account. Indeed, estimating overall body size for these dinosaurs can be difficult when the back of the skull (the are containing the orbit and the various fenestra) are unknown. In spinosaurids the back of the skull may be flatter, taller, and longer, and the slight differences impacting estimates of overall body size. (Likewise, if the skull of Therizinosaurus was anything like that of its smaller relatives, it could throw a monkey wrench into size estimates as it would have a comparatively tiny skull for its body size and potentially break the correlation between skull length and body length.)
We’ll leave Therizinosaurus to the side, however, and concentrate on the big, bad killers like Tyrannosaurus, Giganotosaurus, Carcharodontosaurus, and Spinosaurus. Although claims as to the “biggest” will likely persist continue, all of these animals were approximately 42 feet long and may have weighed about 14 tons. Some were slightly longer and heavier than others (i.e. Giganotosaurus and Carcharodontosaurus may have been a little longer and heavier than Tyrannosaurus), but they could all be grouped in the same heavyweight size class. On several continents, then, predatory theropods independently arrived at about the same size, suggesting that they represent the “upper limit” of size and weight for primarily carnivorous, bipedal dinosaurs. Perhaps they could have become even larger if they became quadrupeds, but this would have required drastic changes to their morphology and behavior, and the variation to initiate such an evolutionary shift did not arise among predatory dinosaurs with big, heavy heads and small forelimbs (nor do I see much reason why it should).
Rather than quibbling over which theropod was the biggest, it’s important to recognize that there were several “ways” to be a large predatory dinosaur. Although all similar in size, Tyrannosaurus more than likely had very different feeding habits than Spinosaurus, and while Giganotosaurus may be closer to Tyrannosaurus in shape, it was dealing with a different group of prey animals than the North American “Tyrant King.” Still, the reasons why Spinosaurus appears to be so large is somewhat vexing. For Tyrannosaurus and Giganotosaurus, an evolutionary arms-race where predators and prey keep getting larger seems intuitive, but why would an animal that seems well-adapted to eat fish get to be so big?
I have little doubt that Spinosaurus was not a strict piscivore, but perhaps its size has something to do with partitioning of prey resources and competition with abelisauroids and carcharodontosaurids, the three types of theropods often found in close association to one another in Africa and possibly elsewhere. The real answer still awaits further analysis, but the varying impacts of convergence and contingency in these large predatory dinosaurs is exceedingly fascinating.
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 Size and Affinities.” Journal of Vertebrate Paleontology, Vol. 25 (4),pp. 888-896
Therrien, F.; Henderson, D.M. (2007) “My Theropod is Bigger Than Yours … Or Not: Estimating Body Size From Skull Length in Theropods.” Journal of Vertebrate Paleontology, Vol. 27 (1), pp. 108-115