As I write this the US premiere of Jurassic World is just around the corner. I had gone back and forth regarding this post given that we currently know very little about the film and as such the interpretations written about here and elsewhere may well be pointless by the time the film premieres.
Ultimately I decided to post this anyway since the overall thrust of the article should remain true regardless of how the film pans out.
Now there has been a lot of buzz around Jurassic World since it was first announced last year. The buzz has been mixed, but fairly positive. I suspect this was, in part, because everyone was happy to hear that the godawful military dinosaur idea was shelved in favour of a more “traditional” JP franchise storyline. Nonetheless the movie has still drawn its fair share of detractors, including myself. Most of the people who are unhappy with the film are either paleontologists, or hardcore dinophiles. Many of the problems leveled at the film have to do with the portrayal of the extinct animals. The problems are actually myriad ranging from pterosaurs capable of picking up humans using grasping feet, mosasaurs that are twice the size of blue whales, sauropods covered in elephant skin rather than scales (a problem not unique to Jurassic World), everything about Velociraptor, and of course Indominus rex. My biggest beef with the film is that the dinosaurs are not being shown as dinosaurs so much as monsters. However, after The Lost World: Jurassic Park came out it became pretty evident that Spielberg’s original vision of portraying dinosaurs as animals had been shelved in favour of the more entertainment-friendly movie monster approach. However, for what seems like a majority of the detractors, the biggest gripe with the film has to do with a lack of feathers on pretty much all the dinosaurs. This seems to be a common theme these days with a particularly vocal group of dinophiles and paleontologists strongly pushing for the feathering of every dinosaur in sight and insisting that all media that portrays scaly (erroneously called: “naked”) dinosaurs is inaccurate. Never mind the fact that a feathered, pack-hunting, 2 meter tall Velociraptor mongoliensis is still every bit as inaccurate as a scaly one.
Anyway, I digress. Dealing with the overwhelming amount of internet drama surrounding Jurassic World (and the media depiction of dinosaurs in general) is a topic for another day. My reason for writing this post is centered around one particular criticism that popped up a few weeks ago.
The scene in question is shown above. It is a still from the Jurassic World TV spot linked above. It features a variety of herbivorous dinosaurs wandering around the park as visitors shoot by on motorized hamster balls. It is a very short scene that lasts about two seconds before moving on, yet in that time frame so many dinophiles became outraged.
Why? Well, as one can plainly see in the image, that Stegosaurus is dragging its tail. This is obviously a throwback to the Victorian era version of dinosaurs as sluggish, cold-blooded animals. It’s a sign that the filmmakers cared more about nostalgia than accuracy, etc.
Remember, this was from a two second scene featured in a TV spot. Still, with even that little bit of information it’s apparent that this Stegosaurus is not dragging its tail. The animal is walking in tall grass. One can see that its feet are lower than its tail. In fact in the scene one can see the tail freely swinging back and forth. So no, this is not evidence of tail-dragging Victorian throwbacks.
What about the droop, though?
The droop is a more interesting question. The Stegosaurus in Jurassic World definitely has its tail lower to the ground than its previous incarnation (in The Lost World: Jurassic Park). Is the droopy tail a sign that the creators of Jurassic World were using outdated information for their dinosaur reconstructions?
The way I see it there are three possible scenarios for the droopy tail.
- The filmmakers cared more about nostalgia than accuracy and used an older version of Stegosaurus.
- Wu and his team genetically engineered “slower” dinosaurs to appeal to the older crowd and better emulate the public’s perception of dinosaurs.
- This particular Stegosaurus happened to have its tail low to the ground, and it’s simply a sign of anatomical and behavioural variation in the living animal.
I think option one can be thrown out. It’s possible that the droopy tail was made as an homage to the pre-Dinosaur Renaissance dinosaurs, but nothing else in the film suggests that would be true. Further, given the precedent set in The Lost World: Jurassic Park, it seems unlikely that Trevorrow would do this, or that it would be greenlit by Spielberg.
Option two has precedence in the canon as Wu explicitly states in Jurassic Park the book:
“Why not push ahead to make exactly the kind of dinosaur that we’d like to see? One that is more acceptable to visitors, and one that is easier for us to handle? A slower, more docile version for our park?”
Hammond frowned. “But then the dinosaurs wouldn’t be real.”
“But they’re not real now,” Wu said. “That’s what I’m trying to tell you. There isn’t any reality here.”
So it’s possible that the Stegosaurus shown in the scene above is actually representative of the “slower, more docile version” of the park. Unfortunately, if that is true, then it would mean the filmmakers are aiding in perpetuating the myth that cold-blooded animals are slow.
However, I suspect that it is the third option that is the most likely. The reason for that can be seen both later in the original TV spot (19 seconds in) where we see a Stegosaurus walking with a higher-raised tail, and in the original trailer for the film, which showed another Stegosaurus with its tail held to nearly the same height seen in The Lost World: Jurassic Park. That all three tail heights are seen in the same film strongly suggests that we are simply seeing behavioural variation, not a return to Victorian ideals of dinosaurs.
All of these interpretations, however, beg the question of whether these tail positions were anatomically possible. For that we need to go back a bit into the history of Stegosaurus and its reconstructions.
First discovered by Marsh (1877), Stegosaurus has had a long and sordid history in paleontology. Much of the controversy surrounding the animal had to do with plate arrangement (flat/vertical, one row/two rows, staggered/linear, dynamic/static, etc.), and brains. The infamous cliche about dinosaurs having brains the size of a walnut can be attributed to Stegosaurus. The controversy about brains stems from a sacral expansion of the neural canal that was once thought to house accessory ganglia referred to as a “second brain”. Tail controversies are much less common for Stegosaurus, with tail spike number and orientation being the biggest issues (Carpenter 1998, Carpenter & Galton 2001). Tail orientation itself has rarely come up.
Early restorations, notably Marsh (1891) and Lull (1910) featured a “tail dragging” version of Stegosaurus. I put tail dragging in quotation marks because for both mounts the tail is shown higher off the ground than the feet. Nonetheless one can assume that Marsh and Lull had intended to show the animal as a tail dragger, and indeed much of the fleshed-out reconstructions at the time reflected this (e.g., Charles Knight’s 1912 depiction, or Sir Ray Lankester’s 1905 depiction). Having the tail drag along the ground also influenced orientation of the tail spikes, resulting in most mounts showing the spikes running parallel to the vertebral spines. It’s worth noting that at no point were any of these reconstructions based on the anatomy of the animal. They appear to have just been chosen based on interpretations of dinosaurs at the time.
It’s not until the late eighties—in particular 1986—that we first see the high-raised tail in Stegosaurus. Where? Robert Bakker’s infamous Dinosaur Heresies. As far as I can determine Bakker was the first person (or at least the first paleontologist) to depict Stegosaurus with a high-raised tail (feel free to correct me in the comments if you know of an earlier version). Bakker’s Heresies was in many ways the critical turning point for the Dinosaur Renaissance. With a new interest in portraying dinosaurs as highly active animals, post 1986 depictions of Stegosaurus largely showed the animal in that “Bakkeresque” pose with its tail shooting ramrod straight off the hips.
The funny thing is Bakker never offered any anatomical justification for his restoration. In fact there was no justification for this new positioning of the tail in Stegosaurus until 1998 when Ken Carpenter re-analyzed the skeleton of Stegosaurus. That means that every depiction of Stegosaurus with a high-raised tail prior to that moment was all conjectural. Note: that includes 1997’s The Lost World: Jurassic Park.
So what did Carpenter find? In his 1998 paper, Ken Carpenter talked about tail position in relation to spike arrangement in Stegosaurus stenops. The reason for the reinterpretation came about during a remounting of the Denver Museum of Nature and Science’s Kessler Stegosaurus (DMNH 1438). Carpenter examined the connection between the terminal sacral vertebra and the first caudal vertebra (the sacrocaudal joint). He noticed that the caudal face of the centrum of the terminal sacral vertebra was deflected dorsally whereas the centrum of the first caudal was longer on its ventral side than its dorsal side. Taken together these results suggested that the base of the tail arced upward from the sacrum. Carpenter pointed out that his reasoning followed Gilmore’s reasoning for reconstructing the tail of Diplodocus carnegii (Gilmore 1932). So from the base the first caudal vertebra was angled away from the plane of the sacrum. This orientation of the sacrocaudal joint is similar to what is observed in extant terrestrial lizards such as iguanas and varanids. In fact, when Gilmore (1932) was reinterpreting the tail in D. carnegii he made mention of this similarity.
The upward curvature of the tail in the sauropodous dinosaurs bears a striking resemblance to that of the large extant lizard Varanus komodoensis.
Now that’s the base of the tail. What of the rest? Well, according to Carpenter (1998):
With the rest of the caudal vertebrae articulated so their centra faces are parallel, the tail is almost at the level of the sacrum throughout its length.
Carpenter placed the tail of Stegosaurus stenops in its osteological neutral pose (ONP). Much has been written about ONP in dinosaurs (Stevens and Parrish 1999, 2005; Taylor et al. 2009; Taylor 2014), but all of that has focused on the neck vertebrae of sauropods. One of the key questions that has come about from studies on ONP is how important it is to the animals themselves. In others words is the osteological neutral pose a pose that animals habitually keep their vertebrae in? Getting ONP in fossils can be difficult as taphonomic distortion can really screw up the alignment of the vertebrae, such as the zigzagging of the spinal cord that resulted from arranging the caudal vertebrae of Kentrosaurus aethiopicus in ONP (Mallison 2010). Orientation of the sacrum can have huge effects on interpretations of tail orientation based on ONP as Coombs (1995) showed for Euoplocephalus (downward deflected sacra of 30° result in tail dragging for ONP tails).
Judging from the vast array of posts from the SVPOW crowd, along with the aforementioned papers, it would seem that ONP may not mean as much as initial interpretations would have us believe. Further, given that tails don’t need to acquire food, nor do they act as a hub for all the senses in the body, we should expect to see even more freedom in movement for these structures.
Carpenter offered other evidence for a ramrod straight tail in Stegosaurus, including the presence of bifid neural spines (seen in the image below) and the length of plate bases over the tail. I’m not sure how useful the latter bit of evidence is as we don’t really have any animals alive today that have large, vertically projecting osteoderms jutting out of their backs. The plates in stegosaurs were embedded in the skin making it difficult to determine how well they should match up with the underlying vertebrae. The bifid neural spines would have provided extra areas of attachment for the m. transversospinalis, which are intrinsic back/tail muscles. They would have functioned in maintaining intervertebral integrity. Interspinous and supraspinous ligaments would have attached along the top of the neural spines as they do in extant amniotes (Tsuihiji 2004, Schwarz-Wings 2009). These ligaments have been associated with passive support for the vertebrae, and Carpenter argued similar for their function in keeping the tail held out straight. However, bifurcation of the neural spines seem more in line with expansion of the m. transversospinalis muscles (Wedel and Taylor 2013), which would not offer passive support. Increased muscle mass between the vertebrae, and a laterally offset intervertebral ligament system instead suggest that the tail was enhanced in favour of increased flexion, with the offset intervertebral ligaments acting as springs to bring the tail back in line after lateral flexion. This would aid in tail swinging, which Carpenter (1998) acknowledged could have been an alternative/complementary use of these structures.
The centra in Stegosaurus are amphiplatyan, which means both articular ends of the centrum are flat, or flattish surfaces. Humans have this same type of vertebrae too. On the outset they appear to be rather restrictive in their movements compared to the ball and socket arrangement seen in most reptilian vertebrae. Indeed, Gilmore (1914) even says as much for Stegosaurus:
…the caudal vertebrae are joined by closely fitting zygapophyses, which are present nearly to the distal end of the tail; moreover, the articular ends of the centra are rather abruptly truncated, not rounded or beveled, as in those animals having flexible tail. These structural details, combined with the series of plates along the dorsal side, must have made the tail of Stegosaurus a heavy, stiff appendage, incapable of more than cumbersome lateral movements and wholly unsuited for use on an active enemy.
Yeah, you read that right. Gilmore did not think that Stegosaurus did much to defend itself with its tail. New data since then have strongly suggested that Stegosaurus spikes were routinely used in defense (Carpenter 1998, Carpenter et al. 2005). Ah, but what of the seemingly stiff tail vertebrae? Recent work presented as last year’s Society of Vertebrate Paleontology meeting (Fronimos & Wilson 2014) suggest that the ball and socket arrangement may have evolved—counterintuitively—to better hold vertebrae together. Indeed, as the authors of that study pointed out in their alligator example the most flexible part of the crocodylian body (the tail) is actually composed of fairly flat-sided vertebrae in the last 2/3rds of the tail.
The centra in the tail of Stegosaurus are fairly short proximally, but gradually elongate towards the tip, a trait that is actually pretty common among reptiles. Keeping the vertebrae aligned with each other is simple enough as long as the prezygapophyses of one vertebra overlap the postzygapophyses of the previous vertebra. The amount of overlap provided by the zygapophyses gives us an idea of how much displacement can be put on each individual vertebra. As I mentioned above, Carpenter found that aligning all the vertebrae to their centra produced that ramrod straight tail. In contrast, Marsh and Lull kept the zygapophyses overlapping in their droopy tail reconstructions. So unlike the Bernissart Iguanodon—which had their tails broken to achieve the stance in their mounts (Norman 1980)—giving Stegosaurus a droopy tail does seem to be anatomically possible.
Okay, so it’s possible, but is it probable?
For that we need to look into the biomechanics of droopy vs. raised tails. As a reptile, the tail in Stegosaurus would have been a functional fifth limb. The muscles responsible for retracting the hind legs (m. caudofemoralis longus and brevis) attach to the proximal part of the tail. Like all muscles, the caudofemoralis muscle complex can only ever pull. To get maximum force and contraction from the muscle the distance between origin and insertion should be as straight as possible. The best way to achieve this is to hold that portion of the tail that houses the caudofemoralis complex as straight as possible. So how far did the m caudofemoralis longus stretch in Stegosaurus? In extant reptiles the m. caudofemoralis longus attaches to the underside of the transverse processes and the lateral faces of the haemal spines/chevrons (Persons & Currie 2011). The caudofemoralis complex eventually tapers off and terminates long prior to the end of the tail. Although not universal, one can use the point in which the transverse processes disappear (or descend from dorsolateral to lateral) as a starting proxy for the extent of the caudofemoralis (to be more exact we would need to look for evidence of the intermuscular septum scar on the chevrons, which is out of the scope of this blog post). For Stegosaurus this would suggest that the caudofemoralis complex extended for about the first 1/3rd of the tail. So for the most effective hind limb force production the first 1/3rd of the tail should be held straight. That still leaves 2/3rds of the tail free to droop if need be. Turning once more to the extant realm we see this type of tail extension during walking/trotting in most lizards such as the Komodo dragon in this video (2 minute mark).
Note how the tail does start to droop once the portion of the tail containing the caudofemoralis complex is gone. It was likely that Stegosaurus and other quadrupedal dinosaurs did similar things with their tails. The higher stance of dinosaurs compared to squamates would have resulted in the droopy parts of the tails rarely touching the ground, or maybe even lightly dragging. If they did lightly drag these marks would be unlikely to preserve, as data from extant Komodo dragon trackways have shown (Padian & Olsen 1984). Keeping the tail off the ground when moving is also useful for reducing drag force created by the friction between the resting tail and the ground. Willey et al. (2004) showed the effects of “tail braking” on locomotion in alligators. It would seem likely that many/most dinosaurs would have had their tails well off the ground when walking, or at least have the heaviest parts off the ground to reduce this tail braking effect. Again, behavioural studies of extant terrestrial reptiles seem to bear this out (e.g., Irschick and Jayne 1999).
So, biomechanically, there is a good reason for Stegosaurus to have held its tail straight. However, this reason is only valid during the parts of its life where it was moving its body from one area to the next (i.e., walking, trotting, running, tiptoeing through the tulips, etc.). If Stegosaurus was just standing around grazing there would be no biomechanical requirement for the tail to be held in the air. This holds true for most dinosaurs (theropods are the big exception since their tails were also acting as counterbalances to their torsos). It’s a thing that I think a lot of paleo art tends to gloss over. Animals that are just standing around are likely to pick the most energetically “cheap” body positions, and that probably meant tails that drooped on the ground. Once again we would be unlikely to see evidence of this in the fossil record since trackways almost always depict animals on the move where having a tail held high is important.
So just to wrap things up:
- The Jurassic World promo material does not show a tail-dragging Stegosaurus
- Tail droop and occasional drag are not impossible anatomical positions for Stegosaurus
- As animals we should expect to see dinosaurs holding their tails in a variety of positions afforded to them by their anatomy
With all that in mind I will commend Jurassic World for showing off dinosaurs acting like animals (in at least some scenes). A separate scene in the trailer even showed a Triceratops lazily lounging about before getting up to move off. I think that these views of dinosaurs are useful as they remind us that these were not mythical monsters from a bygone age, but living animals that were doing more than screaming around the Mesozoic with their mouths open all the time.
As an aside for anyone interested, I suspect that Jurassic World will wind up as a fun splash in the pan summer blockbuster. It will have a sizeable opening weekend, coming in at number one at the box office, followed by a precipitous decline with each successive week. I predict that it will top out around $150–175M domestic gross. I would be genuinely surprised if it was able to crest over $200M. The days of the JP franchise being a huge moneymaker are long past us. [Editors note: Well colour me surprised.]