Changing the Face of T. Rex’s Rear-end: A Guide to Illustrating Theropod Tails
by W. Scott Persons
Art is a lie that helps us realize truth. -- Picasso
Smocks and Lab Coats Should Be Friends
Appearing in this month’s edition of The Anatomical Record is a paper written by myself and my graduate advisor, Dr. Phil Currie. The paper offers a new synthesis and new data on the tail musculature of theropod (the two-legged and mostly carnivorous dinosaurs, like T.rex). A key point the paper makes is that most predatory dinosaurs had robust muscular tails, similar to those of modern reptiles. One muscle in particular, the M. caudofemoralis, appears tohave been supersized in most theropods (though not in deinonychosaurs and modern birds). The M. caudofemoralis is an unusual tail muscle. The skeletal component of a tail is an extension of the vertebral column, and in most theropods the M. caudofemoralis was anchored to the first 15-20 tail vertebrae. Moreover, the M. caudofemoralis was also attached, via a long tendon, to the femur (the upper leg bone). When it contracted, the M. caudofemoralis would have pulled on the leg and swung it backwards. This backwards pull appears to have provided the major oomph in atheropod’s locomotive power stroke. So, a bigger M. caudofemoralis indicates a bigger locomotive oomph and supports higher running speeds, increased endurance, and all aroundgreater athleticism for Tyrannosaurus and its kin. (For more on the M. caudofemoralis checkouta guest post I did for David Hone’s Archosaur Musings back in December: http://archosaurmusings.wordpress.com/2010/12/06/guest-post-bulking-up-the-back-end-why-tyrannosaurus-tail-mass-matters/)
Image 1. A photo of the author measuring the tail of a Gorgosaurus at the Royal Tyrrell Museum of Palaeontology.
The paper in The Anatomical Record discusses the methods used in making thisdiscovery and its implications for theropod physiology, but the final point Dr. Currie and I chose to make was how this new information ought to change the way theropod tails are illustrated.
When Craig Dylke approached me with the idea of doing this guest post and sharing the implications of my research with the paleo-art community, I was delighted at the chance.
Applying the results of new paleontological research to the way we visualize and depict prehistoric life is an important final step in the scientific process, and it is one that is often neglected. For me, paleontology’s ultimate goal is to give our imaginations the power to travel back in time. If that sounds whimsical, you have misunderstood my meaning. Our goal should not be to imagine a fantasy. We should aim to form a vision of the various worlds of prehistory with as much accuracy as possible, using all available evidence as a strict guide . . . and that is
exactly what good paleo-art attempts.
From an educational perspective, it is good practice for paleontologists to make a point of keeping artists informed of new research developments and their implications for the appearance and behavior of prehistoric life (and we should state and explain these implications directly without forcing artists to figure them out themselves). For better or worse, the way the general public thinks about dinosaurs is not directly influenced by scientists, or even by popular sciencewriters. Most people have not read a dinosaur book since they were eight, and even the vast majority of the interested public has never read page one of a scientific research paper. But they have all seen dinosaur art, and it is primarily on art that the public’s conception of dinosaurs is based. So, if you want to improve the public’s understanding, improving paleo-art is the way to do it.
Tail Anatomy 101
Image 2. The tail of Coelophysis.
In most theropods, the tail accounted for over half the animal’s length, so getting its appearance right is non trivial. Before diving into how theropod tails looked, we should briefly cover the skeletal and muscular components of the tail. The skeleton inside most dinosaur tails was made of two kinds of bones: vertebrae and chevrons (see Image 2). The vertebrae were aligned end-to-end all the way down the tail and had four major parts, which are diagramed in Image 3. Chevrons are usually (but not always) narrow wedge-shaped bones. In theropods, the chevrons were positioned below the vertebrae with the upper surface of each chevron straddling two vertebrae and the lower tip of each chevron projecting downward. The skeleton of the tail serves three major functions: it protects the spinal nerve cord, it provides rigidity, and it gives muscles a solid framework for attachment.
Image 3. A vertical cross-section through a typical reptilian tail. Together a vertebra and chevron have a shape like an obese lowercase ‘t’.
Naturally, figuring out what the skeletal portion of theropod tails looked like is the easy part. We can accomplish that just by looking at the fossilized bones. But fossilized muscles are extremely rare. Fortunately, bones do sometimes preserve a record of the muscles that attached to them in the form of surface attachment scares. Attachment scars (see Image 4) may be left both by muscles and muscle septa (thin tissue layers that separate muscles). By combining bone surface observations and knowledge of the muscle arrangement patterns seen in modern animals(thankfully, tail muscles do have a reasonably consistent arrangement across terrestrial vertebrates), it is possible to confidently reconstruct the tail musculature of theropods.
Image 4. Close-up on the tail of Ornithomimus.
Arrows indicate the prominent attachment scar left by the septum that separated the M. caudofemoralis and the M. ilio-ischiocaudalis.
I’ll give you the basics. Tail muscles can be divided into two groups: epaxial (those positioned above the caudal ribs) and hypaxial (those positioned below the caudal ribs) (see Image 3). The hypaxial muscles include the M. caudofemoralis and the M. ilio-ischiocaudalis,and understanding the shape and arrangement of these two muscles is critical to accurately illustrating theropod tails. At the base of the tail (near the hips) the M. caudofemoralis is nestled against the centrums and chevrons and it bulges out laterally from the side of the tail (the way our calf muscle bulges out from the back of our lower leg). But (unlike our calfmuscle) the M. caudofemoralis is covered by another muscle: the M. ilio-ischiocaudalis.The M. ilio-ischiocaudalis attaches to the undersurface of the caudal ribs, raps around the M.caudofemoralis, and attaches to the undersurface of the lower tips of the chevrons. The epaxialmuscles and the M. ilio-ischiocaudalis are continuous all the way to the tip of the tail, but, as mentioned previously, the M. caudofemoralis is not. The M. caudofemoralis usually only extends across a third of the tail, and, as it tapers out, the M. ilio-ischiocaudalis takes over its attachment sites on the vertebrae and chevrons (see Image 5).
Image 5. Top-down view of Tyrannosaurus, with M. caudofemoralis muscles restored and sideview of Tyrannosaurus with all tail muscles restored. (Original skeleton illustration by Greg Paul.)
Finding a Model
In trying to accurately illustrate the tail of a theropod it would be helpful to have a living model or two that could serve as a guide for the correct overall shape. Birds are often the best modern analogue for theropods (after all, as their descendants, birds are technically a group of highly specialized theropods). However, due to the need to reduce weight that is imposed by flight, modern birds only have a short series of tail vertebrae and greatly reduced tail muscles. Multi-ton terrestrial mammals are often another reasonable analogue, but the likes of elephants and rhinos all have relatively tiny, fly-swatter tails. The best tail models for theropod illustrators are the distantly related lizards and (if we remember to ignore the rows of scutes) the more closely related crocodilians. Modern reptiles do have the same basic tail anatomy as dinosaurs. So, when you set to work on the back half of your theropod sketch, take a moment to study a photo of a croc or Komodo dragon.
Image 6. Rear-view of a Komodo dragon.
A word of caution: theropod dinosaurs were a unique form of life, and, as with any analog, modern reptile tails have their limitations. The biggest difference between the average theropod tail and the tails of modern reptiles has to do with the position of the caudal ribs on the basal vertebrae. In reptiles, the caudal ribs tend to be low on the vertebrae, but in most theropods the caudal ribs are positioned higher and may be angled upwards (see Image 7). This means that
the hypaxial tail musculature (particularly the M. caudofemoralis) was relatively more massive in theropods.
Image 7. Comparison between a tail vertebra of an Allosaurus (A) and an Alligator (B). Arrowspoint to the caudal ribs.
Common Mistakes Art involves a lot of imitation, and mistakes made by one artist have a tendency to be copied and perpetuated by others. I’d like to point out three of the most common mistakes madeby paleo-artists when illustrating theropod tails. The artwork I will use as examples are all done by paleo-artists that I have tremendous respect for. Using examples from illustrators who don’t care about accuracy would be far less instructive, and these artists all put great care into their work and obviously try to get the anatomy of their dinosaurs as correct as possible (some of the artists are also accomplished scientists).
1. Where’s the Beef?
Image 8. Top-down view of Tyrannosaurus dorsal silhouette with traditionally thin tail (A) vs. modern Alligator dorsal silhouette with beefy tail (B).
In the wake of the Dinosaur Renaissance, it has become fashionable to depict theropods as dynamic and agile animals (as well it should), but, when it comes to the tail, this fashion has committed an anatomical faux pas. Theropods are often illustrated with tails that are thin and laterally compressed at the base. I suspect this is because such tails appear less bulky, more aerodynamic, and better suited for running. Take a look at the tail of a modern crocodilian or terrestrial lizard and you will see just the opposite. These reptiles have robust tails that budge out past the hips. Sometimes this bulge can be exaggerated by fat deposits(which I suspect theropods mostly lacked), but the majority of this tail bulge is the result ofthe large M. caudofemoralis. With an even larger M. caudofemoralis, most theropods should be depicted with even beefier tails. Ironically, although slim tails may look superficiallyfaster, because the M. caudofemoralis is the primary hind limb retractor muscle, reducing tail girth would have a negative impact of theropod athleticism. If you remember nothing else from reading this, remember that theropod tails should be beefy!
Image 9. Two passing tyrannosaurs depicted with extremely narrow tails. Original image by Greg Paul.
Image 10. A crocodile correctly painted with a tail broader than its hips and a youngAllosaurus incorrectly painted with a tail slimmer than its hips. Original image by James Gurney.
2. Muscle Contours
Image 11. Carcharodontosaurus and Deltadromeus with contours highlighting fictitious tailmuscle arrangements. Original image by Mark Hallett.
Including contour lines that show off underlying muscles can add credibility to an illustration. We may look at it and think: “Wow, that artist must really know his/her anatomy.” But, when it comes to the tail, most artists have just been making stuff up and throwing in contours for muscles that don’t exist. If you look at the tails of most healthy reptiles (the highly specialized tails of chameleons are an interesting exception) you will see that boundaries between individual muscles are usually not visible from the outside. In some reptiles, a single muscle contour can be seen running between the epaxial and hypaxial muscles (see Image 12), so it is not unreasonable to include this in some theropod illustrations (but remember the contour should be faint and relatively higher on the tail).
Image 12. Photo of a young green iguana. Arrow points to the single muscle contour linebetween the epaxial and hypaxial muscles.
3. M. caudofemoralis Sandwich
The M. caudofemoralis has a unique tapering shape and was exceptionally massive in theropods, but it was covered by the M. ilio-ischiocaudalis. Often, the M. caudofemoralis is depicted as a muscular hill sandwiched between two flat muscle zones. Details of the shape of the M. caudofemoralis would not be visible without x-ray vision. Think of a hot dog. The sausage affects the girth of the hot dog, but, looking at it from the side, the bun obscures most details of the sausage’s shape.
Image 13. Aerosteon and Gorgosaurus with robust M. caudofemoralis distinctly visible between two improperly flat muscle zones. Original images by Todd Marshall and Scott Hartman respectively.
To sum up: the tails of most theropods likely resembled those of modern reptiles, with relatively larger hypaxial muscles (and relatively smaller epaxial muscles). At the base, the tails of most theropods would have been as broad or broader laterally as they were tall. At and near the point where the M. caudofemoralis tapered out, the tails would be laterally compressed, and towards the posterior tip, the tails would, as the neural spines and chevrons steadily shrunk, return to being roughly round in cross-section. I would like to conclude by sharing two illustrations created in conjunctionwith the theropod tail research study. One is by Scott Hartman (check-out his web site:http://www.skeletaldrawing.com/), and the other is by Lida Xing (who’s art is also featured in the latest edition of National Geographic). I hope these images will serve as guides for your own illustrations, and I look forward to seeing beefier and better theropod tails in the future.
Image 14. Tyrannosaurus by Scott Hartman with a well-rounded tail.
Image 15. Tyrannosaurus by Lida Xing with a slightly less robust (but still plausibly proportioned) tail.