Tails of Woe


Welcome back!  This will be a shorter article that continues the concern for tails that we established in the inaugural T. rex tail post a couple weeks ago.  There is an all too common error that artists make when they attempt to impart a sense of to dynamic motion to their dinosaurs - and in particular to the dromaeosaurs.  They flex the tail up at the base so sharply that it would break the tail...if not break the pelvis!

Tail Tales: Break Dancing

We all like lively dancing dinosaurs; after a century of seeing moribund dinosaurs in swamps it's understandable that modern artists want to convey the "awesomeness" of their subjects.  If you ask me it can go too far sometimes - animals don't live their lives at 90 miles an hour - but we can all grasp the excitement of making a dynamic composition.  One way to impart motion is to have the tail doing something dramatic.  Alas, enhancing your dancing dinosaur this way without considering the anatomy may lead to an image where the animal has its tail disarticulated, or worse.

I mentioned dromaeosaur images tend to be among the worse offenders, and I know some of you are thinking "I've read that dromaeosaur tails can flex upwards at a 90 degree angle at the base of the tail!!!".  And it's true, the first several tail vertebrae are modified in such a way as to provide an expanded degree of flexibility (for up and down motion...not so much side to side), which means they have the ability to tilt the tail up sharply, to intimidate a rival, or just to better fit on your piece of paper.  But it's important to note that this tilt up takes place over the course of several vertebrae, meaning it can't happen like this:


At least it can't happen more than once unless the animal has good health insurance.  Now I certainly don't want to pick on Chris Srnka here - he's a fine artist and a lot of people make this mistake - this was just a great image to demonstrate the problem.  If you look at the image in handy-dandy X-ray format (as provided by Photoshop.

That red arrow is the steepest possible angle the tail could take emerging from the pelvis.  Why?  Dinosaur pelves have many vertebrae built into the sacrum (adding vertebrae to the sacrum is actually one of the characters that define what is and what isn't a dinosaur).  That sacrum fuses together and to the pelvis in adults, but even in juveniles there are no moving parts involved.


Front is to the left, the tail would be to the right (Carpenter & Wilson, 2008)

See?  There's just nothing that could move, even hypothetically.  In the case of dromaeosaurs the tail flexes up by spreading that 80 or so degrees of motion over 6+ joints, so none is flexing more than 14 degrees.  Here is a diagram of it:


So no more dromaeosaurs with tails growing out of their sacrum, please.

This isn't just a dromaeosaur problem either.  Many artists try to arch the tail base up on dinosaurs who don't naturally do this, and in so doing end up disarticulating the tail (or breaking the sacrum).  Even

Greg Paul's early Daspletosaurus painting fell prey to this temptation.

(Daspletosaurus, copyright Greg Paul, image from here.)

The problem is a bit more subtle in this painting, but the line of the vertebral column should extend gently down from the pelvis, while in this case it is flexed up right at the sacrum/tail juncture.  This would require a 25-30 degree flexure right at the first tail vertebrae (or else some flexing of the sacrum), which isn't going to happen without making the animal wince in sharp pain and reach for some Advil poste haste - and tyrannosaurs have horribly adapted arms for taking pain killers.

There are some important exceptions here.  Many sauropods, stegosaurs, and hadrosaurs have a bit of an upwards arch naturally at the tail base as it exits the pelvis.  Obviously those should have a bit of an arch (how much depends on the species in question).  But for most other dinosaurs, an arch of that magnitude isn't possible that immediately after the pelvis.


Sauropods like Mamenchisaurus have a natural flex in the tail base...but it still happens after the pelvis!

So remember, dinosaur tails may be flexible (depending on the group), but they aren't silly putty.  The vertebrae still need to articulate, and any motion you put into the tail needs to start after the hips, as the sacrum just can't bend.

Till next time, don't create your own tails of woe!


Barsbold, R. (1983). Carnivorous dinosaurs from the Cretaceous of Mongolia, Transactions of the Joint Soviet-Mongolian Paleontological Expedition v19, pp 5–119.

Carpenter, K. & Wilson, Y. (2008) A New Species of Camptosaurus (Ornithopoda: Dinosauria) from the Morrison Formation (Upper Jurassic) of Dinosaur National Monument, Utah, and a Biomechanical Analysis of Its Forelimb, Annals of Carnegie Museum, v76 n4, pp 227-263.

T. rex: "Baby got back"

A few months ago Scott Persons and Phil Currie published a paper that clarified the shape of the tail of T. rex.  It's a paper I'm fond of, in part because I did a reconstruction for it.  No, I'm not going to post it here; it's already staring at you from the banner of this blog.

 In the paper, Persons and Currie did some comparative anatomical sleuthing to better understand the size and shape of the caudofemoralis muscle in tyrannosaurs.  For those of you who aren't familiar with the caudofemoralis muscle, it runs down much of the tail and inserts on the femur; when the muscle contracts, it pulls the leg back.  This makes it a Very Important Muscle (TM).  If you are adventurous (or just happen to live in Louisiana) it's worth noting that if you eat alligator meat, this is the part you usually end up eating.

Their paper demonstrated that many scientists and paleoartists have been underestimating the size of the caudofemoralis by a wide margin.  As a result artists were reconstructing the tail incorrectly.  Here for example is an Allosaurus (drawn bysome yahoo) that has a tail that is too skinny.

In that yahoo's defense, the image is almost 15 years old.  But you can still see many examples of this sort of tail reconstruction in paleo-art published today.

So how are we to fix it?  The P&C paper came with a lovely diagram showing the cross-sectional anatomy of an alligator tail:


(From Persons & Currie, 2010)

Unfortunately, alligators don't have tail vertebrae with the same proportions as a T. rex.  That's not really a surprise (alligators are aquatic after all) but it might make it more difficult to visualize a tyrannosaur tail in cross-section.  So without further ado, here is a quick visual reference on how to do this in tyrannosaurs:

The red muscle is the caudofemoralis.  It starts out fairly small, but as it gets closer to the base of the tail it expands greatly, pushing the other muscles in the tail out of the way in the process.  As it enters the leg (below most of the leg muscles) it joins another head of the caudofemoralis (yellow) that actually originates from a shelf on the upper hip bone (the illium).

This model of tail muscles probably applies to almost any dinosaur that doesn't have an absurdly reduced tail (birds, I'm looking at you!).  And the greater amount of muscle would better power dinosaurs into all of those gee-whiz activities people like to draw them in.  So I guess my advice to paleoartists in this case is: "cover your butt".


Persons, W., & Currie, P. (2010). The Tail of Tyrannosaurus: Reassessing the Size and Locomotive Importance of the M. caudofemoralis in Non-Avian Theropods. The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology DOI: 10.1002/ar.21290