Skeletal reconstructions: Schematic vs Realistic

Above you see two skeletal reconstructions of the basal sauropodomorph Panphagia.  The one on the bottom was published with the original description, while the one on top I just finished a couple days ago.  What is worth noting is how extremely different they are.

Some of the differences can be chalked up to errors in scaling (compare the length of the tails).  Other differences stem from the underlying anatomical assumptions, such as how rib cages articulate and the placement of the shoulder blade (definitely topics for future posts).

But if you look closely you'll see that the very shapes of the bones differ.  And in every case where the known bones differ in shape it is the skeletal that was published in the original description that is inaccurate. This strikes me as odd, since the bones themselves were figured correctly in the same paper.*

At least that's odd if you assume that the skeletal reconstruction on the bottom is meant to be a literal representation of the bones, when in fact it appears to be a schematic skeletal diagram.

What, you may ask, is a schematic diagram? Here's the Wikipedia definition: 

A schematic diagram represents the elements of a system using abstract, graphic symbols rather than realistic pictures. A schematic usually omits all details that are not relevant to the information the schematic is intended to convey, and may add unrealistic elements that aid comprehension.

The artists among you may have just suffered an involuntary shiver.  And you should; as detailed in my three part series on the history of skeletal reconstructions, it has been the norm rather than the exception for peer-reviewed papers to publish schematic skeletal diagrams.  That is, rather than attempt to represent the bones exactly as they were in life, the skeletal emphasizes key features of the anatomy (sometimes literally "emphasizing" them) while perhaps demonstrating other key information, such as which bones are preserved.  This is actually a common practice across the sciences.  Any of you remember reading about cell structures in your high school biology class?  Those illustrations of cells are not intended to be realistic, they're meant to convey the salient information effectively to the reader so you can learn the parts.

Even within the very papers we eagerly search through to discover those skeletal drawings there's a universal type of schematic image: the stratigraphic column.

No one mistakes these for literal representations of the outcrop.  No artist would think that they could translate this image into a lovely landscape illustration that would match what you would see if you traveled to the dig site.  This sort of pictorial short-hand is both common and necessary in science.

As a scientist you want your diagrams to convey the important information, whether it's the placement of the specimen within its local stratigraphy, or the major anatomical characters and the completeness of a specimen.  That data is generally seen as more vital to a description than whether the diagram is showing the information necessary for an artist to use.

That's not to say that paleontologists don't want their published skeletal diagrams to be realistic, it's just frequently not the top concern.  But the way people view skeletal reconstructions has a profound impact on how we view dinosaurs.

Over the last two decades Greg Paul's skeletal reconstructions redefined our expectations of skeletal drawings, and the consistency with which he produced them has (in)famously refined and narrowed the range of depictions of dinosaurs, a fact that Greg Paul himself has recently lamented. As we enter a sort of Post Paulian Period, one of the lasting expectations of laymen, artists, and even many scientists is that skeletal reconstructions should by default be seen as realistic portrayals of extinct animals; a sort of virtual x-ray of extinct life forms.

And why not?  Greg Paul may not want people to base their paleoart on his skeletals, but he's produced more skeletal reconstructions than anyone else, and he obviously intends his skeletals to be realistic depictions of extinct animals, right?  

Right?!!!

To be frank, the answer is both yes and no.  Any line illustration is necessarily somewhat schematic, since you are sacrificing details for the clarity of solid lines.  Greg Paul himself documents some of the areas where his skeletals are more schematic than literal (interested readers are directed to pages 226-228 of Predatory Dinosaurs of the World).  Among the shortcuts Greg takes are simplifying the gastralia basket (something I do as well) and rendering the limbs in side view as if they are in a perfect parasagittal plane, even though in life the elbows and knees would bow out.

Does that mean that paleo artists need to throw their arms up in the air and quit?  Not at all.  We may  not be able to establish a black and white dichotomy between schematic and realistic skeletal drawings, but there are clear differences in practice.  The short cuts Greg takes are intended to balance saving him time while having a minimal impact on how realistic the skeletal is overall.

As an example I offer up my experience when describing Supersaurus with my colleagues; not surprisingly I was working on a skeletal reconstruction of the critter as part of my contribution.  As with all of my skeletal reconstructions, I wanted the supersaur skeletal to be an accurate and realistic representation of the animal, not a purely schematic one.  Alas, Supersaurus is only known from two specimens, and both of them leave a lot to be desired in the category of completeness.  This made the process a lot more difficult than it is when restoring an animal known from more complete remains.  But all was not lost.  We spent a lot of time evaluating which species were best suited to pattern missing parts after, and did lots (and lots) of cross-scaling; not just of individual bones, but also in proportional relationships.  That is to say I was able to constrain the unknown portions of the animal from comparative and phylogenetic data.  The upshot?  Have a look at this:

What is particularly noteworthy is that the two distal vertebrae were not known when I first made this diagram.  We only had ~10% of the tail, but with careful scaling and proper selection of taxa to model the bones on, I didn't have to make any changes to the skeletal after the additional bones were excavated and prepared.  

Obviously things don't always work out this well - sometimes it's less clear which taxa should be used to constrain missing elements, or an animal might have truly novel proportions.  But missing data and margins of error are simply a fact of life in paleontology.  What I hope is clear is that regardless of the possibility for error, this skeletal is intended to be a realistic portrayal of the animal, not a purely schematic one.

While the degree to which a skeletal drawing is schematic or realistic isn't always black and white, the impact they have on paleo art is.  When an artist bases a life reconstruction on a skeletal they need to have the real proportions and shape of the animal, which are just not available in schematic skeletal diagrams.  And it's not just art - for better or worse researchers sometimes cull data such as relative limb proportions from published skeletal drawings.  If, for example, a researcher was interested in comparing relative tail lengths in basal dinosaurs, it would matter very much which skeletal of Panphagia was evaluated.

For some dinosaurs only a single skeletal reconstruction exists.  Sometimes when when there are two or more they are quite different.  So how is one to know which (if any) of the skeletals are meant to be more literal, and which are schematic?  This brings me to the main issue of the day:

We need more transparency in skeletal drawing labels!

The two drawings of Panphagia at the beginning of this post are quite different (especially if you are trying to draw the beast), yet they both convey accurate and useful information.  They just aren't intended to convey the same types of information (although there is overlap).  When writing a professional paper, which one of these styles is "better" depends on the needs of the authors, the time, ability, and access to the data that the illustrator has, and a host of other practical concerns.  Far be it from any of us to dictate that one type of skeletal diagram is suitable in all cases.

What is needed is for authors (and their scientific illustrators) to label their skeletal diagrams more precisely.  Skeletal drawings are not like the schematic diagrams of stratigraphic columns, as there are large bodies of published skeletal diagrams that are intended to be realistic portrayals.  Because there are multiple visually similar types of skeletal diagrams, we need proper labeling so that the viewer's expectations match the authors' intent.

Proper labeling is a basic part of science; papers are rightly rejected for not documenting the confidence interval in a study, or for failing to be precise in the use of significant figures.  In fact most of these practices become second nature long before getting a graduate degree.  So why not with skeletal drawings?

There actually are a few stumbling blocks.  For one, there aren't any published guidelines to differentiate between the two.  Another problem is that not all authors are actually aware of the difference - I know of cases where the artist has been largely left to produce a skeletal on his or her own with little instruction ("it's like taxon A but with a bigger nose and longer tail").  Yet other researchers simply don't think any extinct animal can be reconstructed with this degree of accuracy ("too many assumptions") so they feel all skeletal reconstructions are schematic.  This last view may not be correct, but many of the techniques that render it incorrect are not published in a peer-reviewed journal.

But that doesn't mean we can't start to work to make it better.  To facilitate this (or at least a healthy discussion) I will make the following suggestions for researchers and illustrators:

For researchers:

1) No matter how talented an illustrator, people cannot consistently render accurate proportions without measurements.  If measurements are not available (or not made available) the diagram must be assumed to be schematic.

2) People can (obviously) not draw accurate bone profiles if they have never seen the bones in question.  Ideally an illustrator would see the original material, but if this isn't practical be sure to get photos, the more the merrier.  If this is also not practical, label the skeletal diagram as schematic.

3) Sometimes there just are not enough bones to justify a new skeletal reconstruction, so skeletals of related animals are modified just enough to show of the new bone (or bones).  Please label them "Schematic skeletal of Taxon A, modified after Smith (2010)".

4) Remember there is no more shame in using a schematic skeletal diagram then there is in using a strat column.  But please label the convention you are using!

5) If a skeletal is intended to be a realistic reconstruction of the animal, then it should also be treated as a form of data that needs updating.  If new finds or a more complete description uncovers some inaccuracy, update/amend the image it in a future publication, as you would with a description or phylogenetic analysis.

For Illustrators: 

1) Ask up front what sort of skeletal drawing is being asked of you.  You should also know what you need in order to produce the requested image - if you don't have enough information to produce the type of drawing that is desired, ask for it.

2) Illustrations are often used outside of professional papers - in books, museum displays, on this fancy world wide web thingy.  Maintain proper labeling wherever possible (I realize that illustrators may have little control over some projects, but maintain best practices whenever possible - this will also encourage book editors and museum directors to adopt more explicit labels).

3) If you are producing a realistic skeletal reconstruction, take responsibility for updating it as necessary.  If there isn't time to make changes, put that in the label (e.g. "Executed before additional information about the elongated neural spines was available.")

4) There's no shame in making a schematic diagram.  It's a useful contribution to science, so don't feel like you're producing a "second class citizen" of the illustration word and decide to not label it precisely.  

5) Finally, if you are trying to do a life reconstruction of a dinosaur, be sure to find out if the skeletal drawing available to you is schematic or not.  If it is, you may need to do more work before illustrating it.  That is, unfortunately, the nature of our work.

Conclusions?

I feel like this issue has been a "dirty little secret" in both scientific and paleo art circles.  Hopefully this provides some food for thought.  A lot of work needs to be done if we're going to continue to move the "science" in scientific illustration forward, but more accurate labeling of images should be something that can be universally embraced, and something we should all be aware of.

* I want to be very clear that I'm not criticizing the illustrator of the original Panphagia skeletal reconstruction (there isn't specific credit given in the Martinez & Alcober paper, so I presume it was done by one of the authors).  It's a perfectly good schematic, and demonstrates the key features of Panphagia as well as which bones are preserved.  This also in no way should cast aspersions on the paper itself, which is an excellent example of the value of publishing longer format descriptions in journals like PloS ONE, rather than the glorified abstracts required by certain high impact journals.

I chose this example because it was recent enough and high profile enough to make an excellent jumping off point for the larger discussion of schematic skeletals, not because there is anything wrong or unusual about it.

Back to our regularly scheduled blog posts...

Apologies for my multi-week absence.  I had to finish up some large projects.  I'm embarking some new cool ones as well, and I'll tell you about them as soon as I can.  In the mean time, I'll be getting back to posting blogs on dinosaur anatomy, as well as another series I have in mind on skeletal poses.  If you have any ideas of topics you'd like to see, please leave them below.  In the meantime, enjoy this skeletal drawing of Stokesosaurus, a tyrannosauroid that lived during the Late Jurassic of North America:

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!

References:

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.

A History of Skeletal Drawings: Part 3 - Dino Renaissance to the present

We saw in Part 2 that the modern convention of skeletal illustration had largely been invented by the 1950s.  Alas, it didn’t immediately catch on like wildfire, and in other ways the 1950s represents a nadir in terms of published skeletals.  Yet starting in the 1960s would see steady progress up to the modern era.  How did it all happen?  Let’s take a look!

Dinosaur Renaissance, Skeletal Evolution:

Most accounts of the Dinosaur Renaissance start in the late 1960s.  And it’s true that this period saw rapid change in the peer-reviewed literature on dinosaur biology; John Ostrom, Robert Bakker, Peter Galton and others helped drive a period of time that saw paleontologists unite dinosaurs back into a single evolutionary group, add birds to that group, and re-evaluate the metabolism and growth rates of dinosaurs. Certainly the renaissance witnessed a push to make dinosaur reconstructions more accurate, and a revival of interest in functional morphology.  Yet the seeds of that change were sown in the early 1960s.

Of course scientific ideas also have historical backdrops.  Ostrom is generally credited with ushering in the idea of endothermic dinosaurs with his 1969 paper (enticingly titled: Terrestrial vertebrates as indicators of Mesozoic climates).  Yet it was preceded by papers by L. Russell (1965), Currey (1965), and Wieland (1942), all discussing endothermic dinosaurs.  Not that their work diminishes the impact that Ostrom and other architects of the modern era had in the 1970s, but as both scientists and scientific illustrators it behooves us to always be aware of context.  So let’s jump back to see the beginning of the skeletal renaissance:

(Young's 1954 Mamenchisaurus. Not real renaissance-y.)

Much of the worlds' economy was recovering nicely in the post-war era, and the world had just seen its first black-silhouetted skeletal drawings.  Yet there seems to have been a general malaise in the realm of dinosaur reconstructions in the 1950s.  Professor Young continued to push out an outlined skeletal or two, but if anything his outlines seemed to get less accurate as time wore on. By 1960 he dropped the convention altogether in his description of Shansisuchus.

Young’s example is in many ways symbolic of the problems that plagued the field.  There were no current anatomical guidelines for producing soft-tissue outlines; earlier attempts by Romer and others to reconstruct muscles were ignored or forgotten.  Even the articulation of major skeletal elements varied between researchers without apparent rhyme or reason.  In short, there was implicit cynicism about even attempting scientific rigour with skeletal reconstructions.

(So Young to be so cynical!)

The 1960s brought the beginnings of revolution.  Not the flower-powered one you may be thinking of, but anatomical reconstructions that deserve our admiration and respect. The first (and perhaps most overlooked) of the proto-renaissance workers was Alick Walker.  Often maligned due to his stance on bird origins, Walker was a true comparative anatomist, and took great effort in his descriptive monographs.  He also happened to be an excellent scientific illustrator.

When he published his dissertation on Stagonolepis it contained not one, but two multiple view skeletal reconstructions, demonstrating both the anatomy and the armor of the aetosaur (armored one shown below).  Perhaps even more importantly the text of the paper includes an entire section entitled "Reconstruction of the skeleton" in which he explicitly lays out the assumptions he worked from.

(Walker's 1961 Stagonolepis, an uncelebrated progenitor of the modern era) 

In many ways this humble aetosaur is the first skeletal reconstruction of the modern era.  Not because the anatomical details are unassailable, but because great care was taken to provide all of the relevant information for others to study and improve upon. And really, who doesn't like multi-view skeletal reconstructions?

Walker's career was typified by a small number long descriptive works, so his skeletal reconstructions were not great in number.  He did produced a few more skeletals of note; his 1964 Ornithosuchus (while in a Godzilla pose) was well proportioned and fairly modern.  He also continued the Huene/Wright school of schematic skeletal reconstructions for his Hallopus paper.  This style of skeletal reconstruction was about to make a comeback, and it’s hard to imagine that Walker’s use of it wasn't influential.

(Walker's 1970 Hallopus in the Walker/Wright style)

Of course Walker wasn’t the only one publishing archosaur papers in the 1960s.  John Ostrom named a new species of Parasaurolophus in 1963, and Ewer’s monograph on Euparkeria provided a skeletal of a sprightly Euparkeria in a bipedal dash.  Neither was as inspired as Walker’s reconstructions, but Ostrom's work was to have a larger impact on the public conscience in 1969, when a skeletal reconstruction of his new dromaeosaur was published:

Ostrom's 1969 skeletal reconstruction of Deinonychus may have been inspired by his changing views on dinosaur metabolism, and its discovery is rightly seen as pivotal event in the Dinosaur Renaissance.  But things were moving quickly now on the skeletal drawing front.  1969 also saw Dale Russell publish an excellent description (and skeletal reconstruction) of Troodon.  The reconstruction was well proportioned, and he appears to have taken a page from the Huene/Wright/Walker playbook when he had to outline missing portions of the vertebral column. 

(Sure Russell's 1969Troodon looks nice, but how would it look with a giant head and lacking a tail?)

Dale Russell published several new skeletal reconstructions during the 1970s (and up till today), and they were excellent examples of an exciting new trend.  In the past it was notable if a skeletal drawing got either the shape of the bones right, or the general proportions.  Russell somehow managed to both provide accurate skeletal proportions and accurate representations of individual bones.  If you compare Dale Russell’s Daspletosaurus to that of Greg Paul, you'll see that the the only substantive difference is that Russell arched the back up, while Paul arched his down (Greg is correct in this case).

Hot on the heals of Dale Russell's debut, Peter Galton started a prolific career of skeletal reconstructions in 1970.  Like Russell, his reconstructions were accurate at both capturing the general proportions of the skeleton, as well as the specific shapes of individual bones.

(Galton's 1970 Edmontosaurus and 1971 "prosauropods" are accurate in both proportion and detail.  What a show off!)

Let's pause to catch our breath; Galton and Russell were producing some of the finest dinosaur skeletal reconstructions ever seen, Ostrom was finding crazy new theropods and stirring the pot on dinosaur energetics...honestly it feels like an embarrassment of riches were suddenly heaped upon dinosaur paleontology in the blink of an eye.  Yet things were about to get kicked up a notch; Ostrom’s student (and frequent Galton collaborator) Robert Bakker published a series of papers - not to mention skeletal drawings - throughout the 1970s that really put the "Bam!" in the Dinosaur Renaissance

Much has been said about Bakker’s scientific role, but here we will concern ourselves with his contributions to the evolution of modern skeletal reconstructions.  That story begins in his 1971 reply to critics in the journal Evolution, where he provides multi-view skeletal reconstructions of Struthiocephalus and Centrosaurus.  Interestingly, they are comprised of solid black bones, with only a partial outline around the centrosaur.  In some ways a style reminiscent of Scheele’s alternate skeletons in Prehistoric Animals (only without the eyeballs!).

(Image from Bakker's 1971 Evolution reply.)

In 1974 Bakker and Galton wrote a paper with the modest goal of reuniting ornithischia  and saurischia back into a single Dinosauria.  Not only where they very successful, but in the process they provided a figure with not one, or even two, but three skeletal reconstructions with black profile silhouettes.  The animals are all posed similarly for ease of comparison - pushing off on their left foot in a fast run.  The pose isn't identical to the one eventually adopted by Greg Paul, but the influence seems clear.

(From Bakker & Galton's 1974 Nature paper.)

About this time a young Greg Paul had started to experiment with making skeletal and muscle reconstructions.  In a trip to Robert Bakker’s lab in the late 1970’s he saw Bakker’s silhouetted multi-view skeletal reconstruction of a galloping Triceratops.  Clearly it made an impression on him, and he started to put together skeletal reconstructions in a similar style soon thereafter.

(GallopingTriceratops, Copyright Robert Bakker, image from here.)

Bakker continued to produce skeletal reconstructions, including ones in the black-silhouetted mold.  His 1986 book The Dinosaur Heresies is filled with different twists on this convention.  One consistency Bakker has shown is a preference for stippling or line shading to convey depth to the bone structure, rather than the more minimalist white on black adopted by Greg Paul and others.

Bakker’s book did much to inspire another generation of dinosaur enthusiasts (not to mention researchers), but it was another book that really introduced the modern skeletal reconstruction a wider audience: Greg Paul's Predatory Dinosaurs of the World.  For many of you I expect reading this part is sort of like watching Star Wars: The Phantom Menace since you already know how the story has to end.  And 20 years later it's easy to be cynical about it.  But the impact of that book on people interested in dinosaur anatomy, especially artists, is hard to overestimate; at the time no one had ever assembled such a large collection of modern skeletal reconstructions between the covers of a single book. It wasn't sheer quantity that impressed; the skeletal reconstructions were of closely related animals (theropods, it turns out) and were all in the same pose following the same set of anatomical assumptions.  That made it possible to observe subtle differences in proportions, morphology, etc., without having your eye be mislead by inconsistencies of interpretation (shoulder blade placement, for example).

(Elaphrosaurus in PDW, copyright G.S. Paul.)

Professional reaction to the science in the book was mixed, but the long term impact on the field of skeletal reconstructions is incontrovertible.  Combined with Paul's 1986 article on how to draw dinosaurs and his 1989 article explaining his technique for producing skeletal reconstructions, the popularization of the style would extend far and wide.  Established researchers began to adopt the convention as well; just two years later in his 1990 memoir on basal archosaur relationships Paul Sereno adopted the black silhouette convention, as well as posing his skeletals consistently in a left-foot-pushing-off pose.  His half dozen new dinosaur species described over the coming decade would all sport similar skeletal reconstructions.

Sankar Chatterjee, who had been producing line skeletal reconstructions for some time, also adopted the black silhouette convention (albeit with no consistent pose) in 1997 and has largely stuck to it as well.

(Chatterjee's "optimistic" 1997 reconstruction of Protoavis)

It was in this environment that I started attempting skeletal reconstructions.  I started in the mid 1990s (there was a little known dinosaur movie that had helped to reignite my interest in dinosaurs), but it would take 4-5 years before I was producing skeletals I would view now as “professional”, and not until 2002 that I started to adopt a format that used the inset rigorous version.  That format evolved from an idea that I picked up from Russell Hawley. 

(This hack's artwork keeps popping up on my blog!)

Today of course the black silhouette is the norm rather than the exception. When new animals are described is has become de rigueur for a skeletal reconstruction to be published along side it, and more often than not it has a black silhouette around it. The primitive theropod Tawa is an excellent example of this.  From established paleoartists like Mark Hallett, to more recent ones like John Conway, Jaimie Headden, and others too numerous to mention, it has become the standard way to represent skeletons.  I should also note that many excellent artists around the globe have adopted this technique, with strong showings in South America and continental Europe in the last five years.  Last year Greg Paul released a field guide of all of his dinosaur skeletals, making it feel like a true golden age for the study (and representation) of dinosaur anatomy.

Yet all is not well, and storm clouds have recently gathered on the horizon.  Several weeks ago Greg Paul made a public statement that outlined stricter and less generous guidelines on who could use his work as a basis for life reconstructions (a refined official statement can be found on his website that I expect supersedes any other posts), and laid claim to the pose that his skeletal reconstructions popularized.  Many of his complaints reflect a reasonable desire to not be ripped off by others who simply copy what he has done and call it their own.  Some other requests are perhaps more dubious, but not being a copyright lawyer I will refrain from taking them up here. The take home message is that it is already impacting the field of skeletal reconstructions, and not always in good ways.

This concludes the history of skeletal drawings.  For those who would have liked a more detailed look at the last decade, all I can say is we're still living that history, and I'm somewhat gun-shy about making claims about how influential a given event or artist when so little time has passed.  I doubt many realized the importance of Knights book on animal illustration for paleontology, nor did many recognize Walker's work on Stagonolepis as anything more than an excellent monograph.  It's after the fact that it becomes clear where the inflection points are in history.  So hurry up and do your best work, so that future generations of bloggers may include our names in a favorable light.

Instead it's time to look to the future, post Paulageddon.  It's time to take a hard look at what it means for skeletals, and what we can do as a community to produce some good from all of this.  I will probably have a couple of anatomy posts in the interim, as they are quicker to produce (and let's face it, what the blog is really about), but fear not; skeletal reconstructions and the issues that surround them will be on a reoccurring theme here at SD Blog.

Happy paleoarting!

References

Bakker, R.T. (1971) Dinosaur bioenergetics - A reply to Bennett and Dalzell, and Feduccia, Evolution. v28, n3, pp 497-503.

Bakker R.T. & Galton, P.M. (1974) Dinosaur monophyly and a new class of vertebrates, Nature v248, pp 168-172.

Currey, J.D. (1962) The histology of the bone of a prosauropod dinosaur. Palaeontology. v5, n2, pp 238-246.

Ewer, R.F. (1965). Anatomy of the thecodont reptile Euparkeria capensis Broom, Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. v248, n751, pp 379-435.

Galton, P.M. (1970) The posture of hadrosaurian dinosaurs, Journal of Paleontology. v44, n3, pp 464-473.

Ostrom, J. (1969) Terrestrial vertebrates as indicators of Mesozoic climates. Proceedings of the North American paleontological convention, Field Museum of Natural History. 

Paul, G.S. (1986). The Science and Art of Restoring the Life Appearance of Dinosaurs and Their Relatives: A Rigorous How-To Guide. Dinosaurs Past and Present Volume II (eds. Czerkas, Olson), Natural History Museum of Los Angeles County Press.

Paul, G.S. (1988). Predatory Dinosaurs of the World: A Complete Illustrated Guide. Simon & Schuster, pp 464.

Paul, G.S. & Chase, T.L. (1989). Reconstructing extinct vertebrates. In The Guild Handbook of Scientific Illustration, Van Nostrand Reinhold, pp 239-256.

Russell, L.S. (1965) Body Temperatures of Dinosaurs and its relationships to their extinction. Journal of Paleontology, v39, n3, pp 497-501.

Walker, A.D. (1961). Reptiles of the Elgin area: Stagonolepis, Dasygnathus and their allies, Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. v244, n709, pp 323-373.

Walker, A.D. (1970) A revision of the Jurassic reptile Hallopus victor (Marsh), with remarks on the classification of crocodiles. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. V. 257, N. 816, pp 103-204.

Wieland, G.R. (1942) Too hot for the dinosaur! Science, New Series, V.96 N.2494, pp359.

Young, C. (1960) The pseudosuchians in China, Palaeontologica Sinica. New Series C, N. 19.

A History of Skeletal Drawings: Part 2 - Bone Wars to the 1950's

As we saw in Part 1, the 19th century saw the rise of many of the modern conventions we see in skeletal reconstructions.  Yet the last two decades of the century closed without any innovations, and at times a retreat from proportional accuracy.  In short, skeletal reconstructions became more schematic in nature.  Yet the creation of the modern silhouette skeletal reconstruction evolved in the first half of the 20th century. Why did skeletal drawings see an initial fall? And when did the modern form get invented?  Let’s take a look...

The Fall and Rise of Skeletal Restorations

Before we begin with the doom and gloom, I want to take care not to give off the wrong impression about the Bone Wars.  It was a fascinating and productive time in the history of paleontology, and the results were a treasure trove of fossils that still delight artists and scientists more than a century later.  Yet the time period had a chilling effect on the field of producing skeletal reconstructions.  Nobody outlines were put on skeletal reconstructions for decades, and the skeletals that were published often were erroneous in their general proportions.

(Marsh's Stegosaurus, sporting a bodacious mohawk)

So what happened?  There seem to have been several factors at play.  As the 19th century closed Europe was coping with the social, economic, and political issues that would eventually lead to WWI.  As intellectual giants like Cuvier and Owen died, scientists of similar calibre were not stepping into the field to replace them.  The U.S. on the other hand was rapidly pushing back their western boundaries post Civil War and was eager to demonstrate its growing scientific prowess.  With the increase in publications during the Bone Wars, the bulk  paleontology publications started to shift to North America.  The U.S. had a different (and shorter) set of academic traditions than Europe, and one area it lacked was in comparative anatomists.  Cope, for example, was very interested in evolutionary mechanisms (his 1897 book The Primary Factors of Organic Evolution is a fascinating insight into how trivialized natural selection had become to most biologists by the late 19th century), but he did not spend his free time dissecting modern animals to learn their soft tissue anatomy.

The specimens themselves were also frequently different, especially in their mode of preservation.  Many (though not all) of the fossils we looked at in Part 1 were of the type that are embedded in slabs, or otherwise excavated with their buried position largely intact.  The fossil finds at Como Bluff and Hell Creek were the opposite; they usually come out of the ground one bone at a time.  Quarry maps were a nascent field and not generally as precise as we see today.  It's also worth noting that most of the dinosaurs named during the bone wars were not actually mounted until midway through the first decade of the 20th century. All of these factors would have made it more difficult to judge and measure proportions.

Cope's Camarasaurus on the left, Compsognathus on the right. I admit it, Cope had it harder (images from here and here) 

So in short; both the fossils and the scientists that starred in the Bone Wars were very different from what had come before, and the results were less than satisfying skeletal reconstructions.  And it would take several decades before things would fully recover.

That’s not to say there weren’t skeletal reconstructions that were published.  But for the first couple decades they tended to resemble those popularized in the Marsh and Cope monographs.  Below are the first two published skeletal reconstructions of everyones favorite dinosaur,

Tyrannosaurus rex:

(Godzilla... I mean T. rex , from the 1905 and 1906 papers on the beast.) 

One significant change that did arise in the opening years of the 20th century was photos. Thanks to the relentless march of technology photographic cameras became more widely available to scientists (and their assistants) in the early 20th century.  As a result many of the monographs that traditionally would have included illustrated skeletal reconstructions instead sported photographs of the mounts.  An early example is Brown’s 1905 description of Champsosaurus.

(Not a stinkin' drawing, but at least it comes in two different views)

While the appearance of photography may have hindered the number of skeletal drawings executed, there were certainly benefits; individual bones could be represented more accurate (and eventually at less cost) than traditional plate illustrations. In the 1910s several skeletal reconstructions were made not from observation of the bones directly, but from projections made from photographs. Our good friend T. rex demonstrates this new fad in Osborn’s 1917 paper:

(Still posed like Godzilla, but the bones are proportionately accurate!)

Several classic skeletal reconstructions were published in the 1910s, including Brown’s 1914 “Monoclonius” and Lambe’s 1917 Gorgosaurus, but all looked eerily like those you see above.  In the highly productive 40 years of American paleontology that started with the Bone Wars very little was done to expand the boundaries of the formal skeletal drawing; in the end the largest innovation was to start using photographs as a way to get the proportions correct...as they had largely been before the Bone Wars.

Over in Europe there were a few hints of what might have been.  As discussed in Part 1, Seeley’s Dragons of the Air did contain modern looking skeletal reconstructions of pterosaurs (some with outlines) in 1901.   In 1909 Othenio Abel drew such a giant skeletal reconstruction with body outline for the Frankfurt Senckenberg Museum of Natural History to help the staff understand how the bones of Diplodocus went together.  The extremely colorful Baron Franz Nopcsa was taking an interest in the physiology of dinosaurs and would eventually contribute outlined skeletal reconstructions.

(From Ilja Nieuwland's The colossal stranger (1910) - kindly supplied by the author. Original published with kind permission by the Senckenberg Museum of Natural History, Frankfurt am Main, Germany)

Had the situation in Europe been more conducive to paleontology, perhaps we would have seen a continuation of the thought begun in the 19th century.  But it was not to be, and World War I put paleontology on the back burner for most of the decade.  Ideas must have been fomenting on both sides of the Atlantic, as almost immediately after the war we see a strong re-emergence of the outlined skeletal reconstruction, and widespread interest in paleobiology.  Nopcsa produced a remarkably wrong reconstruction of Tanystropheus, which he mistook to be a pterosaur relative and namedTribelesodon in 1922.   Heilman also contributed outlined skeletal drawings in his The Origin of Birds.  But the real champion of the Roaring ‘20s was Friedrich von Huene.   The entire decade was filled with his skeletal reconstructions; in addition to reviving the body outline, von Huene also demonstrated a  strong concern for proportional accuracy.  Some of his sauropods show a fairly modern feel.

(Huene's remarkably modern-lookingTitanosaurus skeletal reconstruction)

This time the United States was not totally left behind.  Richard Lull published what was probably the first American skeletal reconstruction with an outline in his 1921 redescription of Nodosaurus, and a couple of years later Gilmore did the same for his 1924 description of Stegoceras.

(Huene's Procompsognathus standing and crouching - note the style of the crouched illustration)

Alfred Romer made a different sort of contribution to the field, when he started to publish his systematic restorations of the muscles of extinct animals.  It’s easy to imagine his comparative anatomical work making the ghosts of Cuvier and Owen smile.  The idea of reconstructing muscles explicitly like this was mostly a haphazard affair for decades, but it was the sort of work necessary if the outlines around skeletons were ever to be more than artistic doodles.

(Romer's restoration of the thigh muscles of T. rex)

The 1930s saw the Great Depression, and with it a corresponding decrease in paleontological publications.  Once again attempts to scientifically restore the functional anatomy of dinosaurs stagnated.  In Lull’s 1933 Revision of the Ceratopsia there is a wealth of photographs and illustrations of fossils, but only a single skeletal reconstruction:

(The lone(ly) skeletal drawing in Lull's revised Ceratopsia monograph)

Von Huene continued to publish, but at a reduced rate.  Gilmore put out a landmark osteology on Apatosaurus, with a skeletal reconstruction reminiscent of the monographs of Cope and Marsh (which to some degree was probably intentional).  Loris Russell did a Romer-esque full-body muscle reconstruction of Chasmosaurus, but for the most part it was a lost decade.

(L. Russell's flayed Chasmosaurus, understandably cringing in terror)

World War II further depressed the rate of paleontological publications, yet what was published shows that things had started to perk up. In China C. C. Young showed he was influenced by Huene's skeletal reconstructions, although anatomically-speaking the outlines sometimes appeared to be afterthoughts.

(Well, at least he's trying - frome Young, 1942)

When Lull and Wright put out their monograph on hadrosaurs in 1942 it was in stark contrast to Lull’s ceratopsian monograph of the previous decade; it was chock full of skeletal reconstructions, including several illustrations by Nelda Wright that hearkened back to Huene's 1921 crouching Procompsognathus. She polished the idea, and the images were to directly influence the dinosaur renaissance in the 1970s. 

(Nelda Wright channels her inner von Huene)

In the mid 1940s came an event that would have massive influence down the road.  The famous paleoartist Charles R Knight put out a book called...well, actually there’s a bit of a small mystery there. Most everyone refers to the book as Animal Drawing: Anatomy and Action for Artists. But the hardback copy I have from 1947 is actually called Animal Anatomy & Psychology for the Artist and Layman. Really, here’s a scan of the cover:

See?

As near as I can tell the title that is common today was given to the reprints done in the 1950’s, and has carried over to the present.  Regardless, inside the book - which has nothing to do with prehistoric animals at all - are images that can amaze and astound us in the present.

(Chaz Knight's skeletal of an Asian elephant)

That’s right, full blown skeletal reconstructions with black silhouettes for the muscles was invented by none other than Charles Knight, paleoart superstar!

(More of Knights black-silhouetted skeletal drawings)

The book also contains many lovely drawings of the musculature of living animals.  Given the preeminent role played by Knight in the history of paleoart, the work in Drawing Animals seems both brilliantly ahead of its time and also a lost opportunity.  Romer had started to publish his muscle restorations of dinosaurs (and other extinct animals) in the 1920s - one can easily imagine a world where fully illustrated muscle reconstructions and black-silhouetted skeletal drawings of dinosaurs had been with us for 6 decades.  

Of course the marriage of Knights paleoart and his drawing conventions for living animals did not take place.  The Knight-look was first applied (with several variations) to paleontology by William Scheele in his 1954 book Prehistoric Animals.

(Scheele's Struthiomimus....sloooowly pushing off on its right foot)

At times Scheele seemed to be channeling a bit of Nelda Wright as well, as he experimented with different forms of outlined skeletal reconstructions throughout the book.

(Scheele's Ornitholestes, complete with eyeball (!) in the skull)

To understand how rapidly this unfolded, remember that from the 1880s to 1940 very modest progress was made with skeletal reconstructions.  Most came from Europe (and particularly from von Huene).  Perhaps the kindest thing that could be said 60 years after the Bone Wars began was that skeletal drawings had gotten back to the variety and accuracy seen leading up to the Bone Wars.  Yet during and immediately after WWII (despite an overall decrease in the number of paleo publications) we saw rapid innovation.  Outlined skeletal drawings show up on three different continents, new forms are invented and/or resurrected by Nelda Wright, Charles Knight, and those conventions are synthesized a little later William Scheele. 

What happened?  There is no obvious answer for this burst of creativity.  Knight had actually been moving away from paleoart after the death of Henry Fairfield Osborn in 1935.  It seems clear that Scheele was influenced by both Knight and Wright, but there are no direct records and now it's too late to ask anyone.  The rest of the 1950's played out fairly quietly. But the stage had been set for the great flowering of skeletal reconstructions we have today. In Part 3 we'll (finally) get to the modern period, and see how skeletal drawings were influenced by the Dinosaur Renaissance, and vice versa.

References:

Knights, C (1947) Animal Anatomy & Psychology for the Artist and Layman.

Rudwick, M.J.S. (1995) Scenes from Deep Time. University of Chicago Press, 294 pages.

Seeley, H.G. (1901) Dragons of the Air. William Brendon & Son, Plymouth.

A History of Skeletal Drawings: Part 1 (pre-20th century)

The ponderous creature you see above is Plateosaurus.  The skeletal reconstruction could probably be several posts in itself, but first I’d like to spend some time pondering how we got here.  No, no, I don’t mean what link you clicked on to reach my blog, but rather the larger journey taken by scientists and the scientific illustrators that collaborate with them.  We're going to spend lots of time dissecting modern skeletal reconstructions and laying out the do’s and don’ts of restoring extinct animals, but first it’s worth reviewing how we arrived at this juncture...

We are enjoying a true renaissance in reconstructing dinosaurs (and other extinct forms).  But every renaissance is rooted in a larger context, and given the discussion going on in our profession right now I think it behooves us to pause and look back at that history.  With a better understanding of how the modern form appeared we should also take a moment to gather ourselves and contemplate future of the format, and what can be done to best secure it (well, in Part 2 anyways).

The Beginning: Skeletal reconstructions prior to the 20th century

In terms of peoples' general awareness, I imagine we could sum up the origins of the modern skeletal drawing something like this:

In the beginning there was Bob Bakker, and he decided to reconstruct some skeletons against a black silhouette, and he saw that it was good.  On the second day he showed the format to Greg Paul, who took the format and multiplied it, and they saw that it was good.  And they told the other scientific illustrators to go forth and populate the field with reconstructions, and everyone saw that it was very good indeed.

Obviously both men made immense contributions to the field, and continue to popularize our modern conception of skeletal drawings, but they don't exist in a vacuum.  In fact, there are so many candidates for a discussion of where anatomical illustrations began that we need to set some ground rules in order to avoid a semester’s length art history lesson.  For example we could examine ancient anatomists, but (not surprisingly) they were largely concerned with human anatomy, for both medical and artistic reasons.  Many of these classical works are lost to us (as are as many of the anatomical works produced by Byzantine and Muslim researchers during medieval times) but the other problem is people aren't extinct.

(A Leonardo da Vinci skeletal reconstruction - Fatally flawed by its non-fossil subject matter.)

So instead let’s pick things up where paleontology gets involved.  While the modern field of comparative anatomy is usually traced to the 17th century, it wasn't until the late 18th century that Georges Cuvier added extinct animals into that equation...and after all, that’s what we’re here for, right?

As many of you are aware, Cuvier was the gentleman who demonstrated that, unlike diamonds, species don't last forever.  As head-bludgeoningly obvious as that may seem today, it was quite the intellectual coup in the late 1700s.  Flush with his success in establishing extinction, and excellent anatomist that he was Cuvier started to provide reconstructions of fossil animals fairly quickly.

(One of Cuvier's extinct elephants.)

It should be noted, however, that Cuvier was scooped in creating the first skeletal reconstruction of an extinct animal; that honor lies with Juan Bautista Bru, who did an illustration of a mount of a giant ground sloth in 1793 (later to be named Megatherium by Cuvier).

(The first Megatherium in a very stiff looking pose. Still, being the first ever skeletal reconstruction of an extinct animal we need to cut J. Bru some slack.)

And thus was established the basic idea of using a skeletal reconstruction to convey the anatomy of an extinct form.  Cuvier may not have originated the idea, but he clearly popularized it.  Modern conventions are evident even in these first attempts; the animals are posed in lateral view, and the pose is selected so as to better show off the anatomy and minimize parallax.  In the early 1800s Cuvier introduced a subtle refinement, the use of a human to show scale:

(From Cuvier's 1821 Théorie de la terre.)

The illustration forsakes true side view, but in exchange makes an attempt to convey the size of the animal to the reader. Cuvier also made the next conceptual leap in reconstructing skeletons - that of restoring an outline around the skeleton to suggest the life appearance of the animal. Sadly, this was an innovation he kept to himself; as lovingly detailed in the book Scenes from Deep Time Cuvier never published these illustrations out of fear that they would make his work seem too speculative.  Yet the unpublished skeletal reconstruction of Anoplotherium is so far ahead of its time it would not look out of place in a modern publication:

(Why Georges, why???)

Cuvier’s fear of damaging his scientific reputation meant someone else would have to introduce the concept of a body outline to skeletal reconstructions.  And as it turned out it there was a group of fossils that were ideal for inspiring the reconstruction of outlines: Pterosaurs.

When pterosaurs were first discovered paleontologists weren't quite sure what to think of them. The first fossils were mistakenly thought to be as from aquatic animals. Even as further discoveries clarified the anatomy of pterosaurs, the very idea of "flying reptiles" flew in the face of Victorian era assumptions of what a reptile was supposed to be. Cuvier, always a paleontological busy-body, was the first to suggest pterosaurs flew in 1801. This ignited a debate that would rage for several decades, and as a result reconstructions of the wings became Real Science (TM) as a way to demonstrate the flying hypothesis.

(Almost an outline!) 

In 1812 Samuel von Sömmerring did just this, providing a skeletal reconstruction that included his view of how the wings should be restored. While this isn't a full outline, the fact that pterosaurs had such extensive wing coverage means the it's largely just the head and neck that don't have an outline around them.

This debate was to rage on until the 1830s, and included several rounds of debate as to the proper way to restore the behavior and habitat of pterosaurs...but the important point for our story is that some of those scientists published the first complete outlines with their skeletal reconstructions, like Wagler's swimming Pterodactylus in 1830 (below).

(Not totally correct - but definitely an outline!)

It's worth noting that while Wagler was not correct about the aquatic habitat, the skeleton still shows great attention to the basics of proportion, and getting overall size and shape of the skeleton correct.

It's possible that restoring outlines around skeletal reconstructions could have remained a niche affair for pterosaur workers, but starting in the late 1840s Sir Richard Owen started to adopt the conventions and many of his vertebrate paleontology papers are teeming with outlined skeletals, like the Palaeotherium below:

Owen not only popularized the convention within scientific publications, but his work made an impression on the general public as well. Perhaps the most famous (and certainly one of the most lavish) example is Owen's skeletal reconstruction of Megalosaurus that served as a guide for sculptor Waterhouse Hawkins when he created the famous sculptures at the Crystal Palace.  The illustration is not only one of the earliest dinosaur skeletal reconstructions, it also is one of the first to have the good sense to indicate which bones are actually preserved (in this case, not too many!).

At a time when Darwin was still mulling over his abstract on the impermanence of species, many of the tenets of modern skeletal reconstructions had become widespread.  Darwin’s friend and bulldog Thomas Henry Huxley actually produced one of the first modern-looking dinosaur skeletals in his 1876 work defending evolution in Popular Science Monthly.

(Huxley's 1876 reconstruction of Compsognathus)

So 3/4s of the way through the 19th century it was common to provide skeletal reconstructions of extinct animals.  They were done habitually in side view, great care was paid to reconstructing the proportions of the animals, and there was lively variety in terms of providing scale and conveying the completeness of a specimen.  At this point I feel we could have expected ever more refined attempts at reconstructing skeletons, but a funny thing happened: war broke out.

The great bone wars of North American lead to a then unimaginable treasure troves of dinosaur fossils, and subsequently many important additions to our knowledge of dinosaur anatomy and diversity.  Yet it also lead to stagnation in the portrayal of skeletal reconstructions, and even a move backwards away from accuracy.  The great monographs of Cope and Marsh are often viewed as a sort of golden age in pure descriptive paleontology.  They are filled with dozens of plates of individual bones rendered at great time and expense.  Yet gone are the attempts to place an accurate outline around skeletons; the skeletal drawings themselves are often careless with the number of vertebrae restored and are generally less accurate in capturing the actual proportions of the animals they represent.

With a few notable exceptions (i.e. the Scaphognathus in Seeley’s popular book Dragons of the Air) the business of trying to reconstruct dinosaurs would have to wait until after WW1. And that is where we'll pick up in part two, as we trace the fall and rise of the modern skeletal reconstruction across the 20th century, and up to the present.

(From Dragons of the Air, 1901, page 163.)

References:

Cuvier, G. (1821) Théorie de la terre.

Cuvier, G. (1825) Recherches sur les ossemens fossiles, vol. 1, p. 248.

Huxley, T.H. (1876) Professor Huxley's Lectures I. Popular Science Monthly, vol. 10, November.

Owen: R. (1860) Palaeontology: or A systematic summary of extinct animals and their geological relations. A. & C. Black. 420 pages.

Rudwick, M.J.S. (1995) Scenes from Deep Time. University of Chicago Press, 294 pages.

Seeley, H.G. (1901) Dragons of the Air. William Brendon & Son, Plymouth.

Note: I'd like to thank Mike Hansen, who drew my attention to the role that pterosaur skeletal reconstructions played in the origin of body outlines. You can see his DeviantArt page here.

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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".

Reference:

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