The evolution of (my) Acrocanthosaurus


WithSpinosaurus temporarily out of the way, we're going to take a closer look at Acrocanthosaurus. This series is going to include a post on what we know and don't know about Acrocanthosaurus, how to restore the skeletal in multiple views, and how to restore the muscles. At the end of that series I'll also comment on some areas of soft-tissue variation that artists should keep in mind when they envision "their" Acrocanthosaurus.

First though, I wanted to take a moment to look at how my own reconstruction of Acrocanthosaurus has changed over the last decade. With any luck some of my earlier errors in methodology might help others who want to do skeletals. Also I hope to provide some insight to how I update skeletals over time, and the importance of revising your work as new data is published.

Although Acrocanthosaurus is hardly the best known theropod, it provides more than its share of challenges when attempting to reconstruct it. In my case, I attempted the original skeletals during what I'd call a methodological nexus - it was one of the first skeletals I attempted within an entirely digital environment, and to some degree the initial reconstruction suffered as a result. I had developed many techniques when I executed skeletals in pen and ink, and most of those translated fairly well during the years (roughly 2000-2004) when I used a hybrid method of digital and ink work. Alas, attempting to work entirely inside a computer forced me to rediscover how to accomplish the same things inside of Photoshop, and as a result the first couple of attempts were actually a step backward in some ways. Luckily I stuck with it, and the results are now far better then anything I accomplished in the "analog" world.

In the beginning...

Acrocanthosaurus itself is not a new dinosaur. It was described in 1950, and it was noted at the time that the specimen was a carnosaur with enlarged neural spines on the back. What really brought Acrocanthosaurus to my attention, however, was the reconstruction of NCSM 14345, the "Fran" specimen, which was prepared, molded, and mounted by Black Hills in the 1990s. One of those specimens became part of a travelling display that I contributed to, and I was inspired by the fully restored mount, as well as my ability to take lots of photographs of the mount.

Alas, that was in the "olden days" when cameras captured light on sheets impregnated with silver nitrate rather than CCDs, and I never did scan in those photos. I still had them in 2003, but they appear to be one of many casualties of moving around frequently.

I was also attending the University of Wyoming, and their Geology library had supplied valuable gifts: descriptions of Acrocanthosaurus specimens by Jerry Harris (1998), and Currie & Carpenter (2000). Armed with proto-pdfs (read: xeroxed copies), lots of photos, and an abundance of enthusiasm I sat down to create my first all-digital skeletal reconstruction.

The tomb of the unknown skeletal...

Directly above, what you don't see is my first Acrocanthosaurus skeletal. I'm not being shy, I just didn't back up the original very carefully, and eventually the hard drive it was on suffered a systemic failure. Cloud storage was a pie-in-the-sky dream at the time, so I've lost several "original" skeletal files during the course of computer failures over the years. Luckily I've been anal retentive enough to keep current versions of skeletal files on multiple hard drives, so I haven't totally lost a reconstruction (at least, not a digital one).

What I can say is that in many ways my first attempt was an unmitigated failure. For starters, I didn't actually have the papers I mentioned above in their entirety. I'd read them in Laramie, but at the end of the semester I only photocopied the parts I thought I'd need (hey, it took time and money to copy texts in those days!). And of course the photocopies didn't always do justice to the original figures.

Instead I had to rely strongly on the photographs I had taken of the Fran mount. Even that was fraught with uncertainty, as I hadn't been able to take orthographic photos of the mount. Also, there were errors in the fully restored mount, but without all of the text from Currie & Carpenter I didn't always recall what was restored and what was cast from original bone. The results were predictably poor.

You don't  have to take my word for it though. I posted that first attempt to the Dinosaur Mailing List in early July of 2003. Jaimie Headden and Waylon Rowley made quick and (deservedly) pointed replies. I had missed several characteristic parts of the tail and neck vertebrae, and a bunch of other minor details had simply been missed.

What happened? For one thing I really didn't have enough data on hand. But more importantly, I was distracted by having to solve the technical problems of a new medium. Digital skeletal reconstructions have many advantages, but at the time they were lost on me as I struggled to figure out how to simply do the things I'd taken for granted in the "analog world".

The Acro strikes back


With a bit of work, I quickly produced the skeletal above, which addressed some of the basics. It wasn't a horrible attempt - it accurately captures almost all of the proportions, and many of the changes since have been either due to newer research getting published, or on changing interpretations in soft tissue reconstruction.

One thing mistake in this earlier version is that the neural spines were too tall - I had based their height on the mount of the Fran specimen, but it turned out that neural spines were not complete in that specimen (at least not the ones in the back, sacrum, or the front part of the tail). A review of other specimens ruled out such a grandiose neural spine height, and that also has implications for paleoartists wondering how to illustrate the back.

Two important papers were published on Acrocanthosaurus after that early skeletal. One was on the forearm, which showed that the arm couldn't actually reach as far forward as I'd illustrated it (and also clarified details of the size and shape of all the elements). Another paper was published in the last year that updated our understanding of the skull of Acrocanthosaurus. When I was asked if I could supply the skeletal to an upcoming book, it was clear that I had to revamp the skeletal before sending it along...

Back to the future


Which brings us back to my current skeletal. In addition to the new skull and arm data, the newfangled Acrocanthosaur naturally sports my new skeletal pose. So how crazy different is it? In some ways it's pretty similar - the overall dimensions really didn't change, and that's a good thing. A bunch of more subtle soft-tissue changes took place (at least some of which will get to star in their own post in the future), but a couple of obvious ones involve the tail and the neck.

The tail in the newer one has a heavier base, reflecting the research by Persons and Currie on tyrannosaur tails that I wrote about last year. The tail also is a bit less bouncy. The current skeletal is moving at a more sedate pace, and its tail is no longer bouncing along like a dropped garden hose. The neural spines on the tail have also been updated, with the spines on the front of the tail (near the hips) shorter to reflect other specimens, and the ones in the middle a bit taller, again based on other specimens.

Looking at the neck, the silhouette is no longer shrink-wrapped as tightly to the skeleton. I will probably address the "why" part of that in a later post, but suffice it to say that the newer Acro would have an easier time wolfing down large pieces of meat it tore off a carcass.

A final set of changes were made in the pectoral girdle (the shoulder region). I added a furcula which, while not found in Acrocanthosaurus, now appear to be a primitive feature found in all theropods. Additionally, I used to illustrate a large cartilaginous presternum that split the coracoids apart (you can see it poking out the bottom and front of the chest area on the older skeletal).

Reflecting the articulated chest regions of other non-maniraptoran theropods, I changed it to a smaller presternum, that would have ended sooner and allowed the coracoids to almost contact one another along the midline. This slightly changes the outline in side view, but in front or top view has the effect of narrowing the front of the torso across the shoulders.  Now if only someone would produce multiple-angle skeletals of Acrocanthosaurus to show this off...

Dawn Thief


A quick post (and a new skeletal) today, while I apply a bit more polish to the Acrocanthosaurus series. To satisfy all of your skeletal-drawing-based-amusement needs I give you Eoraptor lunensis, one of the most primitive dinosaurs yet discovered.

How primitive? So primitive that we can't actually answer that question with any certainty right now. Eoraptor is one of those taxa that bounces around a lot in different studies. When it was originally describe, it was thought to be one of the most primitive theropods known. Other studies suggested it might be more primitive than the split between theropods and sauropods. Recently, some have even found it to be on the line leading to sauropods!


All three positions have shown up in recent studies, so for now at least the answer is "we're not sure". Why all the trouble figuring out who is Eoraptor's closest relative? Basically what it boils down to is that Eoraptor is so primitive that this is what the common ancestor of theropods and sauropodomorphs would look like. No matter where Eoraptor ends up on the dinosaur family tree, the difference between it and animals at the other position will be very small indeed.

If you compare Eoraptor to Panphagia (which I examined in this post), which is a well-supported basal sauropodomorph, you can see just how similar the two are. The problem is sort of like being handed photographs of all of the Kennedy's when they were two years old and being asked which one was closest in age to JFK - there's a correct answer, but it's awfully hard to tell from the information you have.

As for the skeletal reconstruction, Eoraptor didn't present nearly as many challenges as some other taxa do. For one, it's know from fairly complete remains. It would have been nice to have a more detail description of the animal in print (the original papers leave something to be desired along those lines), but luckily the specimen is available in detailed orthographic photos, which do a nice job of supplementing the published data.

That's all for now!

The Great Skeletal Repose of 2011: A Retrospective




The Good




Does that mean I'm thrilled with the outcome?


The Bad





An end and a beginning





Revisiting the Fisher King


I know, I know, my last post promised a series on reconstructing Acrocanthosaurus in multiple views - bear with me, as this is actually part of that series. Remember that both animals have stuff sticking up on their backs, so I want to be able to compare and contrast those elongated neural spines...and how those differences should impact reconstructions of the animals. But to do that I had to update this skeletal, as new information had rendered the older one no longer tenable.

Besides, Spinosaurus is cool! For one, it's the only dinosaur in the Jurassic Park series to tangle with a T. rex and emerge victorious (no matter how unlikely that outcome was). It's probably the longest theropod we know of, and may have been the heaviest as well. Yet counter-intuitively it shows specialization for piscivory (fish-eating)...maybe in JP3 the spinosaur mistook the T. rex for a really large lung-fish?

Tongue firmly out of cheek now, Spinosaurus has lit up imaginations partially due to its size, but also because there was so much you had to imagine to try and reconstruct the animal. Until the last decade or two it was sort of a theropod Rorschach test where you could project any sort of oversized monster theropod onto its scant (and now lost) remains. This brings a thrilling "Sherlock Holmes" quality when trying to imagine the living animal, but for most of the last century serious attempts to reconstruct Spinosaurus have been more frustrating than titillating.

Darren Naish has an excellent write up of the history (and tragedy) of of the type specimen of Spinosaurus, which I won't duplicate here. The long and short of it is that WWII claimed the fossils as another victim of the conflict. The already-meager remains lost, paleontologists were stuck with the original description and some somewhat uninspired sketches as the only link to the past.

A series of fortunate events occurred in the latter half of the 20th century that allowed for a more accurate interpretation of Spinosaurus to emerge. For one, other spinosaurids were found. Baryonyx from the U.K., and Nigerian Suchomimus, started to paint a more complete picture of what these animals were like. They had bizarrely long snouts that seemed to resemble a gharial as much as a traditional theropod. Suchomimus even had a smaller version of the enlarged neural spines on the back:


The amusingly-named Irritator from South America further clarified the relationships and anatomy of spinosaurids. But the real breakthrough was the re-discovery of several photographic plates of the original material. While Spinosaurus wasn't the most complete specimen, having photographs at least made it possible to ensure that what was found is incorporated accurately into a reconstruction.

Among other details, the image also shows what had been the basis of attempts to restore the shape of the elongate sail or hump on the back: Stromer's original interpretation for the position of the elongated neural spines. In particular, notice that the tallest one is set directly in front of the sacrum here, while the only associated tail vertebra (at the far left of the picture) has a very short spine. That has lead most people to infer that the spine started quickly after the neck, grew to ridiculous heights over the pelvis, and then quickly dropped off again. Indeed, this is the interpretation that I used in my first attempt, and has been widely seen in such disparate and reputable scientific endeavors as Jurassic Park 3, the Carnegie Collection of "museum quality replicas", and Greg Paul's reconstruction in his Princeton Field Guide to Dinosaurs.

And they're in excellent company (whereby I arbitrarily define myself as "excellent company"). I had been concerned with Stromer's original interpretation for the placement of the tallest neural spinse - no vertebral body (centrum) was preserved, but the change in the angle of the spine seemed pretty extreme compared to the previous dorsals, especially right in front of the sacrum. My solution was to assume it was a sacral neural spine. This largely preserved the traditional appearance of the "sail", but provided a bit of breathing room for the change in orientation.


Luckily for us, Andre Cau and Jamie Headden were busy mulling over this specific issue, and came to a much more likely conclusion, that the backward-oriented neural spine was actually an anterior caudal. Looking at a host of dinosaurs with elongate neural spines, they noted that in general you never seen backward-canted spines in front of the hips, you always see them after it. There is a bit more detail to the argument (which I encourage you to read on their blogs), but in essence they make a very compelling case.

And so it was back to the virtual drawing board. I made some other corrections from my previous attempt - there had been some scaling issues with the neck vertebrae that had given my reconstruction a thinner Baryonyx-like profile in the neck. Also, it appears that the necks of these animal don't have as much of the traditional theropod S-curve, so that was changed as well (although I still don't buy the extreme hang-dog look that Greg Paul has started to restore his spinosaurs with). The results are a stockier animal, with a more elongate sail (or hump):


Looking at the rigorous reconstruction, it's clear that there's still quite a bit of uncertainty in the skeleton, although not all of the missing parts are created equal. Much of the pelvic girdle is known from Irritator, as is the back of the skull. Also, some unpublished specimens shed light on this, even if they aren't documented well enough to be official parts of the reconstruction. Still, there's a bit of ambiguity about the exact limb proportions, the length of the tail, and the exact shape of the sail.

Speaking of which, how should those tall neural spines be restored by artists doing life reconstructions? Is it a sail, was it supporting a hump of tissue like a bison, or was it simply a muscular ridge? We'll get back to that subject in a bit, after looking at Acrocanthosaurus.

Until then, best wishes to one and all for a wonderful 2012!