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

Well, it's 2012, so the Great Skeletal Repose of 2011 must officially come to an end. Most of the bipedal skeletals in my collection have been reposed much like this Velociraptor. I had plenty of things to say, and we saw some great discussion by guest writers, but when it comes right down to it, the issue feels incomplete without some sort of summary as to how I got here, and what's left to do. So let's take a quick look at where things stand now...

To some degree, the walking pose shown above was selected by acclimation. When I started to show off different types of poses, by far the most popular was the walking pose you see above. And I want this pose to be one that other scientific illustrators can feel free to adopt, so widespread can only enhance that proposition. But it wasn't just a popularity contest - there are several practical reasons why this walking pose became the winner:

The Good

1) Utility - Greg Paul of course will continue to use his pose, and a number of previously published skeletals by other authors (including all of mine up until last year) had adopted the same pose. By selecting the walking pose the retracted left leg remains unchanged, allowing for a easy comparison of the proportions. This was probably the biggest factor.

2) Functional Aesthetics - The old pose of animals sprinting along at a lively clip tended to impose a specific hypothesis of activity on the animals. While in some cases dashing along may have been quite likely, it still required that a research who wanted to use the skeletals to swallow that hypothesis, whether they agreed with it or not. With Velociraptor that wasn't unlikely, but with larger theropods it became needlessly controversial, and with graviportal taxa like therizinosaurs the results could be laughable.

3) Laziness - I prefer "efficient" over "lazy", but no matter what label you place on it, this pose required a minimal amount of alteration to my existing skeletals. And the stark reality is that when you have to repose hundreds of technical illustrations that time adds up. Quickly. Of course this reason would not have been sufficient if it weren't for the more important points 1 & 2, but it sure was a nice bonus.

Does that mean I'm thrilled with the outcome?

Yes and no. Since I'm starting with a blank slate, it would have been fun to come up with something truly distinctive. Yet the allure of the new wasn't worth sacrificing how useful the skeletals were for comparative purposes. Also, thanks to lots (and lots) of time spent pondering the issue, and to the extra stimulus provided by Mike Habib and Jon Conway's excellent guest posts, I've come to some unsettling questions that are still unresolved.

The Bad

1) Whither goest utility? - As I wrote way back in the halcyon days of...spring 2011, after I announced that I would relinquish the old pose and select a new one I got many requests that I refrain from this. Almost all of them were concerned with losing the ability to easily contrast my skeletals to others after the repose. I was (and am) sympathetic to that plight, and selected a pose that minimized the "damage". 

Yet I also had a disconcerting realization: Both Greg Paul and myself had been varying the pose of the forelimbs for years and no one ever bothered to complain about the loss of utility. Sometimes we've both illustrated maniraptorans with the arms flexed into their folded-wing pose, sometimes not. Less advanced theropods obviously never adopted such a pose, so their arms often hang listlessly. My tyrannosaurs went from a similar "hanging out" pose to one that reflected the work done by Lipkin and Carpenter. Why was the issue never broached? Are forelimbs less important for comparative purposes than hind limbs? Or do we naturally gravitate towards the larger structure because of how our visual cortex's work? I don't know, but I'm unsatisfied by the discrepancy.

2) Is variety the spice of life? - Jon Conway also brought up a good point in his guest article, that there isn't a single pose that best serves every need. This is certainly true, and while I still feel that within groups making poses similar is useful, I also have to admit that in some ways the job has just begun.

3) That job has just begun - Oh yeah, and another thing. Turns out there are quadrupedal dinosaurs too. Who'd have thunk it, eh? Despite the obvious and objective superiority of theropods, prosauropods, and basal ornithiscians, there's still a lot of four-footed critters in my skeletal collection, and I'm going to have to come up with a pose for them as well. Two actually, since the graviportal species will need a pose that is different from the quadrupeds with flexed limbs. Ah well, that just means that 2012 will also need to have a Great Skeletal Repose as well.

An end and a beginning

So we've come full circle. I've adopted a pose for the bipedal dinosaurs, but still have to come up with (two!) new poses for quadrupeds. I still am very much interested in soliciting outside opinions on the subject, but I also want the blog to move back to posts about anatomy and reconstruction, rather than a continuing series of posts on the technical issues behind selecting a pose. So expect to see the occasional progress report on the quadrupeds, but don't expect it to dominate space on the blog this year.

If you have a strong opinion on the subject, don't hesitate to email me (or use that Gchat thing). In the mean time, I really do have a series of upcoming posts on Acrocanthosaurus and Spinosaurus anatomy, as well as the trials and tribulations of reconstructing skeletals in multiple views.

Stick around, won't you? 2012 should be an interesting year.

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!

Happy Holidays!

Happy holidays to one and all. We'll talk about that Acro skeletal and multi-view reconstructions in the New Year. Until then, be safe and enjoy the holiday season!

Please (properly) label your scale bars: Exhibit A

As many of you know, I spend a lot of my time doing skeletal drawings. Not everyone does them, but I don't think I demand any special considerations in the papers I use as reference. Many of the critiques about measuring your dinosaur posted over at SVPOW are similar to what I think when I read a paper.  Anyhow, in terms of typos making a scale bar useless, I think this next image speaks for itself:

That is all.






(If you're having trouble seeing what I'm talking about, read the image caption carefully)

Falcarius: bizarre sickle-cutter

The truly strange looking animal above is Falcarius utahensis. It's an early, omnivorous member of the theropod clade known as therizinosaurs. Not only does it look weird, it's also a bit different from other skeletals you may have seen on the web. Join me after the break for a bit of a discussion about Falcarius, and the challenges I faced with this reconstruction.

I should warn you, this won't be a tutorial on how I make my skeletal reconstructions. That would certainly be a fun series, but it would require quite a lot of time to do properly, so for now it'll have to wait. But there are still several points worthy of discussion.

First off, the animal was discovered in a bone bed of disarticulated individuals. The good news is that most of the individual elements are known, but the down side is the bones aren't all from the same sized animals. That means that cross-scaling is needed to restore the skeleton, but even that presents a challenge; the usual method of cross-scaling involves double-checking the results against the proportions of close relatives. Alas, in this case the fossil record for the base of the therizinosaur family tree isn't well known, and what is known makes it clear that Falcarius has very different proportions than it's closest known relative: Beipiaosaurus.

(Image copyright Greg Paul)

When the original description of Falcarius was published in 2005, it came with the skeletal drawing at left. Obviously I don't agree with those proportions now, but at the time it had been done when fewer bones had been excavated, prepared, and described in detail, so Greg Paul had to try and scale them based on a smaller amount of material to compare with.

In fact, given the difficulty of restoring the proportions I intentionally avoided doing a Falcarius skeletal reconstruction for several years. I might have avoided it all together, but towards the end of my tenure at the WDC we mounted a cast of Falcarius that Gaston Design produced. Working on that skeleton I was able to not only measure and photograph all of the elements, but spend time looking at how the individual elements were matched up. Some parts of the cast's vertebral column are from different sized individuals (an unavoidable consequence of trying to piece together a skeleton from several different individuals). In other cases, vertebrae I had assumed to be from different sized animals were in fact crushed.

In addition to the hands-on data, Lindsay Zanno had been hard at work publishing more detailed information on Falcarius (this is actually notable, as not all researchers are as timely with getting more detailed descriptions of a new animal into print).

As the information piled up I felt that a skeletal was possible to be done. I still didn't tackle it though, as there were plenty of "low-hanging fruit" skeletals that could be done from less-challenging animals.

As luck would have it, I ended up being asked to produce a skeletal of Falcarius for a display in the new Utah Museum of Natural History building (side note: the new UMNH building just opened, and houses one of the most impressive natural history displays in North America, go see it!).

Since I was working with the UMNH, I got valuable input from several of the researchers who worked on the specimens. They were able to provide additional information - I won't go into the nitty-gritty of it (although you may if you would like), but I wanted to point out that the end result was quite a surprise to me. And little is more satisfying than when you are really surprised at the end of a skeletal reconstruction.

Resulting skeletal in hand, you can compare it to the most recent studies of the therizinosaur family tree, as well as the excellent research being done by Lindsay Zanno and Peter Makovicky on the origin of plant-eating in theropod dinosaurs, and Falcarius starts to tell an interesting tail about the order in which therizinosaur traits appeared.

Falcarius appears to already be specialized for browsing for high forage. Given the lack of an enlarged gut for fermentation it probably preferred to seek out higher-quality plant matter, like fruiting bodies or seeds. The partially upright stance appears concurrently with a widening of the passage through the pelvis (not visible in side view) allowing move guts into that area, causing the center of gravity to sit further back despite the elongation of the neck.

The large hand claws (from which the authors derived the name "sickle-cutter") may have allowed Falcarius to pick up small prey, but they also may have served as defense for a fairly slow animal with small teeth. The first toe is low and long enough to start interacting with the ground, perhaps to provide balance and stability when browsing high.  All of these features would be carried to extremes in advanced therizinosaurs, but they seem to already be playing the same (albeit incipient) functional roles in Falcarius.

So with Falcarius we have an animal that at first glance appears inexplicably strange, but when viewed through the lens of where it was coming from (long-bodied small-headed meat eaters) and where it ends up (the upright, pot-bellied therizinosaurs) the combination of traits start to make a lot of sense.

Isn't science grand?

References:

Kirkland, J. I., Zanno, L. E., Sampson, S. D., Clark, J. M. & DeBlieux, D. D., 2005. A primitive therizinosauroid dinosaur from the Early Cretaceous of Utah. Nature, v435, pp 84-87.

Zanno, L. E. 2006. The pectoral girdle and forelimb of the primitive therizinosauroid Falcarius utahensis (Theropoda, Maniraptora): Analyzing evolutionary trends withing Therizinosauroidea. Journal of Vertebrate Paleontology, v26 n3, pp 636-650.

Zanno, L. E. 2010. Osteology of Falcarius utahensis (Dinosauria: Theropoda): characterizing the anatomy of basal therizinosaurs. Zoological Journal of the Linnean Society. v158, pp 196-230.

Zanno, L. E. & Makovicky, P. J., 2011. Herbivorous ecomorphology and specialization patterns in theropod dinosaur evolution. Proceedings of the National Academy of Sciences. v108 m1, pp 232-237.

Gauging stance in "wide-gauge" sauropods

In 1999 Jeff Wilson and Matt Carrano published an excellent paper addressing the phenomena of "wide-gauge" sauropod trackways.  For years researchers had been working to explain why sauropod trackways seemed to come in two very different flavors - some of them were very closely spaced...so much so that they would actually overlap on the midline of the track.  Other sauropod tracks seemed to show animals walking with their feet spread much further apart.

What were paleontologists to make of this?

One explanation was that the trackways were made by the same type of sauropods that were engaging in different behaviors.  In other words, perhaps sometimes a sauropod would walk with its legs close in, while at other times it would use a wide-gauge stance.

Wilson & Carrano proposed that instead the trackways were made by sauropods with different skeletal adaptations.  They mustered quite a few lines of evidence, but perhaps the best was that there was a group of sauropods - titanosaurs - that in fact had a much wider pelvis than other sauropods.  The paper created a framework for later workers to use when attempting to correlate track makers with fossilized trackways, and is generally a towering success.

But I did want to take issue with one figure of the paper - one that pops up repeatedly at SVP.  It is figure 5, demonstrating their interpretation of hing leg stance:

That's Camarasaurus on the left and Opisthocoelicaudia on the right.  The clever reader may have already surmised from the title of this post that I think the animal on the right has its legs spread too far apart.  But I have a larger issue: both animals have their legs spread much too far apart.

Remember that narrow-gauged trackways actually have their feet fall so close together that they frequently overlap along the midline.  There's no way even sauropod "A" could make those tracks in the stance as figured.  And this is why I'm bringing this up, because animals generally don't walk around with their legs acting as perfectly vertical beams.  If you spend time watching large animals walk away from you, you'd see something like this:

(Elephant image from here, rhino image from here.)

People also move like this, with our vertical limbs generally sloping in toward the midline when we walk or run.  There are probably several reasons for this (including mechanical efficiency) but for our purposes here let's just setting on the fact that it happens.  Large, straight-limbed graviportal animals tend to walk with the limbs angled inward, not down (and certainly not angled out).

And the trackways also demonstrate this.  If you place place sauropods over the actual trackways in question, you end up with a stance more like this:

In this case I've put a diplodocid (Supersaurus) on the left, while the animal on the right is scaled to the pelvic dimensions of Opisthocoelicaudia as seen in the original paper.  Both animals have the hind legs mostly vertical but gently sloping inward.

This is not to say that sauropods never adopted a pose with their legs spread out a bit; Wilson & Carrano point out that titanosaurs have adaptations that may have allowed them to evert their hind limbs more effectively.  They suggest that titanosaurs may have done so when rearing up, or during other activities that require greater stability.

I don't take issue with that, and those sorts of differences in the legs and pelvis may make it possible to tease out further behavioral differences between sauropod groups.  But when walking around in their day to day lives both the footprints and modern analogs make a strong case that the limbs should be vertical, and if anything sloping in towards the midline rather than spread away from it.

Reference:

Wilson, J. A, & Carrano, M. T. 1999. Titanosaurs and the origin of "wide-gauge" trackways: a biomechanical and systematic perspective on sauropod locomotion. Paleobiology, 25(2), pp. 252-267.