Revisiting the Fisher King

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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:

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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.

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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):

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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!

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

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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.

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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

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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:

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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:

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(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:

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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.

Um hey, Scientific American? Bird knees bend the same way as everyone else.

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Ok, time for a quick anatomy lesson: Despite what you may have heard, bird knees do not bend backward.  Nor, in fact, do the knees of any tetrapod perform this trick.  Given the role of the knee in locomotion, it's not even clear how such a reversal could evolve after the initial "knee bend" direction was settled upon several hundred million years ago.

Why bring the anatomical equivalent of a fairy tale?  Well, it's a fairly common misconception.  So common, in fact, that it was recently enshrined by none other than Scientific American.  So let's see if we can clear this up with some simple diagrams:

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Really, it's not.

It's a surprisingly common mistake.  When looking at living birds many people fail to realize that part of the leg is hidden on a bird; the upper leg barely moves, and along with the knee it is actually buried up under the feathers of the wing and body.  Birds also have quite long ankles, leaving their ankle joints in roughly the position we'd expect the knees to be on a human.  Like this:

The key here is that people are plantigrade animals, while all theropods (including birds) are digitigrade.  That means that human ankles are flat on the ground, and in our case our knees are roughly in the middle of our legs.  In birds and other digitigrade animals (most dinosaurs and many mammals, like dogs, deer, and horses) it's only the toes that contact the ground.  The ankle joint is well up off the ground, and the knee is is actually in the upper 1/3rd of the leg.  And in birds the thigh is actually even a smaller part of the leg, and as mentioned above is also mostly hidden under feathers.  Here's a look comparing the same leg portions of a human (in two poses), a dog, and an extinct bird:

(Presbyornis copyright Scott Hartman, other skeletals modified after Charles Knight.)

(Presbyornis copyright Scott Hartman, other skeletals modified after Charles Knight.)

In the diagram the thigh bones (femora) are all colored red, the shin bones and proximal ankle bones are colored blue, the "foot bones" (the distal tarsals and the metatarsals) are green, and the individual toe bones are yellow.  Notice that the femur of the extinct bird Presbyornis is small and very high, and the knee (the joint between the red and blue bones) is up where it would be hidden by the body and wings.  But please also notice...the knee is bending the same way as ours.  And the same way as everything else that has a spine and walks on land.

Which brings us back to the article posted by Scientific American.  I don't want to go too far with "gotcha" blogging, but Scientific American is generally one of the more highly regarded popularizers of science on the web and in print.  I took a look the other pieces published by the author, and she seems like a solid reporter that just happens to have made a mistake.  Journalists don't get science degrees (and even if they did that would only be one subject), so this should not be construed as an attack.

But at the same time, this is a serious error.  It's like reporting that September has 32 days in it, or that the Red Sox clinched a playoff spot this week.  Only worse, as it's perpetuating a myth that gets passed around as "common knowledge".  I attempted to bring this to the attention of the relevant parties shortly after it was reported, both on Google+, where SciAm blog editor Bora Zivkovic has been making effective use of the new social network, and the author's Twitter account (which is frequently used).

Despite near real-time feedback, days have gone by with no correction. Making a mistake is understandable, but failing to correct it is not.  Scientific American has written hundreds of articles on the state of science education, and has often been an effective advocate for ways to improve it.  But the authority they derive comes from their attention to scientific detail, so I hope we will now see a quick correction without further delay.