September 11, 2014

In the paper describing the new giant titanosaur Dreadnoughtus, Lacovara et al. (2014) use the limb bone allometry equation of Campione and Evans (2012) to derive a mass estimate for the holotype individual of 59.3 metric tons. This is presumably the “middle of the road” value spat out by the equation; the 95% confidence interval on either side probably goes from 40 to 80 metric tons or maybe even wider.

I decided to see if 59 metric tons was plausible for Dreadnoughtus by doing Graphic Double Integration (GDI) on the published skeletal reconstruction and body outline (Lacovara et al. 2014: fig. 2). The image above is the one I used, so if you like, you can check my numbers or try your hand at GDI and see what you get.

First up, I have to congratulate Lacovara et al. for the rare feat of having everything pretty much to scale, and a properly-sized scale bar. This is not always the case. Presumably having a 3D digital model of the reconstructed skeleton helped — and BTW, if you haven’t downloaded the 3D PDFs and played with them, you are missing out bigtime.

Here are my measurements of various bits in the picture and the scale factors they give:

Meter scale bar: 37 pixels – 1.0 meters – 37 px/m
Human figure: 66 pixels – 1.8 meters – 37 px/m
Scapula: 62 pixels – 1.7 meters – 36 px/m
Humerus: 58 pixels – 1.6 meters – 36 px/m
Femur: 70 pixels – 1.9 meters – 37 px/m
Cervical: 45 pixels – 1.1 meters – 41 px/m (not included in average*)
Neck: 407 pixels – 11.3 meters – 36 px/m
Post-cervical vertebral column: 512 pixels – 13.8 meters – 37 px/m
Total length: 922 pixels – 26.0 meters – 35 px/m
AVERAGE 36 px/m

* I didn’t include the cervical because when I measured it I sorta guessed about where the condyle was supposed to be. That was the odd measurement out, and I didn’t want to tar Lacovara et al. for what might well be my own observer error.

Here’s the chopped-up Dreadnoughtus I used for my estimate. Just for the heck of it, for the first time out I assigned all of the body regions circular cross-sections. We’ll come back to how realistic this is later. Here’s what I got for the volumes of the various bits:

Neck: 13.9
Body: 32.1
Tail: 4.0
Limbs: 6.8
TOTAL: 57.0 m^3

Okay, this is looking pretty good, right? Lacovara et al. (2014) got 59.3 metric tons using limb allometry, I got a volume of 57 cubic meters using GDI. If Dreadnoughtus was the same density as water — 1 metric ton per cubic meter — then my estimated mass would be 57 tons, which is crazy close given all of the uncertainties involved.

BUT there are a couple of big buts involved. The first is that a lot of sauropods had distinctly non-round body cross-sections (Diplodocus, Camarasaurus). So assuming circular cross-sections might inflate the body well beyond its likely volume. Second is that sauropods were probably much less dense than water (discussed here, here, and here, and see Wedel 2005 for the full scoop). What are the implications for Dreadnoughtus?

Round and Round

It turns out that circular cross-sections are probably defensible for some parts of Dreadnoughtus. By playing around with the 3D PDF of the assembled skeleton I was able to get these orthogonal views:

I don’t remember what the pixel counts were for the max height and max width of the torso, but they were pretty close. I measured at several points, too: front of the pelvis, max extent of ribcage, mid-scap. This is probably not super-surprising as the fatness of titanosaurs has been widely noted before this. Here’s a cross-section through the torso of Opisthocoelicaudia at D4 (Borsuk-Bialynicka 1977: fig. 5) — compare to the more taconic forms of Diplodocus and Camarasaurus linked above.

Okay, a round torso on Dreadnoughtus I can buy. A round neck and tail, not so much. Look at the skeletal recon and you can see that even with a generous allowance for caudofemoralis muscles on the tail, and diapophyses on the cervical vertebrae, no way were those extremities circular in cross-section. Just off the cuff I think a width:height ratio of 2:3 is probably about right.

But there are some body regions that probably were round, or close enough as to have made no difference, like the head and limbs. So I actually toted up the volume three times: once with circular cross-sections throughout (probably too fat), once with a 2:3 width:height ratio in the neck, trunk, and tail (probably too thin, at least in the torso), and once with the 2:3 ratio only in the neck and tail (my Goldilocks version). Here are the numbers I got:

Air Apparent?

Now, for density. Birds are usually much less dense than water — lotsa cited data in this hummingbird post, the punchline of which is that the average whole-body density of a bunch of birds is 0.73 g/cm^3. Why so light? In part because the lungs and air sacs are huge, and account for 15-20% of the whole-body volume, and in part because many of the bones are pneumatic (= air-filled). For a really visceral look at how much air there can be in the bones of birds, see this post, and this one and this one for sauropods.

In my 2005 paper (almost a decade old already — gosh!), I found that for Diplodocus, even a fairly conservative estimate suggested that air inside the bones accounted for about 10% of the volume of the whole animal in life. That may be higher than in a lot of birds, because sauropods were corn-on-the-cob, not shish-kebabs. And that’s just the air in the bones — we also have several lines of evidence suggesting that sauropods had air-sacs like those of birds (Wedel 2009). If the lungs and air sacs occupied 15% of the volume of the whole animal, and the air in the bones occupied another 10%, that would give a whole-body density pretty close to the 0.73 g/cm^3 found for birds. Sauropods might have been lighter still — I didn’t include visceral, intermuscular, or subcutaneous diverticula in my calculations, because I couldn’t think of any way to constrain their volumes.

What about Dreadnoughtus? As Lacovara et al. (2014) describe, the cervical, dorsal, and sacral vertebrae and sacral ribs are honeycombed with pneumatic camellae (small, thin-walled chambers). And the dorsal ribs have pneumatic foramina and were probably at least partly hollowed-out as well. The caudal vertebrae do not appear to have been pneumatic, at least internally (but diverticula going into the tail can be cryptic — see Wedel and Taylor 2013b). Diplodocus has a big, long, highly pneumatic tail, but Dreadnoughtus has a much longer neck, both proportionally and absolutely, and pneumatic dorsal ribs. So this one may be too close to call. But I also ran the numbers for T. rex way back when and found that air in its vertebrae accounted for 7% of its body volume (this abstract). Pessimistically, if we assume Dreadnoughtus had small lungs and air sacs (maybe 10% of whole-body volume) and not much air in the bones (7%), it’s whole-body density was probably still closer to 0.8 g/cm^3 than to 0.9. Optimistically, a lot of titanosaurs were radically pneumatic and they have may have had big air sac systems and extensive diverticula to match, so a bird-like 0.7-0.75 g/cm^3 is certainly not beyond the bounds of possibility.

This table shows a spectrum of masses, based on the three body volumes from GDI (columns) and some possible whole-body densities (rows). Note that the columns are not in the same order as in the first table — I lined them up from most t0 least voluminous here. The 57-ton estimate is the max, and that assumes that the neck and tail were both perfectly round, and that despite the lungs, air sacs, and air reservoirs inside the bones, the whole-body density of Dreadnoughtus was still 1.0 g/cm^3, neither of which are likely (or, I guess, that a real Dreadnoughtus was significantly fatter than the one shown, and that all of that extra bulk was muscle or some other heavy tissue). The 28t mass in the lower left corner is also unrealistic, because it assumes a tall, narrow torso. My pick is the 36t estimate at the bottom of the middle column, derived from what I think are the most defensible volume and density. Your thoughts may differ — the comment thread is open.

This last table is just a quick-and-dirty comparison of how the volume of the body breaks down among its constituent parts in Plateosaurus (from this post), Giraffatitan (from Taylor 2009), and Dreadnoughtus (based on my “tall neck and tail” GDI). Dreadnoughtus seems to have a more voluminous neck and a less voluminous trunk, proportionally, than Giraffatitan, but I think a lot of that is down to the very fat fleshy envelope drawn around the cervicals of Dreadnoughtus. We are fortunate to count some fearsomely talented paleoartists among our readers — I’ll look forward to seeing what you all come up with in your independent skeletal recons.

So, what’s the take-home? Based on the data available, I don’t think the holotype individual of Dreadnoughtus massed anything like 59 metric tons. I think 35-40 metric tons is much more defensible. But I’m happy to have my errors pointed out and new data and arguments brought to the fore. Your thoughts are most welcome.

Give a talk that holds attention!

September 9, 2014

I am just back from SVPCA, where I saw fifty 20-minute talks in three days. (I try to avoid missing any talks at all if I can avoid it, and this year I did.) As always, there was lots of fascinating stuff, and much of it not about the topics that I would necessarily have expected to enjoy. Examples include Tom Fletcher’s talk on the evolution of hydrodynamically efficient skin textures in fish, Lionel Hautier’s on the homologies of sloth teeth and Liz Martin’s on the skeletal-mass:total-mass ratio in birds.

Fletcher et al. 2104: figure 3. Flank scale of the osteichthyan Lophosteus: (a) scanning electron microscope (SEM) image of large buttressed tubercles on upper surface; (b) lateral view (surface rendering of mCt scan); and (c) dorsal view (SEM image). Scale bar: (a) 100 mm, (b-c) 0.5 mm

But the brutal truth is that some of the talks were much less engaging. As the fish, sloths and birds prove, it’s not necessarily the fault of the taxa being studied — other factors are more important.

In a moment of frustration during one of the less appealing talks, I made a list of four basic points that contribute to a talk being compelling.

Here they are.

1. Love your taxon. It’s one of the main generators of enthusiasm, and nothing is more engaging than enthusiasm. I’ve seen dinosaur talks given by people who clearly don’t much care for dinosaurs. It comes through and destroys the appeal of the talk. Conversely, at TetZooCon a couple of months ago, one of the highlights was Helen Meredith’s talk “What have amphibians ever done for us?” about a group that doesn’t honestly excite me much — but the amphibians excited her so much that I caught that excitement. Ditto for Lionel Hautier’s sloth talk at SVPCA.

What to do if you don’t love your taxon? Give a talk about something you do love. If you don’t love anything, why are you in this field? Really, without enthusiasm, you’re lost. If you don’t care, neither will we. So care.

2. Show us pictures of your taxon. If you’re a particle physicist, pretty much the only thing you can show in your talk is graphs. But one of the great things about vertebrate palaeontology and comparative anatomy is that the field is just bursting with beautiful, photogenic objects. So for heaven’s sake show them to us! Yes yes, you may legitimately have to show graphs later on when you get to the hardcore stuff, but your best bet to get us interested in (say) voles is to show us that they’re interesting. In palaeo, that means we need to see both bones and life restorations.

3. Engage with the audience. That means you need to know your material well enough that you don’t need to be reading notes. Yes, notes are a help if you’re nervous, but they absolutely kill any sense of connection between speaker and listeners. Do whatever it takes to avoid a monotone.

There are two ways to do this. The simplest is to learn your talk. Write it out longhand if you find that helpful, but then rehearse it enough times that you don’t need the script — so that seeing each slide is enough to send you, Pavlov-like, into the relevant bit of spiel. Then you can be making eye contact and waving your hands around, just like you would if you were explaining something in a pub.

The second way to do it is even better. Don’t just learn your talk but learn your subject. If you get sufficiently familiar with (say) sauropod necks, then you can hardly watch one of your slides come up and not start talking about what it shows you. It’s better to fly blind (even if you risk a crash) than to crawl. (And of course you’re not really blind: preparing the slides burns the narrative of the talk into your mind, so that you know where you’re going even without really trying.)

4. Tell a story. People are wired to love stories. It’s not coincidence that Aesop and Jesus both did their moral teaching principally through stories; nor that Dawkins’ fine explanations of evolution are expressed in pretty much story form. People who are listening to a story want to know what happens next.

“I did a principle component analysis” is not a story. “How separate radiations of anole lizards evolved to fill the same set of ecological niches” is a story. If a principle component analysis is the way you reach the punchline of that story, fine: the point is, you’ve made us care about the PCA before we get to it, because we want to find out what happens to the cute little lizards (which you showed us lots of nice pictures of early on).

Here is the key point that underlies all this, and which I fear students are not always told as clearly as they should be: talks are not papers. A paper by convention is dry. It’s mostly words, equations and (often) graphs. A talk can’t afford to be dry, and by nature is about images and speech. It’s a much more human thing.

Here’s one reason why. We only read papers that we’re already interested in. I’ll read the sauropod papers in JVP, but skip over the fish, sloth and bird papers. That’s because I am already invested in sauropods, and because I know enough about them to make sense of a dry, technical paper. But when we go to conferences, we hear talks on lots of things that we’re not pre-interested in. A good speaker makes us interested. She has to. In short, a paper is directed at a specialist audience, while a talk has to win an audience from among non-specialists.

To be even shorter: talks should be fun to watch and listen to!

Please welcome Rukwatitan (over on Mark Witton’s blog)

September 9, 2014

I just read Mark Witton’s piece on the new new titanosaur Rukwatitan (as opposed to the old new titanosaur Dreadnoughtus). I was going to write something about it, but I realised that Mark has already said everything I would have, but better. So get yourselves over to his piece and enjoy the titanosaurianness of it all!

September 5, 2014

Yay, vertebrae! Lacovara et al. (2014: fig. 1)

Mike and I are in York for SVPCA — more on that soonish — and I just wanted to get out some quick thoughts about the world’s newest giant sauropod.

First off, the paper (Lacovara et al. 2014) is open access, which is great. And, hey, 3D PDFs of the whole skeleton and selected elements — I’m going to be having some fun with those.

And given that this is a short initial descriptive paper, I was really happy to see a reasonably detailed table of measurements. The materials and methods section at the end spells out explicitly how the team arrived at their estimates of the animal’s length and mass. All of that looks very solid, and it’s more information that we often get in these short initial descriptions. So although I will look forward to seeing a complete osteological description of Dreadnoughtus in the future, this first paper is better than a lot of similar papers in that it includes a lot of actually useful information.

As for whether Dreadnoughtus was the world’s heaviest sauropod — how could anyone possibly tell? The femur of Dreadnoughtus is 1.9 meters, which is only three-quarters of the estimated length of the largest partial femur of Argentinosaurus. Now, there is plenty of evidence from both histology and macro-level indicators of skeletal age that the holotype individual was still growing, but how much bigger was it going to get, 10%, 25%? I think that given its size, completeness, and immature state it is fair to discuss Dreadnoughtus in the same breath as Argentinosaurus, Puertasaurus, the largest specimens of Alamosaurus, and other giant sauropods. But I think any claim that it is ‘the’ heaviest is premature until we know how big a fully adult Dreadnoughtus was.

Dreadnoughtus and kin. Lacovara et al. (2014: fig. 3)

Here’s a weird thing: according to Table 1, the 113-cm cervical vertebra of Dreadnoughtus is the longest known among titanosauriforms. But the longest cervical of Sauroposeidon has a 125-cm centrum, and Sauroposeidon always comes out as a titanosauriform in phylogenetic analyses, including the one in the Dreadnoughtus paper. The estimated 2.5-meter femur of Argentinosaurus reported by Mazzetta et al. (2004) is also not listed in that table, although some estimated lengths for other incomplete elements are given. I don’t think there’s any conspiracy here — it is actually quite a challenge to keep up with all of the relevant numbers — but I would like to have seen a bit more thoroughness in reporting measurements of other sauropods where at least some individual elements are larger than in Dreadnoughtus.

Anyway, as we found for the next-most-recent “world’s largest dinosaur” earlier this year, Dreadnoughtus does not extend the known size range of the largest sauropods. Period. Anyone who says definitively otherwise is actually making assumptions about ontogeny and mass estimation that just aren’t justified.

Does that mean that Dreadnoughtus isn’t interesting? Of course not! For one thing, now we can start talking intelligently about the body proportions of these giant titanosaurs. Up until now we’ve had a good idea of what other, smaller sauropods looked like, things like Mamenchisaurus, Diplodocus, and Giraffatitan, and we’ve had reasonably complete skeletons of small titanosaurs such as Malawisaurus and Rapetosaurus, but we haven’t had a very clear idea of the proportions of the largest titanosaurs (sometimes because of conflicting measurements). So now we can start investigating questions involving the biomechanics and hopefully the growth trajectories of giant titanosaurs, which were more in the realm of speculation until now. There are some tantalizing hints toward this in the current paper — for example, the authors mention that a lot of the bones preserve muscle attachments. That would be a fascinating study in its own right, just knowing what the muscle attachments can tell us about the soft-tissue anatomy of Dreadnoughtus, and in turn what soft tissue can tell us about how the muscles and joints worked.

Big and getting bigger: the limb bones of Dreadnoughtus. Lacovara et al. (2014: fig. 2)

There are myriad interesting questions dealing with the ability of the limb bones and vertebrae to support the mass of the body and how that skeletal support changed, both over the lifespan of an animal and over evolutionary time. Now, there is a limit to how much Dreadnoughtus can add here, since it’s only known by two individuals that weren’t radically different in size, but given how bleak the data landscape is for giant titanosaurs, it’s an important addition to our knowledge.

In conclusion, although I have some reservations about overlooked measurements of some other giant sauropods, and although the media-driven Dreadnoughtus-vs-Argentinosaurus pissing contest is pointless, I’m excited about this first paper. And I’m looking forward to more, both more complete descriptive work, and functional and biomechanical analyses building on that. Happy days.

References

Come on, AAAS. You can do better than this.

September 2, 2014

A couple of weeks ago, more than hundred scientists sent an open letter to the AAAS (American Association for the Advancement of Science) about their new open-access journal Science Advances, which is deficient in various ways — not least the absurdly inflated article-processing charge.

Today I learn from email that there has finally been a response — of sorts. Editor-in-Chief Marcia McNutt had a long phone-call with Jon Tennant — one of the hundred-plus authors/co-signers. All we know about that call is (and I quote from Jon’s email account) “it became quite apparent that we would have to agree to disagree on many points”.

All I want to say is this. When a hundred scientists co-sign an open letter, it is TOTALLY UNACCEPTABLE for the response to take the form of a private telephone call with one of those authors.

Come on, AAAS. This is all about openness. Let’s see an open response: a substantive, non-patronising one which addresses the actual points made in the original letter.

Meanwhile, you may like to read this article at The New StatesmanScientists criticise new “open access” journal which limits research-sharing with copyright. In finishes on this very clear note, courtesy of Jon Tennant:

The AAAS should be a shining beacon within the academic world for progression of science. If this is their best shot at that, it’s an absolute disaster at the start on all levels. What publishers need to remember is that the academic community is not here to serve them – it is the other way around.