How bigsmall was Aquilops?

December 12, 2014

Handling Aquilops by Brian Engh

Life restoration of Aquilops by Brian Engh (CC-BY).

If you’ve been reading around about Aquilops, you’ve probably seen it compared in size to a raven, a rabbit, or a cat. Where’d those comparisons come from? You’re about to find out.

Back in April I ran some numbers to get a rough idea of the size of Aquilops, both for my own interest and so we’d have some comparisons handy when the paper came out.

Archaeoceratops skeletal reconstruction by Scott Hartman. Copyright Scott Hartman, 2011, used here by permission.

Archaeoceratops skeletal reconstruction by Scott Hartman. Copyright Scott Hartman, 2011, used here by permission.

I started with the much more completely known Archaeoceratops. The measurements of Scott Hartman’s skeletal recon (shown above and on Scott’s website – thanks, Scott!) match the measurements of the Archaeo holotype given by Dodson and You (2003) almost perfectly. The total length of Archaeoceratops, including tail, is almost exactly one meter. Using graphic double integration, I got a volume of 8.88L total for a 1m Archaeoceratops. That would come down to 8.0L if the lungs occupied 10% of body volume, which is pretty standard for non-birds. So that’s about 17-18 lbs.

Archaeoceratops and Aquilops skulls to scale

Aquilops model by Garrett Stowe, photograph by Tom Luczycki, copyright and courtesy of the Sam Noble Oklahoma Museum of Natural History.

Archaeoceratops has a rostrum-jugal length of 145mm, compared to 84mm in Aquilops. Making the conservative assumption that Aquilops = Archaeoceratops*0.58, I got a body length of 60cm (about two feet), and volumes of 1.73 and 1.56 liters with and without lungs, or about 3.5 lbs in life. The internet informed me that the common raven, Corvus corax, has an adult length of 56-78 cm and a body mass of 0.7-2 kg. So, based on this admittedly tall and teetering tower of assumptions, handwaving, and wild guesses, Aquilops (the holotype individual, anyway) was about the size of a raven, in both length and mass. But ravens, although certainly well-known, are maybe a bit remote from the experience of a lot of people, so we wanted a comparison animal that more people would be familiar with. The estimated length and mass of the holotype individual of Aquilops also nicely overlap the species averages (60 cm, 1.4-2.7 kg) for the black-tailed jackrabbit, Lepus californicus, and they’re pretty close to lots of other rabbits as well, hence the comparison to bunnies.

Of course, ontogeny complicates things. Aquilops has some juvenile characters, like the big round orbit, but it doesn’t look like a hatchling. Our best guess is that it is neither a baby nor fully grown, but probably an older juvenile or young subadult. A full-grown Aquilops might have been somewhat larger, but almost certainly no larger than Archaeoceratops, and probably a meter or less in total length. So, about the size of a big housecat. That’s still pretty darned small for a non-avian dinosaur.

Although Aquilops represents everything I normally stand against – ornithischians, microvertebrates, heads – I confess that I have a sneaking affection for our wee beastie. Somebody’s just gotta make a little plush Aquilops, right? When and if that happens, you know where to find me.

References

Life restoration of Aquilops by Brian Engh. Farke et al. (2014: fig. 6C). CC-BY.

Life restoration of Aquilops by Brian Engh. Farke et al. (2014: fig. 6C). CC-BY.

Today sees the description of Aquilops americanus (“American eagle face”), a new basal neoceratopsian from the Cloverly Formation of Montana, by Andy Farke, Rich Cifelli, Des Maxwell, and myself, with life restorations by Brian Engh. The paper, which has just been published in PLOS ONE, is open access, so you can download it, read it, share it, repost it, remix it, and in general do any of the vast scope of activities allowed under a CC-BY license, as long as we’re credited. Here’s the link – have fun.

Obviously ceratopsians are much more Andy’s bailiwick than mine, and you should go read his intro post here. In fact, you may well be wondering what the heck a guy who normally works on huge sauropod vertebrae is doing on a paper about a tiny ceratopsian skull. The short, short version is that I’m here because I know people.

OMNH 34557, the holotype of Aquilops

OMNH 34557, the holotype of Aquilops

The slightly longer version is that OMNH 34557, the holotype partial skull of Aquilops, was discovered by Scott Madsen back in 1999, on one of the joint Cloverly expeditions that Rich and Des had going on at the time. That the OMNH had gotten a good ceratopsian skull out of Cloverly has been one of the worst-kept secrets in paleo. But for various complicated reasons, it was still unpublished when I got to Claremont in 2008. Meanwhile, Andy Farke was starting to really rock out on ceratopsians at around that time.

For the record, the light bulb did not immediately go off over my head. In fact, it took a little over a year for me to realize, “Hey, I know two people with a ceratopsian that needs describing, and I also know someone who would really like to head that up. I should put these folks together.” So I proposed it to Rich, Des, and Andy in the spring of 2010, and here we are. My role on the paper was basically social glue and go-fer. And I drew the skull reconstruction – more on that in the next post.

One of the world's smallest ceratopsians meets one of the largest: the reconstructed skull of Aquilops with Rich Cifelli and Pentaceratops for scale.

One of the world’s smallest ceratopsians meets one of the largest: the reconstructed skull of Aquilops with Rich Cifelli and Pentaceratops for scale. Copyright Leah Vanderburg, courtesy of the Sam Noble Oklahoma Museum of Natural History.

Anyway, it’s not my meager contribution that you should care about. I am fairly certain that, just as Brontomerus coasted to global fame on the strength of Paco Gasco’s dynamite life restoration, whatever attention Aquilops gets will be due in large part to Brian Engh’s detailed and thoughtful work in bringing it to life – Brian has a nice post about that here. I am very happy to report that the three pieces Brian did for us – the fleshed-out head that appears at the top of this post and as Figure 6C in the paper, the Cloverly environment scene with the marauding Gobiconodon, and the sketch of the woman holding an Aquilops - are also available to world under the CC-BY license. So have fun with those, too.

Finally, I need to thank a couple of people. Steve Henriksen, our Vice President for Research here at Western University of Health Sciences, provided funds to commission the art from Brian. And Gary Wisser in our scientific visualization center used his sweet optical scanner to generate the hi-res 3D model of the skull. That model is also freely available online, as supplementary information with the paper. So if you have access to a 3D printer, you can print your own Aquilops – for research, for teaching, or just for fun.

Cloverly environment with Aquilops and Gobiconodon, by Brian Engh (CC-BY).

Cloverly environment with Aquilops and Gobiconodon, by Brian Engh (CC-BY).

Next time: Aquilöps gets röck döts.

Reference

Farke, A.A., Maxwell, W.D., Cifelli, R.L., and Wedel, M.J. 2014. A ceratopsian dinosaur from the Lower Cretaceous of Western North America, and the biogeography of Neoceratopsia. PLoS ONE 9(12): e112055. doi:10.1371/journal.pone.0112055

In a comment on the last post, on the mass of Dreadnoughtus, Asier Larramendi wrote:

The body mass should be considerably lower because the reconstructed column don’t match with published vertebrae centra lengths. 3D reconstruction also leaves too much space between vertebrae. The reconstruction body trunk is probably 15-20% longer than it really was. Check the supplementary material: http://www.nature.com/srep/2014/140904/srep06196/extref/srep06196-s1.pdf

So I did. The table of measurements in the supplementary material is admirably complete. For all of the available dorsal vertebrae except D9, which I suppose must have been too poorly preserved to measure the difference, Lacovara et al. list both the total centrum length and the centrum length minus the anterior condyle. Centrum length minus the condyle is what in my disseration I referred to as “functional length”, since it’s the length that the vertebra actually contributes to the articulated series, assuming that the condyle of one vertebra sticks out about as far as the cotyle is recessed on the next vertebra. Here are total lengths/functional lengths/differences for the seven preserved dorsals, in mm:

  • D4 – 400/305/95
  • D5 – 470/320/150
  • D6 – 200/180/20
  • D7 – 300/260/40
  • D8 – 350/270/80
  • D9 – 410/ – / -
  • D10 – 330/225/105

The average difference between functional length and total length is 82 mm. If we apply that to D9 to estimate it’s functional length, we get 330mm. The summed functional lengths of the seven preserved vertebrae are then 1890 mm. What about the missing D1-D3? Since the charge is that Lacovara et al. (2014) restored Dreadnoughtus with a too-long torso, we should be as generous as possible in estimating the lengths of the missing dorsals. In Malawisaurus the centrum lengths of D1-D3 are all less than or equal to that of D4, which is the longest vertebra in the series (Gomani 2005: table 3), so it seems simplest here to assign D1-D3 functional lengths of 320 mm. That brings the total functional length of the dorsal vertebral column to 2850 mm, or 2.85 m.

At this point on my first pass, I was thinking that Lacovara et al. (2014) were in trouble. In the skeletal reconstruction that I used for the GDI work in the last post, I measured the length of the dorsal vertebral column as 149 pixels. Divided by 36 px/m gives a summed dorsal length of 4.1 m. That’s more than 40% longer than the summed functional lengths of the vertebrae calculated above (4.1/2.85 = 1.44). Had Lacovara et al. really blown it that badly?

Before we can rule on that, we have to estimate how much cartilage separated the dorsal vertebrae. This is a subject of more than passing interest here at SV-POW! Towers–the only applicable data I know of are the measurements of intervertebral spacing in two juvenile apatosaurs that Mike and I reported in our cartilage paper last year (Taylor and Wedel 2013: table 3, and see this post). We found that the invertebral cartilage thickness equaled 15-24% of the length of the centra.* For the estimated 2.85-meter dorsal column of Dreadnoughtus, that means 43-68 cm of cartilage (4.3-6.8 cm of cartilage per joint), for an in vivo dorsal column length of 3.28-3.53 meters. That’s still about 15-20% shorter than the 4.1 meters I measured from the skeletal recon–and, I must note, exactly what Asier stated in his comment. All my noodling has accomplished is to verify that his presumably off-the-cuff estimate was spot on. But is that a big deal?

Visually, a 20% shorter torso makes a small but noticeable difference. Check out the original reconstruction (top) with the 20%-shorter-torso version (bottom):

Dreadnoughtus shortened torso comparison - Lacovara et al 2014 fig 2

FWIW, the bottom version looks a lot more plausible to my eye–I hadn’t realized quite how weiner-dog-y the original recon is until I saw it next to the shortened version.

In terms of body mass, the difference is major. You’ll recall that I estimated the torso volume of Dreadnoughtus at 32 cubic meters. Lopping off 20% means losing 6.4 cubic meters–about the same volume as a big bull elephant, or all four of Dreadnoughtus‘s limbs put together. Even assuming a low whole-body density of 0.7 g/cm^3, that’s 4.5 metric tons off the estimated mass. So a ~30-ton Dreadnoughtus is looking more plausible by the minute.

For more on how torso length can affect the visual appearance and estimated mass of an animal, see this post and Taylor (2009).

* I asked Mike to do a review pass on this post before I published, and regarding the intervertebral spacing derived from the juvenile apatosaurs, he wrote:

That 15-24% is for juveniles. For the cervicals of adult Sauroposeidon we got about 5%. Why the differences? Three reasons might be relevant: 1, taxonomic difference between Sauroposeidon and Apatosaurus; 2, serial difference between neck and torso; 3, ontogenetic difference between juvenile and adult. By applying the juvenile Apatosaurus dorsal measurement directly to the adult Dreadnoughtus dorsals, you’re implicitly assuming that the adult/juvenile axis is irrelevant (which seems unlikely to me), that the taxonomic axis is (I guess) unknowable, and that the cervical/dorsal distinction is the only one that matter.

That’s a solid point, and it deserves a post of its own, which I’m already working on. For now, it seems intuitively obvious to me that we got a low percentage on Sauroposeidon simply because the vertebrae are so long. If the length-to-diameter ratio was 2.5 instead of 5, we’d have gotten 10%, unless cartilage thickness scales with centrum length, which seems unlikely. For a dorsal with EI of 1.5, cartilage thickness would then be 20%, which is about what I figured above.

Now, admittedly that is arm-waving, not science (and really just a wordy restatement of his point #2). The obvious thing to do is take all of our data and see if intervertebral spacing is more closely correlated with centrum length or centrum diameter. Now that it’s occurred to me, it seems very silly not to have done that in the actual paper. And I will do that very thing in an upcoming post. For now I’ll just note three things:

  1. As you can see from figure 15 in our cartilage paper, in the opisthocoelous anterior dorsals of CM 3390, the condyle of the posterior vertebra is firmly engaged in the cotyle of the anterior one, and if anything the two vertebrae look jammed together, not drifted apart. But the intervertebral spacing as a fraction of centrum length is still huge (20+4%) because the centra are so short.
  2. Transferring these numbers to Dreadnoughtus only results in 4.3-6.8 cm of cartilage between adjacent vertebrae, which does not seem unreasonable for a 30- or 40-ton animal with dorsal centra averaging 35 cm in diameter. If you asked me off the cuff what I thought a reasonable intervertebral spacing was for such a large animal, I would have said 3 or 4 inches (7.5 to 10 cm), so the numbers I got through cross-scaling are actually lower than what I would have guessed.
  3. Finally, if I’ve overestimated the intervertebral spacing, then the actual torso length of Dreadnoughtus was even shorter than that illustrated above, and the volumetric mass estimate would be smaller still. So in going with relatively thick cartilage, I’m being as generous as possible to the Lacovara et al. (2014) skeletal reconstruction (and indirectly to their super-high allometry-derived mass estimate), which I think is only fair.

References

 

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

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)

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)

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

Supersaurus vs Brachiosaurus - BYU 9024 and FMNH P25107

This was inspired by an email Mike sent a couple of days ago:

Remind yourself of the awesomeness of Giraffatitan:
http://svpow.files.wordpress.com/2008/11/mike-by-jango-elbow.jpeg

Now think of this. Its neck is 8.5m long. Knock of one measly meter — for example, by removing one vertebra from the middle of the neck — and you have 7.5 m.

Supersaurus’s neck was probably TWICE that long.

Holy poo.

I replied that I was indeed freaked out, and that it had given me an idea for a post, which you are now reading. I didn’t have a Giraffatitan that was sufficiently distortion-free, so I used my old trusty Brachiosaurus. The vertebra you see there next to Mike and next to the neck of Brachiosaurus is BYU 9024, the longest vertebra that has ever been found from anything, ever.

Regarding the neck length of Supersaurus, and how BYU 9024 came to be referred to Supersaurus, here’s the relevant chunk of my dissertation (Wedel 2007: pp. 208-209):

Supersaurus is without question the longest-necked animal with preserved cervical material. Jim Jensen recovered a single cervical vertebra of Supersaurus from Dry Mesa Quarry in western Colorado. The vertebra, BYU 9024, was originally referred to “Ultrasauros”. Later, both the cervical and the holotype dorsal of “Ultrasauros” were shown to belong to a diplodocid, and they were separately referred to Supersaurus by Jensen (1987) and Curtice et al. (1996), respectively.

BYU 9024 has a centrum length of 1378 mm, and a functional length of 1203 mm (Figure 4-3). At 1400 mm, the longest vertebra of Sauroposeidon is marginally longer in total length [see this post for a visual comparison]. However, that length includes the prezygapophyses, which overhang the condyle, and which are missing from BYU 9024. The centrum length of the largest Sauroposeidon vertebra is about 1250 mm, and the functional length is 1190 mm. BYU 9024 therefore has the largest centrum length and functional length of any vertebra that has ever been discovered for any animal. Furthermore, the Supersaurus vertebra is much larger than the Sauroposeidon vertebrae in diameter, and it is a much more massive element overall.

Neck length estimates for Supersaurus vary depending on the taxon chosen for comparison and the serial position assumed for BYU 9024. The vertebra shares many similarities with Barosaurus that are not found in other diplodocines, including a proportionally long centrum, dual posterior centrodiapophyseal laminae, a low neural spine, and ventrolateral flanges that connect to the parapophyses (and thus might be considered posterior centroparapophyseal laminae, similar to those of Sauroposeidon). The neural spine of BYU 9024 is very low and only very slightly bifurcated at its apex. In these characters, it is most similar to C9 of Barosaurus. However, theproportions of the centrum of BYU 9024 are more similar to those of C14 of Barosaurus, which is the longest vertebra of the neck in AMNH 6341. BYU 9024 is 1.6 times as long as C14 of AMNH 6341 and 1.9 times as long as C9. If it was built like that of Barosaurus, the neck of Supersaurus was at least 13.7 meters (44.8 feet) long, and may have been as long as 16.2 meters (53.2 feet).

Based on new material from Wyoming, Lovelace et al. (2005 [published as Lovelace et al. 2008]) noted potential synapomorphies shared by Supersaurus and Apatosaurus. BYU 9024 does not closely resemble any of the cervical vertebrae of Apatosaurus. Instead of trying to assign its serial position based on morphology, I conservatively assume that it is the longest vertebra in the series if it is from an Apatosaurus-like neck. At 2.7 times longer than C11 of CM 3018, BYU 9024 implies an Apatosaurus-like neck about 13.3 meters
(43.6 feet) long.

Supersaurus vs Diplodocus BYU 9024 and USNM 10865 - Gilmore 1932 pl 6

Bonus comparo: BYU 9024 vs USNM 10865, the mounted Diplodocus longus at the Smithsonian, modified from Gilmore 1932 (plate 6). For this I scaled BYU 9024 against the 1.6-meter femur of this specimen.

If you’d like to gaze upon BYU 9024 without distraction, or put it into a composite of your own, here you go:

Supersaurus cervical BYU 9024

References

 

We’ve touched on this several times in various posts and comment threads, but it’s worth taking a moment to think in detail about the various published mass estimates for the single specimen BM.R.2181 (formerly known as HMN SII), the paralectotype of Giraffatitan brancai, which is the basis of the awesome mounted skeleton in Berlin.

Here is the table of published estimates from my 2010 sauropod-history paper, augmented with the two more recent estimates extrapolated from limb-bone measurements:

Author and date Method Volume (l) Density (kg/l) Mass (kg)
Janensch (1938) Not specified `40 t’
Colbert (1962) Displacement of sand 86,953 0.9 78,258
Russell et al. (1980) Limb-bone allometry 13,618
Anderson et al. (1985) Limb-bone allometry 29,000
Paul (1988) Displacement of water 36,585 0.861 31,500
Alexander (1989) Weighing in air and water 46,600 1.0 46,600
Gunga et al. (1995) Computer model 74,420 1.0 74,420
Christiansen (1997) Weighing in air and water 41,556 0.9 37,400
Henderson (2004) Computer model 32,398 0.796 25,789
Henderson (2006) Computer model 25,922
Gunga et al. (2008) Computer model 47,600 0.8 38,000
Taylor (2009) Graphic double integration 29,171 0.8 23,337
Campione and Evans (2012) Limb-bone allometry 35,780
Benson et al. (2014) Limb-bone allometry 34,000

(The estimate of Russell et al. (1980) is sometimes reported as 14900 kg. However, they report their estimate only as “14.9 t”; and since they also cite “the generally accepted figure of 85 tons”, which can only be a reference to Colbert (1962)”, we must assume that Russell et al. were using US tons throughout.)

The first thing to notice is that there is no very clear trend through time, either upwards or downwards. Here’s a plot of mass (y-axis) against year of estimate (x-axis):

giraffatitan-mass-by-year

I’ve not even tried to put a regression line through this: the outliers are so extreme they’d render it pretty much useless.

In fact, the lowest and highest estimates differ by a factor of 5.75, which is plainly absurd.

But we can go some way to fixing this by discarding the outliers. We can dump Colbert (1962) and Alexander (1989) as they used overweight toys as their references. We more or less have to dump Russell et al. (1980) simply because it’s impossible to take seriously. (Yes, this is the argument from personal incredulity, and I don’t feel good about it; but as Pual (1988) put it, “so little flesh simply cannot be stretched over the animal’s great frame”.) And we can ignore Gunga et al. (1995) because it used circular conic sections — a bug fixed by Gunga et al. (2008) by using elliptical sections.

With these four unpalatable outliers discarded, our highest and lowest estimates are those of Gunga et al. (2008) at 38,000 kg and Taylor (2009)at 23,337. The former should be taken seriously as it was done using photogrammetrical measurements of the actual skeletal mount. And so should the latter because Hurlburt (1999) showed that GDI is generally the least inaccurate of our mass-estimation techniques. That still gives us a factor of 1.63. That’s the difference between a lightweight 66 kg man and and overweight 108 kg.

Here’s another way of thinking about that 1.63 factor. Assuming two people are the same height, one of them weighing 1.62 times as much as the other means he has to be 1.28 times as wide and deep as the first (1.28^2 = 1.63). Here is a man next to his 1.28-times-as-wide equivalent:

two-men

 

I would call that a very noticeable difference. You wouldn’t expect someone estimating the mass of one of these men to come up with that of the other.

So what’s going on here? I truly don’t know. We are, let’s not forget, dealing with a complete skeletal mount here, one of the very best sauropod specimens in the world, which has been extensively studied for a century. Yet even within the last six years, we’re getting masses that vary by as much as the two dudes above.

 

Way back in November 2011, I got this inquiry from Keiron Pim:
I’m currently writing a popular guide to dinosaurs, to be published by Random House next autumn [Ed.: available now at amazon.com and at amazon.co.uk]. I’ve been writing about [Brachiosaurus and Giraffatitan], and have read your 2009 study vindicating the proposal to separate them into two genera.
[…]
I know you consider Brachiosaurus likely to have been bigger (and note that the specimen was not fully grown), with a longer trunk and tail – but most of the sources I can find give both animals the same body length, generally around 26m. Presumably this doesn’t reflect your work, and your calculations are different.

I replied at the time, and said that I’d post that response here on SV-POW!. But one thing and another prevented me from getting around to it, and I forgot all about it until recently. Since we’re currently in a sequence of Brachiosaurus-themed posts [part 1, part 2, part 3, part 4, part 5, part 6], this seems like a good time to fix that. So here is my response, fresh from November 2011, lightly edited.

dscn1253

Well, Giraffatitan has only been recognised as a separate animal at all in the last couple of years, and nearly everything that has been written about “Brachiosaurus“, at least in the technical literature, is actually about Giraffatitan. So existing sources that give the same length for both are probably not making a meaningful distinction between the two animals.

First, on Giraffatitan: Janensch (1950b:102) did a great job of measuring his composite mounted skeleton. His figure for the total length of Giraffatitan along the neural canal is 22.46 m, and is certainly the best estimate in the literature for an actual brachiosaur specimen (and quite possibly the best for any sauropod).

I don’t know where the figure of 26 m comes from, but as Janensch (1961:213) notes, the isolated fibula XV2 of Giraffatitan in the Berlin collection is 134 cm long, compared with 119 cm for that of the mounted skeleton. This is 1.126 times as long, which if scaled isometrically would yield a total length of 25.29 m.  So that is defensible, but 26 m is not, really.

I would advise sticking with Janensch’s published figure of 22.46 m, as it’s based on good material, and also because it forms the basis of my comparative estimate for a Brachiosaurus of similar limb length.

Now in my 2009 paper I estimated with reasonable rigour that the torso of Brachiosaurus was probably about 23% longer than that of Giraffatitan, yielding 4.82 m rather than 3.92 that Janensch gave for Giraffatitan. On much less solid evidence, I tentatively estimated that the tail of Brachiosaurus might have been 20-25% longer than that of Giraffatitan. Given the paucity of evidence I would play safer by going with the lower end of that estimate, which would give a tail length of 9.14 m compared with Janensch’s 7.62 for Giraffatitan. Riggs (1904) tells us that the sacrum of Brachiosaurus is 0.95 m long, which is slightly less than 1.07 m for Giraffatitan. Finally, since we know nothing of the head and neck of Brachiosaurus, the null hypothesis has to be that they were similar in proportion to those of Giraffatitan.

Putting it all together, Brachiosaurus may have been longer in the torso by 0.9 m, and in the tail by 1.52 m, but shorter in the sacrum by 0.12 m — for a total additional length of 2.3 m. That would make Brachiosaurus 24.76 m long, which is 10% longer than Giraffatitan.

Note that all the Brachiosaurus figures are given with much greater precision than the sparse data we have really allows.  I think you could round Janensch’s 22.46 m for Giraffatitan to 22.5 and be pretty confident in that number, but you shouldn’t really say anything more precise than “maybe about 25 m” for Brachiosaurus.

Finally, you correctly note that the Brachiosaurus specimen was not fully grown — we can tell because its coracoid was not fused to the scapula. But the same is true of the mounted Giraffatitan, so these two very similarly sized animals were both subadult. How much bigger did they get?  We know from the fibula that Giraffatitan got at least 12-13% bigger than the well-known specimen, and I’d be pretty happy guessing the same about Brachiosaurus.  And I wouldn’t rule out much bigger specimens, either.

References

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