University of Portsmouth

Mike Taylor

\[Sorry about the late posting today: I had to leave the house at 7:15
to fly to Copenhagen for Christmas lunch --- long story --- and I am
completing today's post from my hotel room.\]

There's no getting away from it: everyone wants to know how big
dinosaurs are. *Xenoposeidon* is based on a single partial vertebra, so
there is no way to be at all sure about the size and shape of the whole
animal; but we knew that everyone would want to know, so in the paper
(Taylor and Naish 2007, natch) we made the best guesses we could.
Details on how we did this follow below.

Then, a few months after the revised manuscript was accepted by
*Palaeontology*, purely as a joke, I put together what I called "the
first scientifically rigorous skeletal reconstruction of
*Xenoposeidon*". The joke is based on a very well-established style for
skeletal reconstructions of complete or nearly complete skeletons, first
popularised by Greg Paul, in which the animal is shown in left lateral
view, with bones drawn in white and the soft tissue outlined as a black
silhouette --- as in these rather beautiful Morrison Formation
diplodocids from Paul (1998: fig. 1b).  F is *Diplodocus carnegii* CM 84
with skull CM 3542 scaled to fit, G is *Barosaurus lentus* AMNH 6341
(with a fair bit of guesswork to fill in the gaps) and E is *Apatosaurus
louisae* CM 3018 with skull CM 11162 scaled to fit:

[![paul1998-fig1b-diplodocids-500px.jpeg](https://svpow.files.wordpress.com/2007/11/paul1998-fig1b-diplodocids-500px.jpeg)](http://svpow.files.wordpress.com/2007/11/paul1998-fig1b-diplodocids.jpeg "paul1998-fig1b-diplodocids.jpeg")

I thought it would be funny to do this for an animal known only from a
single bone, showing the bone floating in the middle of a big black
silhouette. Har har. (You may not find that funny. The key point here is
that Matt and Darren both did, which is all I was aiming for. And it
turns out that [John Hutchinson
agrees](http://scienceblogs.com/tetrapodzoology/2007/11/the_worlds_most_amazing_saurop.php#comment-640197),
which proves it.)

[![xenoposeidon-skeleton-500px.jpeg](https://svpow.files.wordpress.com/2007/11/xenoposeidon-skeleton-500px.jpeg)](http://svpow.files.wordpress.com/2007/11/xenoposeidon-skeleton.jpeg "xenoposeidon-skeleton.jpeg")

In fact the only things that are remotely scientific about this
"reconstruction" are that the bone is in roughly the right place (i.e. a
posterior dorsal) and the human is about the right size. The actual
shape of the "reconstruction" is a total guess: given that
[*Xenoposeidon* could belong in any part of the huge clade
Neosauropoda](http://svpow.wordpress.com/2007/11/15/xenoposeidon-week-day-1-introducing-xeno/#comment-134),
the only reason I went with a brachiosaurid body-shape rather than, say,
a diplodocid was that I like brachiosaurus more. They're just cooler.
(This is how science is done, kids! Don't tell your parents!)

It wasn't until much later, when the publicity was in full swing, that
it occurred to me that if the media were going to use the "skeletal
reconstruction" it would be better to give them one with *all* the
bones, greyed out. I made this almost as soon as it had occurred to me,
and made it available to newspapers and TV stations, but it was a bit
too late in the day --- hardly any of them used it. Here it is, anyway:

[![xenoposeidon-skeleton2-500px.jpeg](https://svpow.files.wordpress.com/2007/11/xenoposeidon-skeleton2-500px.jpeg)](http://svpow.files.wordpress.com/2007/11/xenoposeidon-skeleton2.jpeg "xenoposeidon-skeleton2-500px.jpeg")

Now that all the bones are back in place, it's easy to recognise this as
a knock-off of Matt Wedel's *Brachiosaurus* reconstruction, as discussed
in a much-overlooked [Prehistoric Times
article](http://sauroposeidon.net/Wedel_2000_Brachiosaurus.pdf) and
which has since cropped up
[in](http://sauroposeidon.net/Wedel-Cifelli_2005_native-giant-plus-cover.pdf)
[various](http://www.cafepress.com/brachiosaurus)
[places](http://www.cafepress.com/poseidon). If I'd had my wits about
me, I'd have credited Matt for the original "reconstruction" that went
out to newspapers, but since it was only done as a joke it didn't occur
to me. Sorry, Matt!

So how did we figure out the probable size of *Xenoposeidon*? Since we
had only one vertebra to go on, and since even that was missing the
neural spine and other processes, all we had to work with were the
centrum measurements. We
[reconstructed](http://svpow.wordpress.com/2007/11/16/xenoposeidon-week-day-2-imagining-the-whole-thing/)
the centrum to be 20 cm long, with a cotyle diameter of 16.5 cm (the
average of 16 cm width and 17 cm height). Then we compared that with the
dimensions of vertebrae from the same position in the dorsal column of
other sauropods. For this, we chose *Brachiosaurus brancai* and
*Diplodocus carnegii*, because they are both known from nearly complete
skeletons, and pretty much bracket the range of neosauropod body shapes.

Guessing length is easy: we just assume that the total length of
*Xenoposeidon* would be in the same proportion to the length of one of
its posterior dorsals as in the comparison taxa. The *Xenoposeidon*
dorsal is 60% as long as the 33 cm D7 (seventh dorsal) of the
*Brachiosaurus brancai* type specimen HMN SII (Janensch 1950: 44). If
Xeno were built like a brachiosaurid, then it would be 60 per cent as
long as HMN SII, yielding a length of 15 m based on Paul's (1988)
estimate of 25 m for that specimen. Similarly, the *Xenoposeidon* dorsal
is 74% as long as the 27 cm vertebrae in the *Diplodocus carnegii* type
specimen CM 84 (the average of the lengths of D7 and D8 as stated in
Hatcher 1901:38). Therefore, a *Diplodocus*-like *Xenoposeidon* would be
about 20 m long, based on the widely quoted figure of 27 m for CM 84.

But these figures are subject to a fair degree of uncertainty, and
shouldn't be taken too seriously. Suppose for example that we compared
instead with D9 of *Diplodocus*, which is 29 cm long: the corresponding
length estimate for *Xenoposeidon* would have been reduced to 20/29 of
27 m, which is 18.5 m.

And now to the much more handwavy problem of estimating mass. We can
roughly extrapolate the mass of an animal as being proportional to the
centrum volume, which in turn is proportional to centrum length times
the square of its average cotyle diameter, and this is what we did in
the paper. For *Brachiosaurus brancai*, this means that we estimate
*Xenoposeidon* mass as (20 x 16.5 x 16.5) / (33 x 27 x 27) = 22% the
mass of *Brachiosaurus brancai*; or as (20 x 16.5 x 16.5) / (27 x 29.5 x
29.5) = 23% the mass of *Diplodocus carnegii*, depending on which it
most resembled.

But weight just a moment! (Har har.) What actually *is* the mass of
*Brachiosaurus brancai*? An astonishingly wide range of figures have
been calculated, all based on the same specimen (HMN SII). In
chronological order (and do let me know if I've missed any):

- 78 tonnes (Colbert 1962), based on the volume of sand displaced by a
  scaled plastic model
- 15 tonnes (Russell et al. 1980) based on the dimensions of limb-bones,
  plotted on a best straight line through limb-bone-thickness vs. mass
  for extant animals
- 47 tonnes (Alexander 1985), based on the volume of water displayed by
  a different model from the one that Colbert used
- 29 tonnes (Anderson et al. 1985), using a similar though slightly more
  rigorous method than that of Russell et al. (1980)
- 32 tonnes (Paul 1988), using, I think, graphic double integration or
  something similar (the paper is not very explicit)
- 74 tonnes (Gunga et al. 1995), based on a computer model built using
  data scanned with lasers --- very high-tech!
- 37 tonnes (Christiansen 1997) by suspension of scale models
- 26 tonnes (Henderson 2004) by measurement of computer model built by
  hand

We ignored the Russell et al. (1980) and Anderson et al. (1985)
estimates because they are not actual measurements of anything, and
depend instead on additional assumptions about scaling. We ignored the
Colbert (1962) estimate, too, as it was based on a grotesquely
overweight model. Worse still is the Gunga et al. (1995) estimate, which
starts with ultra-precise measurements of the skeleton and then throws
it away by fleshing that skeleton out in a computer model composed
entirely of circular conic sections. Needless to say this makes the
neck, torso and tail all hugely too wide, and the resulting volume
estimate is near worthless. When they redo this work using elliptical
conic sections, I will be interested to see how much their result comes
down by. That leaves the more reliable estimates of Alexander, Paul,
Christiansen and Henderson, and the average of their estimates 35322 kg
--- which seems correct using the
[well-it's-way-bigger-than-an-elephant-anyway
method](http://drvector.blogspot.com/2006/05/piling-on-tetrapods.html).
Using that figure (as we did in the paper), we got a probable mass of
7600 kg for *Xenoposeidon* --- and this is course is the source of the
statement, much quoted in the media, that it was "about as big as an
elephant". A big elephant.

But remember how vague this is: the lightest of the published estimates
of *Brachiosaurus brancai* mass is 42% of the value we used, and the
heaviest is 2.2 times as great. If we'd used these values in
extrapolating the mass of *Xenoposeidon* we could have arrived at 3200
kg or 16720 kg!

For *Diplodocus carnegii*, we just used the estimate of [Wedel
2005](http://sauroposeidon.net/Wedel_2005_pneumaticity-plus-table-and-figs.pdf),
12 tonnes, and got a mass of 2800 kg for *Xenoposeidon*. There is rather
less divergence in the various published mass estimates for this animal;
but whether that's because people's work on it is more consistent, or
just because not so many people can be bothered estimating the mass of
so relatively uncharismatic a sauropod, I wouldn't like to say.

All this means that *Xenoposeidon* was small --- for a sauropod. In our
poxy extant ecosystems, elephants are considered tolerably large, but in
the Mesozoic they would hardly have raised an eyebrow, and neither would
*Xenoposeidon*. Why the small size? Is it a juvenile? Apparently not: in
archosaurs (that is, crocodiles, birds and all descendents of their most
recent common ancestor, including dinosaurs), the neural arches fuse to
their centra only some way into growth (Brochu 1996), so a vertebra such
as that of *Xenoposeidon* in which the arch is fully fused and the
sutures completely obliterated indicates that the animals was mostly or
fully grown. Just small. Like Paul Simon. That's nothing to be ashamed
of.

Coming up next on SV-POW!'s *Xenoposeidon* week: on day 5, Darren will
talk about the joys and hazards of dealing with the media. Stay tuned.
(And, folks, please do make the occasional comment. Just so we know
someone's listening.)

## References

- Alexander, R. McNeill. 1985. Mechanics of posture and gait of some
  large dinosaurs: Zoological Journal of the Linnean Society 83: 1-25
- Anderson, J. F., A. Hall-Martin and Dale A. Russell. 1985. Long-bone
  circumference and weight in mammals, birds and dinosaurs. Journal of
  Zoology 207: 53-61
- Brochu, Christopher A. 1996. Closure of neurocentral sutures during
  crocodilian ontogeny: implications for maturity assessment in fossil
  archosaurs. Journal of Vertebrate Paleontology 16: 49-62.
- Christiansen, Per. 1997. Locomotion in sauropod dinosaurs. Gaia 14:
  45-75
- Colbert, Edwin H. 1962. The weights of dinosaurs. American Museum
  Novitates, 2076: 1-16
- Gunga, Hans-Christian, K. A. Kirsch, F. Baartz, L. Röcker, Wolf-Dieter
  Heinrich, W. Lisowski, A. Wiedemann and J. Albertz. 1995. New Data on
  the Dimensions of *Brachiosaurus brancai* and Their Physiological
  Implications. Naturwissenschaften 82: 190-192
- Hatcher, Jonathan B. 1901. *Diplodocus* (Marsh): its osteology,
  taxonomy and probable habits, with a restoration of the skeleton.
  Memoirs of the Carnegie Museum, 1: 1-63 and plates I-XIII
- Henderson, Donald M. 2004. Tipsy punters: sauropod dinosaur
  pneumaticity, bouyancy and aquatic habits. Proceedings of the Royal
  Society of London, B (Supplement) 271: S180-S183. doi
  10.10998/rsbl.2003.01.36
- Janensch, Werner. 1950. Die Wirbelsaule von *Brachiosaurus brancai*.
  Palaeontographica (Suppl. 7) 3: 27-93
- Paul, Gregory S. 1988. The brachiosaur giants of the Morrison and
  Tendaguru with a description of a new subgenus, *Giraffatitan*, and a
  comparison of the world's largest dinosaurs. Hunteria 2 (3): 1-14
- Paul, Gregory S. 1998. Terramegathermy and Cope's Rule in the land of
  titans. Modern Geology 23: 179-217
- Russell, Dale, Pierre Béland and John S. McIntosh. 1980. Paleoecology
  of the dinosaurs of Tendaguru (Tanzania). Memoires de la Societe
  Geologique de France 139: 169-175
- [Taylor, Michael P., and Darren Naish. 2007. An unusual new
  neosauropod dinosaur from the Lower Cretaceous Hastings Beds Group of
  East Sussex, England. Palaeontology 50 (6): 1547-1564. doi:
  10.1111/j.1475-4983.2007.00728.x](http://www.miketaylor.org.uk/dino/pubs/taylor-and-naish2007/TaylorNaish2007-xenoposeidon.pdf)
- Wedel, Mathew J. 2005. Postcranial skeletal pneumaticity in sauropods
  and its implications for mass estimates. pp. 201-228 in: Jeffrey A.
  Wilson and Kristina Curry-Rogers (eds.), *The Sauropods: Evolution and
  Paleobiology*. University of California Press, Berkeley


[Sorry about the late posting today: I had to leave the house at 7:15 to fly to Copenhagen for Christmas lunch — long story — and I am completing today’s post from my hotel room.] There’s no getting away from it: everyone wants to know how big dinosaurs are.

 Xenoposeidon

is based on a single partial vertebra, so there is no way to be at all sure about the size and shape of the whole animal;

Xenoposeidon week, day 4: the question everyone is asking … how big was it?

University of California, Merced

Matt Wedel

I see now that Mike has beaten me to the punch in providing your
at-least-weekly dose of sauroponderous vertebrawesome. And a nice job it
is. Still, I feel funny about you not getting a *new* picture
([ahem](http://svpow.wordpress.com/2007/10/04/tutorial-2-basic-vertebral-anatomy/)),
so I'm posting my late entry anyway. For some reason, despite--or
perhaps because of--my ardent devotion to cervicals, I have taken it on
myself to push the anatomical boundaries of SV-POW!
[again](http://svpow.wordpress.com/2007/10/15/the-promised-land/).

[![amnh-diplodocus-500.jpg](https://svpow.files.wordpress.com/2007/11/amnh-diplodocus-500.jpg)](http://svpow.files.wordpress.com/2007/11/amnh-diplodocus-pelvis.jpg "amnh-diplodocus-500.jpg")

Here's a partial pelvis of *Diplodocus* in the American Museum of
Natural History, in left lateral view. The big, vertically-oriented slab
is the ilium. The semicircular concavity on the bottom of the ilium is
the top half of the acetabulum, or hip socket. The bottom half of the
acetabulum was formed by the other two pelvic bones, the pubis, which
pointed down and forward, and the ischium, which pointed down and back.
Behind the ilium you can see five sacral vertebrae, and the sacral ribs
that connect the ilium to the vertebral column. The neural spines of the
middle three vertebrae are fused together. The first and fifth have free
neural spines, but all five are fused together at the centra.

The number of sacral vertebrae increased several times in the evolution
of sauropodomorphs (sauropods and "prosauropods"). Almost all
"prosauropods" have three sacrals, basal sauropods have four, most
neosauropods have five, most titanosaurs and some elderly individuals of
*Camarasaurus* have six, and *Neuquensaurus* is reported to have seven.
It is a little weird that sacral count increases so regularly and
straightforwardly up the tree. The sacrum is the only bony connection
between limbs and the vertebral column (recall that the shoulder girdle
"floats" in a sling of muscles and ligaments), so it's pretty important
mechanically. You might expect that bigger sauropods would have more
sacral vertebrae and smaller sauropods would have fewer, but that's not
the case. Monsters like *Apatosaurus* and *Brachiosaurus* only have
five, and little ole titanosaurs like *Malawisaurus* and *Saltasaurus*
have six ("little" in this case means up to the size of an elephant--big
for an animal, kinda pathetic for a sauropod). Now, granted, there were
much bigger titanosaurs, like *Argentinosaurus*, *Puertasaurus*, and
*Paralititan*, but the sixth sacral shows up early in titanosaurian
history, in the little guys. So there does not seem to be much of a
connection between sacral count and body size. Which raises the
question: what was driving the increase?


I see now that Mike has beaten me to the punch in providing your at-least-weekly dose of sauroponderous vertebrawesome. And a nice job it is. Still, I feel funny about you not getting a

 new

picture (ahem), so I’m posting my late entry anyway. For some reason, despite–or perhaps because of–my ardent devotion to cervicals, I have taken it on myself to push the anatomical boundaries of SV-POW! again.

The sacral and the profane

[![diplodocid-caudal.jpg](https://svpow.files.wordpress.com/2007/10/diplodocid-caudal.jpg?w=500&h=420){height="420"
width="500"}](http://svpow.files.wordpress.com/2007/10/diplodocid-caudal.jpg "diplodocid-caudal.jpg")

We promised non-presacrals and non-brachiosaurids, so here's a
diplodocid caudal vertebra in right lateral view. Most of the neural
spine is blown off. The huge hole in the side of the vert is legit,
though. That's a pneumatic foramen (literally, air hole), through which
air-filled tubes connected to the respiratory system entered the
bone--just like in birds. More on that later. Mike and I are off to the
Society of Vertebrate Paleontology meeting in Austin. Will you get the
not-promised but so far always delivered extra post this week? Time
alone will tell.

By the way, I see that I stupidly did not include any form of scale bar
here. This bone is about the size of a really big toaster. If you
dropped it on your foot, you'd never dance again. Not bad, considering
it comes from about halfway down the tail...


We promised non-presacrals and non-brachiosaurids, so here’s a diplodocid caudal vertebra in right lateral view. Most of the neural spine is blown off. The huge hole in the side of the vert is legit, though. That’s a pneumatic foramen (literally, air hole), through which air-filled tubes connected to the respiratory system entered the bone–just like in birds. More on that later.

Postagens de Rogue Scholar

Xenoposeidon week, day 4: the question everyone is asking … how big was it?

The sacral and the profane

The promised land