Amphicoelias fragillimus - smaller than you think

Amphicoelias fragillimus is often listed as one of the largest dinosaurs, if not the largest. Size estimates are usually superlative, and often includes mass estimates well over 100 tonnes, with the most commonly cited estimates usually around 120-150 tonnes; and length estimates in the 50-60 meter range.

I myself at one point gave a mass estimate of 150-250 tonnes and length of about 70 meters.

I think I was probably wrong, and a recent size comparison with Argentinosaurus demonstrates why:



Since my GDI mass estimate for Argentinosaurus is about 64 tonnes, I knew 100+ tonnes estimates for Amphicoelias had to be wrong.

I did a completely new reconstruction of Amphicoelias fragillimus, with missing parts restored after Amphicoelias altus and compared that to a reconstruction of a posterior dorsal of Argentinosaurus. As you can see, in both total width and centrum length, Argentinosaurus just edges out A. fragillimus in size. I should also point out that the posterior dorsals of diplodocids are usually taller than the anterior dorsals, while the opposite is the case in at least some titanosaurs:


Diplodocus dorsal sequence

Malawisaurus dorsal sequence

This means, of course, that we are not comparing apples and oranges and therefore, the absolute size of a vertebra can be misleading, especially when comparing taxa that are not closely related. There are several things one needs to keep in mind, then, when trying to get an idea of overall size of a sauropod based on vertebral size (or indeed, any animal):

1. Serial position of vertebra (i.e., the 1st dorsal, 5th dorsal, 8th dorsal, etc.)
2. Proportions (length to width, etc.)
3. Phylogeny (diplodocid, brachiosaur, etc.)
4. Absolute size

These points may seem obvious, but often times it appears only absolute size is discussed. In fact, this is probably the least important factor in determining the total size of the animal. An example to compare would be Giraffatitan and Diplodocus. Both Giraffatitan and Diplodocus each have a dorsal vertebra that measures 107 cm in total height. If one were to study these vertebra in isolation without respect to points #1, #2 and #3 that I listed, and only compared absolute size (point #4), one might get the impression that both taxa were approximately the same size. In fact, they are not, as you may well be aware. Rigorous reconstructions show that Giraffatitan is consistently estimated to have been about 25-30 tonnes and about 23 meters long, whereas Diplodocus is usually estimated to be about 10-15 tonnes, and about 25 meters long. Clearly, Giraffatitan was bigger in mass than Diplodocus.

Focusing on points #1, #2 and #3 show why, even though both Giraffatitan and Diplodocus have a dorsal vertebra of the same height, one is clearly larger.

First, the serial position of the vertebrae are different. In Giraffatitan, it is the 4th dorsal that is 107 cm tall, while it is the 10th dorsal in Diplodocus. In Giraffatitan the anterior dorsals are taller than the posterior dorsals, while the opposite is the case in Diplodocus.


Giraffatitan dorsal (top right)

Second, consider the proportions. The dorsal of Giraffatitan is considerably wider proportionally (and in absolute terms) than the dorsal in Diplodocus. In Giraffatitan the width of the diapophyses is the same as the height of the entire vertebra, over 107 cm wide (Janensch, 1950). In Diplodocus, the widest dorsal (the 4th) is only 75 cm across the diapophyses, and is only 49.5 cm in the 10th dorsal (Lull, 1919).

Third, consider the phylogeny. We can use phylogenetic bracketing to get a rough idea of the proportions of an animal, even if known from incomplete material. We know, for instance, that diplodicids tend to be long and skinny, whereas brachiosaurs tend to be shorter in total length, but also more robust and therefore heavier than a diplodocid of the same length.

So what does this all mean for Amphicoelias fragillimus, especially in comparison to Argentinosaurus?

First, consider the serial position of the vertebra. The dorsal of Amphicoelias fragillimus appears to be one of the last, if not the last dorsal, and is commonly cited as the 10th (Carpenter, 2006). The vertebra of Argentinosaurus I have chosen to compare it with was originally described as the 2nd (Bonaparte and Coria, 1993), but subsequent analysis suggests it is a posterior dorsal, possibly the last or penultimate dorsal (Novas and Ezcurra, 2006; Salgado and Powell 2010).

Second, consider the proportions. Amphicoelias is clearly much more gracile than Argentinosaurus. In fact, it appears likely that the centrum would have been shorter and skinnier than in Argentinosaurus. Even the diapophyses appear to be not to have been as wide. Obviously, Amphicoelias is considerably incomplete, even the lone dorsal. This is where the third point comes into play.

Phylogenetically A. fragillimus was a diplodocid, and appears to be close to A. altus. This gives us a way to confidently restore the missing bits. The parts in gray were restored after A. altus (Osborn and Mook, 1921). Also, we can compare it to more complete diplodocids, like Diplodocus itself and compare proportions with it.

Restoring the incomplete vertebra of A. fragillimus off of A. altus, gives these estimated measurements:

* Total height: ~2400 mm
* Diapophyses width: ~1070 mm
* Centrum width (posterior face): ~600 mm
* Centrum length (total): ~490 mm

In Diplodocus, these measurements are (10th dorsal; from Lull, 1919):

* Total height: 1070 mm
* Diapophyses width: 495 mm
* Centrum width (posterior face): 325 mm
* Centrum length (total): 290 mm

Here are some ratios of Amphicoelias to Diplodocus:

* Total height ratio: ~2.24
* Diapophyses ratio: ~2.16
* Centrum width: ~1.84
* Centrum length: ~1.69

Taking the average of these ratios, we get that, on average, A. fragillimus was "only" 1.9825 times larger than Diplodocus in linear dimensions, which would suggest being about 7.79 times heavier, or approximately 78 tonnes, assuming Diplodocus was 10 tonnes (a mass estimate I got by doing a GDI of Gregory S. Paul's multi-view Diplodocus reconstruction). My own mass estimate for Argentinosaurus is about 64 tonnes, and for the largest Alamosaurus specimen about 74 tonnes. This means that there are no sauropods yet known that we can confidently say are heavier than 100 tonnes. Amphicoelias fragillimus still was possibly the largest sauropod, although not nearly by the margin often claimed: only by a few tonnes. Since any mass estimates are imprecise, I'd say it is approximately a tie for first place between Alamosaurus and Argentinosaurus for the largest known sauropod.

So, the whales win - by a wide margin*.

*There are at least 4 living whale species for which specimens are known that reach over 100 tonnes: the blue whale, fin whale, bowhead whale, and Northern Pacific right whale.

References:

Bonaparte, J. F., and R. A. Coria. 1993. Un nuevo y gigantesco sauropodo titanosaurio de la Formacion Rıo Limay (Albiano–Cenomaniano) de la Provincia del Neuquen. Ameghiniana 30:271–282.

Carpenter, K. 2006. Biggest of the big: a critical re−evaluation of the mega−sauropod Amphicoelias fragillimus Cope, 1878. New Mexico Museum of Natural History and Science Bulletin 36: 131–137.

Janensch, W. 1950 Die Wirbelsäule von Brachiosaurus brancai ("The vertebral column of Brachiosaurus brancai") Palaeontographica Supplement VII (1), teil 3, leif 2:31-93

Leonardo Salgado & Jaime E. Powell (2010): Reassessment of the vertebral laminae in some South American titanosaurian sauropods, Journal of Vertebrate Paleontology, 30:6, 1760-1772

Lull, R. S. 1919. The sauropod dinosaur Barosaurus Marsh. Memoirs of the Connecticut Academy of Arts and Sciences 6:1–42.

Novas, F. E., and M. Ezcurra. 2006. Reinterpretation of the dorsal vertebrae of Argentinosaurus huinculensis (Sauropoda, Titanosauridae). Ameghiniana 43(4, Supplement):48R–49R.

Osborn, H. F., and C. C. Mook. 1921. Camarasaurus, Amphicoelias and other sauropods of Cope. Memoirs of the American Museum of Natural History, n.s. 3:247–387.