Was Amphicoelias a rebbachisaur?
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Update (10/22/18): Dr. Ken Carpenter has recently published a new paper supporting the view below (and cites me favorably), but I would also be remiss to not recongize Dr. Andrea Cau for having thought up this idea 2 years before me. Sadly, he was not cited in Carpenter's paper. He and I both were unaware of Cau's work.

The last time I wrote about the size of Amphicoelias, I still used Diplodocus as a comparison. One of the comments that was made was that my size estimate was likely wrong, as Amphicoelias was probably a basal diplodocoid, not a diplodocid proper. After a little investigation, it turned out that two phylogenetic analyses have been published that included Amphicoelias, and both found Amphicoelias to be a basal diplodocoid. Whitlock (2011) was one of the studies, and I reproduce the phylogenetic diagram here (with Amphicoelias highlighted by yours truly):



As can be seen here, Amphicoelias is recovered more derived than Amazonsaurus, but more basal than the most basal rebbachisaur, Histriasaurus. This placement has weak support, and as Whitlock comments, "The recovery of Amphicoelias as a basal diplodocoid agrees with the findings of Rauhut et al. (2005), although the latter result may hinge on one or two key scorings. As mentioned above, re-scoring Amazonsaurus in Rauhut et al.’s (2005) matrix results in the placement of Amphicoelias in a large polytomy with essentially all other eusauropods."

On top of that, it is unclear from Whitlock's (2011) analysis, whether A. altus is only scored, or if A. fragillimus is included as well. So this analysis may only apply to A. altus. But no matter, as when I did some comparisons of the overall morphology and proportions of A. fragillimus, it turns out it agrees quite well with rebbachisaurs.



One of things I have noticed about A. fragillimus was that its neural arch is proportionally tall when compared to diplodocids such as Diplodocus, Barosaurus, Apatosaurus and others, as well as in comparison to the dicraeosaurs. Using the dorsal of Rebbachisaurus garasbae, I completed a new reconstruction of the dorsal of A. fragillimus (see above). The gray parts are directly traced from Rebbachisaurus and you can see how well the neural arch proportions match. Obviously, the neural spine in Rebbachisaurus is considerably wider proportionally than that of A. fragillimus, but the neural spine of Limaysaurus tessonei matches quite well, being remarkably thinner side-to-side than in Rebbachisaurus. A thinner neural spine is also seen in Amazonsaurus.


Amazonsaurus dorsal from Carvalho et al (2003)

Another thing that seems to support the hypothesis that A. fragillimus was a rebbachisaur or rebbachisaur-grade diplodocoid is the apparent extreme pneumaticity of the vertebra, with the etymology of its species name meaning "very fragile". Rebbachisaurs such as Nigersaurus and Tataouinea demonstrate extreme pneumaticity in their skeletons, for instance.

How big is a rebbachisaur-ized A. fragillimus?

As shown in the second image above, the reconstructed total height of the vertebra of A. fragillimus is just under 243 cm tall (this happens to be quite close to the reconstructed height when I previously used Diplodocus as a comparison). Compare this to the dorsal vertebrae height of Limaysaurus, at just over 121 cm tall. Assuming fairly similar proportions, A. fragillimus is about twice as large as Limaysaurus in linear dimensions. My recent reconstruction of Limaysaurus gives a total length of 14.3 m, and a mass of about 6.2-6.5 tonnes (depending on the assumed tissue density). This means a total length of about 28.6 m for A. fragillimus, and a mass of about 51-53 tonnes.


Key: blue figure -A. fragillimus, yellow figure - Supersaurus, green figure - Rebbachisaurus, gray scale figure - Limaysaurus

As seen in this comparison I mocked up, A. fragillimus was probably only slightly shorter in length than Supersaurus (silhouette of Supersaurus based on skeletal from Scott Hartman: www.palaeocritti.com/_/rsrc/12…). The lengths come out the same in the image, but in a fair comparison, the neck and tail would be horizontal (with no dorsal or ventral bending) in the Supersaurus outline, which would probably "add" a few more meters to the "length" of the Supersaurus. For comparison, Supersaurus was estimated to be 33-34 m long and mass 35-40 tonnes by Lovelace et al. (2007), but a GDI mass estimate based on the multi-view skeletal therein gives me a volume of about 32.6 m^3 and a mass of about 26 tonnes (assumed the same density they did of 0.8).

So, a rebbachisaur-like A. fragillimus was probably the biggest diplodocoid, but not the longest, and is smaller than the largest titanosaurs, although not by much. (Also note that the length of the hindfeet of a scaled of rebbachisaur-like A. fragillimus are about 144 cm long, which is only slightly shorter than the biggest Broome & Plagne sauropod tracks which are up to 150 cm long, meaning they might not have been record-breakers either, possibly 60 tonnes or less if they were rebbachisaurs.)

Refs--

Carvalho IDS, Avilla LDS, Salgado L. 2003. Amazonsaurus maranhensis gen. et sp. nov. (Sauropoda, Diplodocoidea) from the Lower Cretaceous (Aptian-Albian) of Brazil. Cretaceous Research 24: 697–713.

Lovelace DM, Hartman SA, Wahl WR. 2008. Morphology of a specimen of Supersaurus (Dinosauria, Sauropoda) from the Morrison Formation of Wyoming, and a re-evaluation of diplodocid phylogeny. Arquivos Do Museu Nacional, Rio De Janeiro 65: 527–544.

Whitlock, JA. A phylogenetic analysis of Diplodocoidea (Saurischia: Sauropoda). Zoological Journal of the Linnean Society, 2011, 161, 872–915.
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Comments (42)
Atlantis536's avatar
Atlantis536Edited |Hobbyist Traditional Artist
Somehow I believe Carpenter made Maraapunisaurus a rebbachisaur just so he can make DeviantArt paleontologists happy.
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palaeozoologist's avatar
palaeozoologist|Hobbyist General Artist
LOL, if only that were true! He was unaware of this post until it was pointed out to him by a reviewer, and added a reference into the paper in the edits after the initial peer review.
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Atlantis536's avatar
Atlantis536|Hobbyist Traditional Artist
Oh, I see! All the while I thought Carpenter himself saw the post and credited you. I also remember seeing a comment by him on SV-POW saying he sent you a copy of Maraapunisaurus' paper via a note on DeviantArt, which may or may not have influenced that thought.
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palaeozoologist's avatar
palaeozoologist|Hobbyist General Artist
Yeah, he had been trying to contact me through DeviantArt, but I haven't been logging in very frequently here.
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TyrannosaurusPrime's avatar
"Mr. Cope, I don't feel so huge....."
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bricksmashtv's avatar
bricksmashtv|Hobbyist General Artist
Congratulations Zach, Carpenter has now officially gifted us rebbachisaurid Maraapunisaurus fragillimus (you even made it into the acknowledgements!): www.utahgeology.org/wp-content…
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palaeozoologist's avatar
palaeozoologist|Hobbyist General Artist
Cool, thank you!
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bricksmashtv's avatar
bricksmashtv|Hobbyist General Artist
no problem!
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Franoys's avatar
Franoys|Student Digital Artist
You must be very proud about this:

svpow.com/2018/10/21/what-if-a…

www.utahgeology.org/publicatio…

Congratulations!
Reply  ·  
palaeozoologist's avatar
palaeozoologist|Hobbyist General Artist
Well, thank you! You have a nice write up on the paper, although I really can't take any credit - I was just speculating at the time -  Dr. Carpenter did the real work. Although it is nice that I haven't turned out to be a David Peters-eque crank! I also found out that I in fact did not originate this idea, as Dr. Andrea Cau came up with the idea 2 years prior to me thinking of it (theropoda.blogspot.com/2012/01…) - and he actually did some phylogenetic work on it. So, I was neither the first nor the most detailed. I do appreciate that you noticed, though! :)
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Franoys's avatar
Franoys|Student Digital Artist
Thank you; I just learnt about the Andrea's proposal yesterday, I will edit this when I can in the write up, but you surely deserve some credit since your proposal was surely heavily used as inspiration for Carpenter's manuscript and he himself thought crediting you was in order.
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ForbiddenParadise64's avatar
The theory of it being a rebachisaurid has also been dropped now though, as Thsopp and Matias have reclassified it as a flagellicaudatan, quite probably with similar proportions to tornieria, and thus very different to this. It certainly seems more plausible for a giant animal to have a long neck than a short one. Here's Broly's new estimate on it: brolyeuphyfusion9500.deviantar… and here is another by Randomdinos: randomdinos.deviantart.com/art… both use basal proportions without rebachisaurid like designs. How has your views changed on the creature, if anything?
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palaeozoologist's avatar
palaeozoologist|Hobbyist General Artist
I'm don't agree that 'Amphicoelias' fragillimus is very close to A. altus (which was included in the cladogram). The overall proportions of the vertebra are quite different between the two. Based on what is described, I still think it is likely that 'A.' fragillimus is probably a rebbachisaur-grade (or possibly dicraeosaurid) diplodocoid.
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ForbiddenParadise64's avatar
The general consensus now is that the two were the same creature at different growths in their life, similar to the situation with Apatosaurus and the giant Barosaurus specimen, with these bits of evidence seeming to suggest that giant sauropods were certainly more common than previously thought and that the giant ones were actually old members of known groups. It's certainly not inconceivable to imagine. Now I still don't agree with the scaling up from Diplodocus lazily, as that is not consistent with proportions. Then again, that is the same paper that tried to make out that Argentinosaurus and Puertasaurus were scaled up saltosaurus without any acknowledgement of their basal morphology, resulting in short and dimunitive looking creatures despite the bones themselves suggesting not only long necks but also vertically orientated ones.

Rebachisaurids seem to be absent from North America while Dicreasaurids are there more often. The problem with this reconstruction is that large sauropods tend to naturally be orientated towards higher browsing, and have had proportionally longer necks as a result, rather than a simple scale up. Plus even in your graph, it is outside of the rebachisaurid and Dicreasaurids groups, so it doesn't guarantee similar proportions. I agree that it was more basal than the competition, but I doubt the idea of such a huge beast having proportions similar to a far smaller one, especially when other basal sauropods are known that didn't have such specialisations. For now Tornieria does seem a plausible comparison.
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palaeozoologist's avatar
palaeozoologist|Hobbyist General Artist
I seriously doubt they were the same creature, the portions of the dorsals are very different if you scale them together, and accounting for ontogeny doesn't explain it, since the A. altus specimen has neurocentral fusion, so it was at least skeletally mature, meaning the proportions would not have changed much.

The large Apatosaurus and Barosaurus notably do not differ much in the proportions of the preserved material.

Confirmed rebbachisaurs are so far absent from N. America.  Note that I did say "or rebbachisaur-grade diplodocoid". It may not be a rebbachisaur 'proper' (all rebbachisaurs are known from the Cretaceous) but likely a basal diplodocoid of some sort. Since rebbachisaurs are the only basal diplodocoids (except Haplocanthosaurus) that have decent material preserved, I use them as a guide. Notably, Haplocanthosaurus has somewhat similar proportions overall to rebbachisaurs, so I think it is reasonable.

Please remember, that this was a thought experiment. Until more material is preserved, all such estimates are speculations of course. That said it is informed speculation, and data-driven and much more likely than scaled up apatosaurs or diplodocus.

At its size, even with a short neck, it would still be a 'high-browser', so not sure that point makes my reconstruction less likely. Even doubling the size of the neck would not affect the mass much.
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bricksmashtv's avatar
bricksmashtv|Hobbyist General Artist
actually Tschopp, et al's analysis scored for A. altus, not A. fragillimus. They used Tornieria as they both assumed it was still a basal Diplodocid. I think Zach has made a convincing argument to the contrary, and that it should properly be addressed rather than just shoved to the side and ignored.
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ForbiddenParadise64's avatar
It's still too dimunitive in proportions, not to mention too shrink-wrapped to have a realistic weight estimate I think- no living animals have such small amounts of soft tissue proportionally compared to these, even if the skeletons are usually of good quality.
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bricksmashtv's avatar
bricksmashtv|Hobbyist General Artist
I definitely agree with that. I'd say it's probably closer to 65 or 70 tonnes (given that there is far too little flesh & I'm pretty sure Zach's SG is too low). Lengthwise though it's pretty close to what I'd expect (neck allometry at most only allows for 30.42m here).
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pds314's avatar
I'd normally laugh at such phylogenetic redistribution, but considering that the actual fossil was tragically lost, and that putting it in amphicoelias results in an animal the size of a 767, this might be a reasonable conclusion.
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ijreid's avatar
ijreid|Hobbyist Digital Artist
Hmm. My thoughts are similar. Why not test it as a dicraeosaurid, with those short neck and taller arches than diplodocids. Also those high processes of the arch seem more similar to dicraeosaurids.
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palaeozoologist's avatar
palaeozoologist|Hobbyist General Artist
Well, I don't have any good references for doing a dicraeosaurid skeletal in mult-view, so mostly lack of references hinders me from restoring it as a dicraeosaurid. But it would be an interesting thing to do, I agree.
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megabass22's avatar
megabass22|Hobbyist Traditional Artist
Seems more plausible than the 60 meter monster with nothing even large enough to compare.
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ElSqiubbonator's avatar
Is that a Spinosaurus-style sail I see on its back?
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palaeozoologist's avatar
palaeozoologist|Hobbyist General Artist
Not quite as dramatic as Spinosaurus, but yes...
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anonymous's avatar
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