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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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:icontyrannosaurusprime:
TyrannosaurusPrime Featured By Owner Oct 23, 2018
"Mr. Cope, I don't feel so huge....."
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:iconbricksmashtv:
bricksmashtv Featured By Owner Oct 21, 2018  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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:iconpalaeozoologist:
palaeozoologist Featured By Owner Oct 22, 2018  Hobbyist General Artist
Cool, thank you!
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:iconbricksmashtv:
bricksmashtv Featured By Owner Oct 22, 2018  Hobbyist General Artist
no problem!
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:iconfranoys:
Franoys Featured By Owner Oct 21, 2018  Student Digital Artist
You must be very proud about this:

svpow.com/2018/10/21/what-if-a…

www.utahgeology.org/publicatio…

Congratulations!
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:iconpalaeozoologist:
palaeozoologist Featured By Owner Oct 22, 2018  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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:iconfranoys:
Franoys Featured By Owner Oct 23, 2018  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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:iconforbiddenparadise64:
ForbiddenParadise64 Featured By Owner Oct 5, 2016
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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:iconpalaeozoologist:
palaeozoologist Featured By Owner Oct 19, 2016  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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:iconforbiddenparadise64:
ForbiddenParadise64 Featured By Owner Oct 20, 2016
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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:iconpalaeozoologist:
palaeozoologist Featured By Owner Feb 20, 2017  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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:iconforbiddenparadise64:
ForbiddenParadise64 Featured By Owner Feb 21, 2017
Oh right this is an old comment. As Broly/SpinoinWonderland pointed out, there is no known D10 or D11 for Rebbachisaurus itself, which makes scaling the vertebrae problematic, not to mention Lamayisaurus had much larger vertebrae proportionally (though more different to Amphi) than Rebbachisaurus did, which skewers things further.
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:iconbricksmashtv:
bricksmashtv Featured By Owner Oct 15, 2016  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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:iconforbiddenparadise64:
ForbiddenParadise64 Featured By Owner Nov 29, 2016
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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:iconbricksmashtv:
bricksmashtv Featured By Owner Nov 29, 2016  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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:iconforbiddenparadise64:
I don't think Plagne was one and definitely not Broome. Not to mention both of those were supposed to be 1.5m wide, not long. Plagne and Broome are like 2m long.
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:iconbricksmashtv:
bricksmashtv Featured By Owner Nov 30, 2016  Hobbyist General Artist
oh I definitely wasn't referring to Plagne or Broome, just fragillimus
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:iconforbiddenparadise64:
ForbiddenParadise64 Featured By Owner Dec 3, 2016
What are your views on them? The former is definitely a Diplodocid (the brachiosaur position has been dropped I think) and the latter was almost certainly a titanosaur.
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:iconbricksmashtv:
bricksmashtv Featured By Owner Dec 3, 2016  Hobbyist General Artist
yeah that's what I go with usually.
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:iconpds314:
pds314 Featured By Owner Mar 7, 2016
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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:iconijreid:
ijreid Featured By Owner Apr 26, 2015  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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:iconpalaeozoologist:
palaeozoologist Featured By Owner May 2, 2015  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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:iconmegabass22:
megabass22 Featured By Owner Jun 15, 2014  Hobbyist Traditional Artist
Seems more plausible than the 60 meter monster with nothing even large enough to compare.
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:iconelsqiubbonator:
ElSqiubbonator Featured By Owner May 18, 2014
Is that a Spinosaurus-style sail I see on its back?
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:iconpalaeozoologist:
palaeozoologist Featured By Owner May 19, 2014  Hobbyist General Artist
Not quite as dramatic as Spinosaurus, but yes...
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:icontheropod1:
theropod1 Featured By Owner Mar 16, 2014  Student Traditional Artist
Hmm, the biggest broome track is 1.7m long according to Tony Thulborn, that’s the measurement we know is lenght.
dml.cmnh.org/1997Aug/msg00778.…

The plagne measurement may be pes width, not lenght.

The scaling of the vertebra looks realistic, but it’s strange Amphicoelias lacks the dorsal transverse expansion of the neural spine that is notable in Limaysaurus and even more in Rebacchiasaurus, and has a far more massive neural arch when compared to the spine. Based on A. altus, it also has a far bigger centrum, comparable to Diplodocus etc. in overall terms.
imo that makes sense considering it’s much closer to those in terms of size and stratigraphy.

Anyway, if Amphicoelias was the size indicated by your restoration, the biggest specimens of Apatosaurus are bigger.
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:iconpalaeozoologist:
palaeozoologist Featured By Owner Mar 16, 2014  Hobbyist General Artist
Well I was going off of measurements that have been published. I believe that of the lengths that have been formally published, the largest (in length) are 1.5 m long. And they are not widths, they are clearly stated to be lengths. See this comment by Tony Thulborn on SV-POW: svpow.com/2009/10/13/how-big-w…

"but it’s strange Amphicoelias lacks the dorsal transverse expansion of the neural spine that is notable in Limaysaurus and even more in Rebacchiasaurus, and has a far more massive neural arch when compared to the spine."

Yes, I noted that, and I also compared it to Amazonsaurus, which does not have transversely expanded neural spines. We should expect some variation between the taxa, after all.

"Based on A. altus, it also has a far bigger centrum, comparable to Diplodocus etc. in overall terms.
imo that makes sense considering it’s much closer to those in terms of size and stratigraphy.
"

Well, A. fragillimus is not close in size to A. altus or Diplodocus and I'm not sure what stratigraphy has to do with centrum size. At the very least, the phylogeny does not demonstrates close affinities to Diplodocus, so I'm really puzzled as to why you think a Diplodocus-like centrum is more likely. It's true that A. altus has more typically diplodocid proportions, but I think I also have demonstrated that A. fragillimus has generally very different proportions from A. altus (see my previous posts on this as well), so I don't think it is necessarily more likely that A. fragillimus had an A. altus-like centrum. Sure, it is possible. And I have done a couple of different reconstructions to show the possibilities. I am not saying A. fragillimus was a carbon-copy rebbachisaur or diplodocid, what I am saying is that arguments that A. fragillimus was the biggest are not that strong.
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:icontheropod1:
theropod1 Featured By Owner Mar 16, 2014  Student Traditional Artist
No, it’s very far  in terms of size, but even further from rebacchiasaurids. On the other hand, Supersaurus, D. hallorum and Apatosaurus, derived diplodocids, are all very large taxa (and in a similar ballpark to what you got for A. fragillimus based on Rebacchiasaurus).

I know the 1.5m from thulborn’s paper are lenght. That doesn’t appear to correspond to the largest known tracks from broome tough. In the case of plagne, the only reported figure seems to refer to width, but it would be very nice to actually see them described.
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:iconyty2000:
yty2000 Featured By Owner Mar 20, 2014
Only the largest Apatosaurus would be close in weight though.
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:icontheropod1:
theropod1 Featured By Owner Mar 20, 2014  Student Traditional Artist
That’s true (their taxonomy is unclear tough), I was referring to specimens.
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:iconyty2000:
yty2000 Featured By Owner Mar 20, 2014
Thanks!
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:iconpalaeozoologist:
palaeozoologist Featured By Owner Mar 19, 2014  Hobbyist General Artist
Yes, it seems the largest sauropods seem to max at about 60 tonnes on average, with some exceptions.

As for the tracks, other than the comment that Tony Thulborn made on the DML (that you referenced), I am not aware of any 1.7 m prints, and certainly not one listed as widths. But I confess I have a very poor knowledge of ichnology.
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:icontheropod1:
theropod1 Featured By Owner Mar 20, 2014  Student Traditional Artist
1.7m regarding the broome tracks was explicitely stated to be lenght. 1.5m in the case of the plagne footprints was the measurement referring to width.
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:iconyty2000:
yty2000 Featured By Owner Mar 16, 2014
Maybe they don't belong to the same genus? To me A. altus looks more diplodocid.
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:iconpalaeozoologist:
palaeozoologist Featured By Owner Mar 16, 2014  Hobbyist General Artist
I would agree that they probably don't belong to the same genus (whatever "genus" means exactly), as there appears to my eyes significant proportional differences between the two (although if they were found to be each others' closest relatives to the exclusion of other sauropods, then I would be comfortable with them being in the same genus). Unfortunately, unless new material pops up, I'm guessing A. fragillimus won't get a new generic name.
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:iconfragillimus335:
Fragillimus335 Featured By Owner Mar 15, 2014  Hobbyist General Artist
I honestly doubt Amphicoelias looked like a Rebbachisaur.  Not because of phylogenetics, but do to size and niche.  Every sauropod that topped the 50 ton mark had elongate necks.  The "Barosaurus/Supersaurus" body plan seems maximized to take advantage, or to simply allow such massive sized.  I don't think the short-necked Rebbachisaur model wold work well on a 30m 50 ton frame.
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:iconyty2000:
yty2000 Featured By Owner Mar 15, 2014
It's only an assumption here that A. fragillimus is an isometrically scaled up rebbachisaur. I think it could be more likely that it's a longer-necked, shorter legged rebbachisaur-like animal than a straight up enlarged rebbachisaur.
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:iconpaleo-king:
Paleo-King Featured By Owner Mar 15, 2014  Professional Traditional Artist
That's a pretty good theory. Even Supersaurus has shorter legs proportionally than some other smaller diplodocids. The legs tend to get more stocky with larger size... maybe except in brachiosaurs. I wonder when we will find a brachiosaur so huge that the leg bones actually get thick for their length.
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:iconyty2000:
yty2000 Featured By Owner Mar 16, 2014
It's only a hypothesis so far.  ;)
I think I read on SVPOW that sauropods' axial skeletons get proportionally longer than limbs through ontogeny.
After I had seen your Fusuisaurus skeletal, I went and read the paper myself. It's axial elements might be a lot bigger than those of Giraffatitan. But the distal femural head is 60cm wide, that's only a little bigger than Giraffatian.
Also Xinjiangtitan, compared to smaller species of Mamenchisaurus, has a much longer neck, but proportionally short femur.
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:iconpaleo-king:
Paleo-King Featured By Owner Mar 16, 2014  Professional Traditional Artist
The hypothesis I agree with. Fusuisaurus was basically built like a tank by the look of things. Judging by the femur, the legs really aren't that impressive for such a huge animal. Question is, were other giant brachiosaurs proportioned the same way? Even for the largest Giraffatitan specimens, appendicular material is surprisingly rare. HMN XV2 is a partial fibula, which is a lot harder to scale from than a femur. There's some indication from the quarry map of the Archbishop (even though as a mature adult it was smaller than the biggest Giraffatitans) that it had smaller legs on a deep body with a proportionally longer neck. So not all brachiosaurs were ridiculously long-legged it seems.

I do notice that giant mamenchisaurs like M. sinocanadorum appear more short-legged than their smaller cousins. And of course Supersaurus is shorter in the legs proportionately than Apatosaurus or Diplodocus.
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:iconyty2000:
yty2000 Featured By Owner Mar 16, 2014
I don't really trust the skeleton of M. sinocanadorum.
The holotype only consists of 3 cervicals and a lower jaw. There is a huge Mamenchisaurid humerus (estimated complete length 180cm) from Junggar basin as well. It is rather long and slender for a mamenchisaur.
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:iconpaleo-king:
Paleo-King Featured By Owner Mar 16, 2014  Professional Traditional Artist
What?!?!? So where did Greg Paul get source material for his big skeletal? Are all those bones just plaster models? They look more like casts of real bones than most of the sculpted replica bones you see in the typical Chinese mounts.

Greg Paul also did the same thing with Huabeisaurus, he reconstructed a whole skeleton based on a few bones instead of leaving the missing parts blacked out like he normally does. Not only that but he got the family and the shapes of some of the bones wrong too. :X Made it look a lot more saltasaur or nemegtosaur-like than it really was. Of course the makers of the museum mount are equally at fault.
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:iconyty2000:
yty2000 Featured By Owner Mar 16, 2014
I have seen a partially accessible version of the paper on M. sinocanadorum partially online somewhere, can't find it now.
If I remember correctly, there's only cervical 2-4, a lower jaw, and maybe a quadratojugal or something. The famous mount from the Japanese dinosaur expo is pretty much a scale up composite from other species of Mamenchisaurus as far as I know. The 35m Mamenchisaur is pretty much a myth.
There's the big humerus I mentioned about. The reconstructed full length is 180cm, based on Mamenchisaurus constructus (they mentioned that it fits with the proportion of that species better than others). The two known M. constructus humeri are 108 and 97cm long. I don't know how big M. constructus is. The body proportion of the giant Mamenchisaur remains unknown.
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(1 Reply)
:iconpaleo-king:
Paleo-King Featured By Owner Mar 15, 2014  Professional Traditional Artist
This is possible, but there's also the possibility that Amphicoelias was an unusually long-necked rebbachisaur-like animal. After all, no sauropod clade appears to be all one way or another... Atlasaurus shows that not all brachiosaurs had very long necks, and Supersaurus ans Barosaurus have far longer necks than the average diplodocid. Furthermore, the huge "Antarctosaurus" giganteus, while probably not a species of Antarctosaurus, does appear to be within the typically short-necked antarctosauridae... perhaps more like a Lirainosaurus-proportioned animal. The thing is, antarctosaurids all have short anterior dorsals, and whose known from neck material (such as Bonitasaura) are short-necked animals. Antarctosaurus wichmannianus appears to be a rather big 60-foot animal, but its skull is clearly designed for ground feeding like a vacuum cleaner, so this was probably a short-necked creature too. so A. giganteus may also have been a short-necked animal. Or it could have just had a longer neck and different skull shape than smaller antarctosaurids. So traditionally short-necked clades do produce very large species or unusually long-necked species from time to time.

I always did have a suspicion that Amphicoelias was probably smaller than the insane estimates originally worked out based on Cope's drawing by Paul, Carpenter and others.... foremost because those reconstructions were twice as big as any of the other mega-sauropods, and having a diplodocus-proportioned animal reach such colossal sizes also looks rather improbable in my book (the thin limbs, narrow body, the neck of rather modest sauropod proportions, etc.).

Also the Supersaurus silhouette here appears to be based on Scott Hartman's restoration of the WDC "Jimbo" specimen. The BYU material is considerably larger, so Supersaurus lay have been longer than A. fragillimus. Just possibly.
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:iconpalaeozoologist:
palaeozoologist Featured By Owner Mar 15, 2014  Hobbyist General Artist
Well, we don't have neck material for many of the sauropods that are thought to have reached more than 50 tons (i.e., Argentinosaurus, Antarctosaurus giganteus, Ruyangosaurus, etc.), so I am not sure how you can possibly say that "every" sauropod that topped 50 tonnes had elongate necks. We simply don't know.

"I don't think the short-necked Rebbachisaur model wold work well on a 30m 50 ton frame."

Not sure why not, as even with its short neck, it still would be longer necked (proportionally speaking) than most other contemporary animals. Also, at its size, its neck would still be longer, in an absolute sense (~6.6 m long in my reconstruction), then most of its contemporaries (Supersaurus and Brachiosaurus being the exceptions). At its size, even a proportionally short neck would still enable it to reach high plants out of the reach of other contemporary sauropods, ornithopods and stegosaurs.

That said, it is certainly possible that it could have had a more elongate neck then other rebbachisaurs, but that would not significantly change its mass (maybe a few tonnes), and maybe add a few more meters of length. So it would have then looked like a long-necked rebbachisaur.
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:iconspinoinwonderland:
SpinoInWonderland Featured By Owner Mar 16, 2014
Well, as for neck length, taking allometry into account would result in it being ~8.42 meters long. This adds ~1.82 meters to the total length, making it ~30.42 meters long in total.

"Mere isometric scaling would of course suffice for larger animals to have longer necks, but Parrish(2006) found a stronger result: that neck length is positively allometric with respect to body size in sauropods, varying with torso length to the power 1.35. "

peerj.com/articles/36/

The mass is trickier since it is also influenced by how much flesh is on that spine. The ones on your Limaysaurus has a triangular cross-section and such. Personally, I suspect that those tall rebbachisaur spines simply raised the back like in hadrosaurs, making the mass estimates higher, but that's just my opinion.
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:iconpalaeozoologist:
palaeozoologist Featured By Owner Mar 19, 2014  Hobbyist General Artist
I agree with the fact that sauropod necks get relatively longer as they get larger, I chose not to take this into account for simplicity's sake. You're length estimate would still be less than that of Supersaurus, however. Increasing the size of the neck will affect the total mass (of course), but not as much as you would think, since sauropod necks take up little volume in proportion to their total size. So it might add a few tonnes or so, which is well within the margins of error when estimating the size of something known from a single vertebra ;)

"Personally, I suspect that those tall rebbachisaur spines simply raised the back like in hadrosaurs, making the mass estimates higher, but that's just my opinion."

I guess I am not totally clear on what you mean by "raised the back". If you mean that the back was strongly arched as in hadrosaurs, you're incorrect. The vertebrae of sauropods and hadrosaurs are a lot different when it comes to their articulations (plus note that hadrosaurs with shallow dorsal neural spines, like Maiasaura still have strongly arched backs). Also, consider that the strongly arched backs of hadrosaurs actually lead them to have very shallow chests, and if such were the case in sauropods, the mass would actually decrease. Sauropods in fact had fairly straight backs, although rebbachisaurs had unusually flexible backs (side-to-side). Rebbachisaurs were heavier than a diplodocid of the same length, due to their proportions, but also had a lower density due to the extensive pneumatization of the vertebra (so would be lighter than a diplodocid of the same volume).
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:iconspinoinwonderland:
SpinoInWonderland Featured By Owner Mar 19, 2014
Fair enough about the neck thing.

By "raising the back", I don't mean arching, I meant that the back in life is uplifted to the height of the neural spines. So you won't really see a ridge or even a hump in the living animal.

Hope that clears things up.
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:iconpalaeozoologist:
palaeozoologist Featured By Owner Mar 20, 2014  Hobbyist General Artist
Are you talking about the musculature of the back? What I'm gathering you're saying is that the musculature would "fill up" the space between the neural spin and the the outer edge of the diapophyses, like in bison.

Otherwise, I am having a hard time understanding by what you mean by the back being "uplifted to the height of the neural spines". If true, plenty of well-muscled animals still have humps and ridges (see many breeds of cattle: cdn1.arkive.org/media/3A/3AF21…. Also, there are a number of animals that due have true sail-like ridges like some species of chameleon and the sailfin lizards (lh4.ggpht.com/D0B9ADtKBBnKp4j-….
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:iconspinoinwonderland:
SpinoInWonderland Featured By Owner Mar 20, 2014
Yes, I mean back musculature.
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