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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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palaeozoologist
Zach A.
United States

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:icondarklord86:
I love your work!
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:iconpalaeozoologist:
Well, I don't know much about morphometrics, so I don't really have any major thoughts. The only thing I will point out is that one of the methods they used in the study was UPGMA, which is a phenetics-based method. To quote Wikipedia, "In a phylogenetic context, UPGMA assumes a constant rate of evolution...and is not a well-regarded method for inferring relationships unless this assumption has been tested and justified for the data set being used." (en.wikipedia.org/wiki/UPGMA) The problem with phenetics is that it does not account for evolutionary relationships, and therefore is inferior to cladistics when trying to account for evolutionary relationships (see: en.wikipedia.org/wiki/Phenetic…. So it tells us that the Triceratops and Torosaurus specimens are morphologically distinct and cluster together (which we already knew), but I'm not sure what that means in regards to whether they are truly distinct species. So for instance, fig. 7 I believe is possibly misleading in this regard. It shows morphological distinctness, but what does this mean for ontogeny vs. species? This doesn't mean I think their study is wrong, it's just hard for me to tell if these methods are being appropriately used.

I would like to see a similar study on closely related living taxa that explicitly test ontogenic differences versus species differences  to test whether these methods are reliable. This may have already been done, but like I said, I don't know enough about it.

One thing that makes me wonder about this is deer. Deer are sexually dimporphic - (in general females don't have antlers, while males do have them) and on top of that, the antlers of male deers continually change as they get older (peramorphosis) (www.deerhunting.ws/skulls.jpg). If we took extremely close related species, but only used young adult specimens of one species, and old adult specimens of the other, could we reliably distinguish the species vs ontogenetic stages? My basic question is this: Can morphometrics reliably distinguish between ontogeny-related difference versus species related differences? I would need to see studies of this sort done on living animals (like deer, for example) for which we do know these differences already and see if they are matched by morphometric methods. Until I see that, it is difficult for me to know whether the study actually demonstrates what it says it demonstrates.

Ultimately, if Torosaurus and Triceratops truly are distinct, juvenile Torosaurus specimens that are clearly juvenile Torosaurus will be found.
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:icontyrannosaurusprime:
Late Maastrichtian Cretaceous Vertebrate Fauna From India has Enormous Implications: thedragonstales.blogspot.co.uk…
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:icontyrannosaurusprime:
A snout of Apatosaurus ajax has been uncovered: www.livescience.com/41133-miss…
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:icontyrannosaurusprime:
A new tyrannosauridae Lythronax has been described: www.plosone.org/article/info%3…
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