Supersaurus, Ultrasaurus and Dystylosaurus in 2019, part 5: what actually is Supersaurus?

June 25, 2019

When I started this series, it wasn’t going to be a series at all. I thought it was going to be a single post, hence the title that refers to all three of Jensen’s 1985 sauropods even though most of the posts so far have been only about Supersaurus. The tale seems to have grown in the telling. But we really are getting towards the end now. This should be the last post that is only about Supersaurus, and then we should be able to finish with one more that covers all three animals.

Supersaurus skeletal reconstruction at NAMAL, based in part on preserved fossil material. Mike Taylor for scale, lying in front of the referred scapulocoracoid BYU 12962.

So: what actually is Supersaurus?

Is Supersaurus the same thing as Barosaurus?

As we established previously, a lot of material has been referred not only to Supersaurus in general, but to the type individual in particular: a cervical, two dorsals, four sacrals, 20 caudals, two scapulocoracoids, an ulna, a carpal, right ilium and pubis, both ischia, and a phalanx. (After Jensen’s original papers, Curtice and his collaborators did much of the work to assemble this list.) And remember, too, that Lovelace et al. (2008) described a completely separate Supersaurus specimen from Wyoming.

So: a problem arises: Matt and I are about as certain as we can be that the big cervical verebra BYU 9024 is Barosaurus. That means there are two possibilities: either the cervical been wrongly referred to the Supersaurus type individual, and our conception of Supersaurus needs to change accordingly; or it was correctly referred, which means that Supersaurus is merely a very big Barosaurus, and the name should be sunk.

I would be a lot more confident about which of these is the right thing to do if Matt and I had had time to look at all the sacral, caudal and appendicular material of Supersaurus during the Sauropocalypse. But our time was very limited (seven museums in nine days) and we had to focus on the presacrals.

What we really want is a solid assessment of all the putative Supersaurus material and a judgement of whether the differences between it and regular Barosaurus might be size- or age-related. We can’t have that (at least, not unless someone with more time on their hands than Matt or me takes it on).

But we are not left without hope. We have the published literature.

Pylogenetic analyses

Lovelace et al. (2008:figure 14). Strict consensus tree resulting from the addition of Supersaurus and “Seismosaurus” into a modified matrix from Harris & Dodson (2004).

First, Lovelace et. al’s (2008) description of Jimbo, the WDC’s referred Supersaurus specimen, included a phylogenetic analysis. This recovered Supersaurus as the sister taxon to Apatosaurus, with Suuwassea as its outgroup, and the BarosaurusDiplodocus clade sister to that broader grouping. That finding would argue against Supersaurus being Barosaurus. (They commented that “It is possible that some similarities between Supersaurus and other apatosaurines result from a size-coupled increase in robustness, but it is worth noting that apatosaurine robustness does not correlate with size, and large diplodocines like Seismosaurus do not exhibit markedly more robust pelvic or costal elements.)

Whitlock (2011:figure 7). Phylogenetic hypothesis presented in this analysis. Cladogram represents a strict consensus of three equally parsimonious trees (273 steps), labelled with relevant clade names. Decay indices reported below each node.

Whitlock’s (2011) more detailed phylogenetic analysis recovered Supersaurus is a somewhat more traditional position, closer to Barosaurus than to Apatosaurus. But still not very close. Supersaurus is here the most basal diplodocine, the outgroup to Dinheirosaurus, Torneria and the Barosaurus+Diplodocus pair. It’s not a result that would immediately make you want to synonymise Supersaurus with Barosaurus.

One problem with both Lovelace et al.’s and Whitlock’s analyses is that they took as read that the WDC specimen really is Supersaurus — the same thing as the BYU specimen. What if it isn’t? Maybe the WDC animal is something different that’s more closely related to Apatosaurus, while the BYU specimen is a big Barosaurus? Is that possible?

Enter Tschopp et al. (2015), whose monumental specimen-level analysis separated Jimbo out from BYU Supersaurus — and so they tested the hypothesis that these two specimens are the same thing, instead of assuming it. Here’s what they found:

Tschopp et al. (2015:figure 118). Reduced consensus tree obtained by implied weighting. Eight OTUs were deleted a posteriori. Numbers at the nodes indicate the number of changes between the two branches departing from the node (for the apomorphy count), where they differ from the trees under equal weights.

As you can see, BYU Supersaurus and the WDC specimen came out as sister taxa in every most parsimonious tree. And Tschopp et al.’s (2015) figure 115 shows that this is true under equal-weights parsimony as well as under implied weighting. So this gives us confidence that the WDC team’s referral of Jimbo to Supersaurus probably is correct after all.

But that Supersaurus duo comes out some way away from Barosaurus, being well outside the DiplodocusBarosaurus node.

These are the only three phylogenetic analyses I am aware of to have included Supersaurus — though if there are others, please shout in the comments. In none of them do Supersaurus and Barosaurus come out as sister taxa, and in fact they are separated by multiple nodes in all three analyses.

More compellingly, Andrea Cau re-ran Tschopp et al.’s (2015) analysis with Supersaurus and Barosaurus constrained to be sister groups (thanks, Andrea!) and found that the best resulting trees were 18 steps longer than the unenforced trees (1994 steps vs 1976). This is convincing evidence that the totality of the Supersaurus material is not Barosaurus.

Is BYU Supersaurus a chimaera?

All of this strongly suggests — it comes close to conclusively proving — that Supersaurus (as defined by all the BYU and WDC material) is not Barosaurus. But if Matt and I are right that BYU 9024 is a vertebra of Barosaurus, then it follows that this cervical doesn’t belong to Supersaurus.

And that, I think, throws the whole material list of BYU Supersaurus into question. Because if the big cervical belongs to something different, then it follows that there are (at least) two big diplodocids mixed up in the Dry Mesa quarry, contra Curtice et al.’s (2001) assertion that all the big bones there can be referred to two individuals, one diplodocid and one brachiosaur.

In which case, how can we know which of the elements belongs to which of the animals?

Are the scapulocoracoids from the same individual?

Can we even trust the assumption that the two scapulocoracoids were from the same animal? Maybe not. In favour of that possibility, the two elements are similar sizes, and were found close together. But there are reasons to be sceptical.

Based on our photos in the earlier post, I was coming to the conclusion that Scap B is much less sculpted than Scap A. But I started to change my mind once I was able to make a weak anaglyph of Scap B. Now, thanks to Heinrich Mallison and the magic of photogrammetry, my set of bad photos have become a 3D model, which is far more informative again.

Here, then, is a comparative anaglyph of the two scapulocoracoids.

Red-cyan anaglyps of both scapulocoracoids of Supersaurus from BYU’s Dry Mesa Quarry, Utah. Top: the holotype BYU 9025, left scapulocoracoid (“Scap A”); Bottom: referred specimen BYU 12962, right scapulocoracoid (“Scap B”), reversed for easier comparison. Scap B rendered from a 3D model created by Heinrich Mallison. Scaled to the same length. (We could not scale them in correct proportion, since the true current lengths of both are unknown.)

These are not obviously from the same individual, or from the same species, or even necessarily the same “subfamily”. A few of the more obvious morphological differences:

  • In Scap A, the acromion process projects posterodorsosally, whereas in Scap B it projects dorsally (i.e. at right angles to the long axis of the scap.)
  • In Scap A, the acromion process is positioned close to mid-length of the whole element, whereas in Scap B it is closer to the proximal end.
  • In Scap A, the acromion process comes to a point, whereas in Scap B is it lobe-shaped.
  • In Scap A, the ridge running running up to the acromion process is broad and becomes rugose dorsally, whereas in Scap B it is narrow and remains smooth along its whole length.
  • Scap B has a distinct ventral bump around midlength, which Scap A lacks (or at most has in a much reduced form).
  • In Scap B, the ventral border below the acromion process distinctly curves down to the glenoid, but in Scap B this ventral margin is almost straight.
  • In Scap A, the glenoid margin is gently curved, nearly straight, whereas in Scap B it has a well defined “corner”, with distinct scapular and coracoid contributions that are at right angles to each other.
  • In Scap A, the dorsal margin of the coracoid is well defined and has a low laterally protruding ridge. This is absent in Scap B, where the coracoid’s dorsal margin is poorly defined.

Now, much of this is quite possibly due to damage — as (I assume) is the excavation in the dorsal margin of the distal part of the scapular blade in Scap A. But when you put it all together, I think they really are rather different, even allowing for variation in limb-girdle bones. Certainly if you found them both in different quarries, you would not leap to the conclusion that they belong to the same species. Jensen’s (1985:701) description of Scap B (BYU 5001 of his usage) as “same as Holotype, BYU 5500” is difficult to justify.

The possibility that the two scaps are from different individuals is also weakly supported by the fact that the preservation looks very different between the two elements — dark and rough for Scap A but light and smooth for Scap B. But I don’t trust that line of evidence as much as I might for two reasons. First, different photography conditions can give strikingly different coloured casts to photos, making similar bones appear different. And second, I know from experience that bones from a single specimen can vary in colour and preservation much more than you’d expect.

At any rate, I certainly don’t think it’s a given that the two scapulae belonged to to the same individual as Curtice and Stadtman (2001) stated. And of course if they do not, then the issue of which is the holotype takes on greater importance — which is why we spent so long on figuring that out.

So what are we left with?

We know — or at least we are confident — that one of the referred BYU Supersaurus elements is Barosaurus. We don’t think the whole animal is Barosaurus, due to the evidence of three phylogenetic analyses. So we think there are at least two big diplodocoids in the BYU quarry, and we can’t know which of the elements belongs to which animal. We can’t even be confident that the two scapulocoracoids belong to the same animal.

As a result, the only bone that we can confidently state belongs to Supersaurus is the holotype — BYU 9025, which we called “Scap A”. All bets are off regarding all the other Dry Mesa diplodocoid elements. They might belong the Scap A taxon, or to Barosaurus. (Or indeed to something else, but we’ll ignore that possibility as multiplying entities without necessity.)

So to the next question: is the holotype element even diagnostic, beyond the level of “big diplodocoid”? I’m not sure it is, but this is where I’d welcome input from people who are more familiar with sauropod appendicular material than I am. At any rate, Jensen’s (1985:701) original diagnosis based on the holotype scap is useless: “Scapula long but not robust; distal end expanding moderately; shaft not severely constricted in midsection”.

The emended diagnosis of Lovelace et al. (2008:530) says of the scapulocoracoid only “scapular blade expanded dorsally; deltoid ridge perpendicular to the acromian[sic] ridge”. but they also include a more comprehensive assessment of the BYU scapulae (p. 534) as follows:

The only known pectoral elements for Supersaurus are the scapulocoracoids from Dry Mesa (Fig.10). Scapulocoracoid BYU 9025 demonstrates a deltoid ridge that is perpendicular to the acromian ridge and the scapular blade is one-half the entire length of the scapulocoracoid. Both of these features are seen in Apatosaurus but not in Diplodocus or Barosaurus, which have relatively short scapular blades, and an acute angle between the deltoid ridge and the acromian ridge. This angle is much stronger in Barosaurus than it is in Diplodocus. The apatosaurine nature of the scapulocoracoids further reinforces the referral of BYU elements to the type scapula, as well as our referral of WDC DMJ-021 to Supersaurus.

This is a helpful discussion (although note that Lovelace et al. are not consistent about which of the scaps they think is BYU 9025). But, notably, nothing here suggests any unique characters of the scapulocoracoid that could serve to diagnose Supersaurus by its holotype.

Putting it all together, it seems that BYU 9025 is the only bone in the world that unambiguously belongs to Supersaurus (because it is the the holotype, and all referrals are uncertain); and that bone is non-diagnostic. I think it must follow, then, that Supersaurus is currently a nomen dubium.

I say “currently”, because there are at least three possible ways for the name to survive. (Four, if you count everyone just ignoring this sequence of blog-posts.) Next time, we’ll talk about those options.

 

References

  • Curtice, Brian D. and Kenneth L. Stadtman. 2001. The demise of Dystylosaurus edwini and a revision of Supersaurus vivianae. Western Association of Vertebrate Paleontologists and Mesa Southwest Museum and Southwest Paleontologists Symposium, Bulletin 8:33-40.
  • Harris, Jerald D., and Peter Dodson. 2004. A new diplodocoid sauropod dinosaur from the Upper Jurassic Morrison Formation of Montana, USA. Acta Palaeontologica Polonica 49:197–210.
  • Jensen, James A. 1985. Three new sauropod dinosaurs from the Upper Jurassic of Colorado. Great Basin Naturalist 45(4):697–709.
  • Lovelace, David M., Scott A. Hartman and William R. Wahl. 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(4):527–544.
  • Tschopp, Emanuel, Octávio Mateus and Roger B. J. Benson. 2015. A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ 2:e857. doi:10.7717/peerj.857
  • Whitlock, John A. 2011. A phylogenetic analysis of Diplodocoidea (Saurischia: Sauropoda). Zoological Journal of the Linnean Society 161(4):872-915. doi:10.1111/j.1096-3642.2010.00665.x

 

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20 Responses to “Supersaurus, Ultrasaurus and Dystylosaurus in 2019, part 5: what actually is Supersaurus?”

  1. Andrea Cau Says:

    Topological (node) distance alone is not a good criterion for dismissing the referral of Supersaurus to Barosaurus. Because:

    1- there is clearly not consensus in diplodocid relationships, because the three analyses differ in too many details. This means that more data is necessary before assessing a robust framework. I am not that into diplodocid phylogeny, but suspect that the Tschopp et al. analysis can considered “superior” being the most taxonomically sampled.
    Surely, no more than one of them could be “true” (and they may even be all wrong), so the mere fact that all the 3 trees do not support Barosaurus-Supersaurus does not mean much.
    2- You should enforce the Supersaurus-Barosaurus node and compare the step difference of this enforced tree with the unenforced trees. If, for example, the step difference is just 1-2 steps, we cannot dismiss the alternative. This in particular relevant for fragmentary specimens scored for a few characters.

  2. Mike Taylor Says:

    You make a good point, Andrea, thanks for that. This may be the prod I need to finally get to grips with TNT.


  3. […] part 5 of the Supersaurus series, I made the point that my photos of Scap A and Scap B seem to show them as being very different […]

  4. Andrea Cau Says:

    I’ve just made a quick test using the Tschopp et al (2015) matrix: enforcing the sister-group between the Supersaurus couple and the Barosaurus genus group is 18 steps longer than the unenforced trees (1994 steps vs 1976).
    So, it seems not that parsimonious.

  5. Lee Braithwaite Says:

    If the cervical vertebra belongs to a Barosaurus and there are at least two large diplodocids in the original material then the question arises as to what Supersaurus cervicals look like.
    The WDC material has 10 cervicals, how different from Barosaurus are they?
    And if the cervicals are ignored for a moment is there enough overlap in the non cervical material to be confident that the two skeletons represent the same taxon?

    LeeB.


  6. […] Last time, I noted that photographs of the exact same object, even under the same lighting conditions, can come out different colours. That is one of the two reasons why I am not persuaded that the very different colours of my photos of the two Supersaurus scapulae is strong evidence that they are from different individuals. […]

  7. Mike Taylor Says:

    Andrea, many thanks for running this forced-sister analysis — something that would have taken me ages to figure out how to do, if I was able to do it at all. I will update the article to reflect this.

  8. Mike Taylor Says:

    Lee, I agree that the WDC cervicals are important here. A great next step would be to visit the WDC and see the Wyoming Supersaurus cervicals in person. Unfortunately, that’s not something I can easily do, as a 100% unfunded palaeontologist based in the UK. I hope some day to visit the WDC on a second Sauropocalypse, but there are no plans as of yet.

  9. Lee Braithwaite Says:

    Perhaps someone from there could take photos or comment here on them.
    One can but hope.

    LeeB.

  10. Mike Taylor Says:

    The good news here is the Dave Lovelace has sent me the photos they took back when they were working on the description. I will ply around with these, and publish some of them (with his permission) is a subsequent post.

  11. LeeB Says:

    Excellent!

    LeeB.


  12. If I may offer my extremely armchair opinion, I think that Supersaurus seems to be diagnostic and that it is at least plausible that the two are of the same species–as you noted, many of the apparent differences could be the result of breakage and others (such as rugosity of the acromial ridge) strike me as plausibly within the realm of individual variation.

    The big similarity is that both scapulae have a blade that flares distally to twice its minimum width, whereas in other diplodocids it never reaches more than 190% the minimum width (with one exception I’ll get to later).

    I suspect the absence of the “bump” on the ventral margin of the blade in Scap A is damage, as Jensen illustrates Scap A with such a bump in his drawing and the margin does look damaged at that point. In that case, Supersaurus shares with Diplodocus hallorum and Barosaurus lentus the presence of such a bump–absent in all other diplodocids except maybe a couple apatosaurines (Tschopp et al. score CM 3018 for this bump but I don’t see it in Gilmore 1936 or your walkaround video). There are other characters that distinguish Supersaurus from most diplodocids: it also has an unusually wide scapular blade, and it shares with Diplodocus hallorum and some apatosaurines a scapula less than six times as long as its minimum length. Supersaurus differs from Barosaurus and Tornieria in having an acromial ridge angled more than 75° from the axis of the blade (it’s slightly more acute in the holotype but overall they’re much more similar to each other than to Barosaurus).

    These differences are all based on characters and tables from Tschopp et al. 2015. I’m sure you guys would agree that the character list from one phylogenetic analysis is not the end of the story, either—there may well be more characters that an expert with access to a lot of diplodocid scapulae could figure out.

    Now, I said I’d get to an exception. There is one apatosaurine specimen, BYU 1252-18531, that is seemingly more Supersaurus-like than the rest. According to the scorings of Tschopp et al., it also has the extreme degree of flaring present in Supersaurus, as well as the ventral bump, but differs from Supersaurus in the classic apatosaurine characters of a medially-beveled glenoid and rectangular coracoid, as well in lacking a fossa posterior to the acromial ridge. Only that last one strikes me as an obvious diffrence—the coracoids of Supersaurus are rectangular-ish and the glenoid of Scap A appears damaged. In both characters it shares with Supersaurus, though, it appears to differ from all other apatosaurine specimens, which I find curious, but as the specimen is undescribed and unfigured I can’t find out any more.

    So overall, Supersaurus can be diagnosed by a unique combination of characters, present in both scapulae: angle between acromial ridge and axis of blade >75°, fossa present on acromion posterior to acromial ridge, ventral expansion of blade near anterior end, scapula length 1.9 times minimum width. Overall, it appears to be most similar to Diplodocus hallorum and BYU 1252-18531.


  13. Slight update to my previous comment—Diplodocus carnegii also appears to have the ‘bump’ on the ventral margin, albeit not an especially prominent one, based on fig. 14 of Holland 1901 (contra Tschopp et al.’s scoring of character 370), making the presence of the ‘bump’ a probable synapomorphy of Barosaurus+Diplodocus, suggesting Supersaurus may be closer to those genera than to Galeamopus or Tornieria, at least based on the scapula.

  14. Mike Taylor Says:

    Thanks, John, this is interesting stuff. But I’m not really finding your proposed diagnosis very compelling at this point. Looking at McIntosh’s (1990:fig. 16.8) line-drawings of sauropod scapulocoracoids, for example, it seems that distal expansion to 1.9 times the minimal width appears in at least some of Cetiosaurus, Haplocanthosaurus, Vouivria (“French Bothriospondylus“), Giraffatitan (“Brachiosaurusbrancai), Euhelopus, Camarasaurus and Rebbachisaurus — being at least as apparent in the drawings for those scaps as in that for Supersaurus (Figure. 16.8o).

    Can anyone remind me: has there been a better comparative assessment of sauropod scapulocoracoids since McIntosh’s encyclopedia entry?


  15. […] the previous installment of this epic, we’ve taken two brief digressions on how little importance we should attach the […]


  16. Thanks. You’re right that the flared distal end is not unique to Supersaurus among sauropods. Most of the specimens with such a flared scapula are outside of Flagellicaudata, however. The widely flared scapula could still be a valid local autapomorphy within Flagellicaudata, then, so perhaps proving Supersaurus’s validity is as simple as proving it is a diplodocid?

    It turns out I may have been wrong about the prevalence of a flared scapula in Diplodocidae, however. In addition to BYU 1252-18531 (which Tschopp et al. refer to Brontosaurus parvus, though it differs from both other specimens referred to that species in multiple characters), the holotype of Apatosaurus yahnahpin appears to possess such a widely flared scapula. Tschopp et al. scored it as having a scapula not flared to twice its minimum width, but Fig. 5 of Filla and Redman 1994, as well as their diagnosis of the species, suggest that it may have had a similarly flared scapula to Supersaurus. It also shares a relatively anteroposteriorly short coracoid with Supersaurus.

    However, the holotype of Supersaurus clearly differs from apatosaurines in the position of the acromion, which is near the midpoint as in diplodocines but not other flagellicaudates. Scap B appears to be intermediate between apatosaurines and diplodocines in this character, but I also wonder how much the exact angle at which the scapula is shown has an effect.

    So…where does this leave us? I’m not sure. There are several characters present in one or both scapulae that definitely merit investigation as potentially valuable for resolving the issue of Supersaurus’s validity. So here’s my updated provisional diagnosis of Supersaurus vivianae: A diplodocid characterized by the following combination of characters: 1) distal dorsoventral width of the scapular blade greater than 1.9 times its minimum dorsoventral width, 2) acromial ridge oriented at an angle greater than 75° from the scapular blade, 3) bulge on ventral margin of scapula at base of the blade, 4) well-developed fossa on acromion posterior to acromial ridge, and 5) coracoid dorsoventral height more than 1.5 times its anteroposterior length.

    Character 2 allows it to be distinguished from Barosaurus (which is, of course, important given the cervical), characters 1 and 5 are shared only with some Brontosaurus species among diplodocids, character 4 allows it to be distinguished from all Brontosaurus specimens except BYU 1252-18531 and character 3 allows it to be distinguished from BYU 1252-18531.

    The position of the acromion is also worth investigating, as it appears to be the most important difference between the two specimens (as it is a diplodocine synapomorphy, thereby making Scap A look like a diplodocine and Scap B look like an apatosaurine). It’s not entirely clear to me how people measure the length of the acromion for this character, though, tbh.

  17. Vahe David Demirjian Says:

    Regarding your question of whether Supersaurus is the same as Barosaurus based on BYU 20815, McIntosh (2005) stated that he once considered Supersaurus a possible gigantic species of Barosaurus. Since Lovelace et al. (2007) note that some BYU diplodocid material cannot be confirmed as belonging to Supersaurus, it’s possible that BYU 9024 could be Barosaurus and there might be two diplodocines in the Dry Mesa Quarry.

  18. Mike Taylor Says:

    Exactly, Vahe! We think that is the mostly likely (though not certain) explanation.


  19. […] If you’ve been following along, you’ll remember that Matt and I are convinced that BYU 9024, the big cervical vertebra that has been referred to Supersaurus, actually belongs to a giant Barosaurus. If we’re right about, then it means one of two things: either Supersaurus synonymous with Barosaurus, or there are two diplodocids mixed up together. […]


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