Saltapotamus, meet Obesethocoelicaudia

August 21, 2020

This just in from John Conway:

John doesn’t say much about it in the tweet where he unveiled this piece: just “A new #painting, of a Saltapotamus”. His website is just a little more forthcoming:

Saltapotamus

Saltasaurus was a small (for a sauropod) sauropod from the Late Cretaceous of Argentina. It had a some armour, and a lot of girth.

This reminds me very strongly of Obesethocoelicaudia, a fat restoration of Opisthocoelicaudia that John kindly did for Matt and me to use in our 2014 SVPCA talk, “Slender Giants”:

(Saltasaurus and Opisthocoelicaudia are both derived titanosaurs, and in most phylogenies they come out as pretty closely related.)

Is this kind of restoration credible? After all, it’s a long way from how we’ve been used to seeing Saltasaurus. Here, for example, is how E. Guanuco restored a group of four Saltasaurus individuals in Powell (2003: plate 78):

In this illustration they are tubby in a Normanpedia kind of way, but nothing very different from how (say) Apatosaurus was being restored not too long before then.

But the truth is that lots of animals have flesh envelopes very different from what you might predict based on the skeleton alone. Exhibit A, the inspiration for John’s new piece: the humble hippopotamus. Skeleton:

And life appearance:

It seems more than reasonable that across a clade as diverse, disparate and long-lived as the sauropods, there would have been some that were similarly heavy with flesh. In fact, I think it would be special pleading to argue that there were not.

Which specific sauropods were obese? That is much harder to tell. Hippos can be very heavy with little penalty as they spend much of their time in the water. Perhaps the same was true of some sauropods. If that’s so, then our quest must be for sauropods whose skeletons show adaptations for a semi-aquatic lifestyle, and on that basis Opisthocoelicaudia may have at least two feature supporting this interpretation: very robust limb bones and (if the interpretation of Borsuk-Bialynicka 1977: figure 5 is to be trusted) a transversely broad torso.

References

  • Powell, Jaime E. 2003. Revision of South American Titanosaurid dinosaurs: palaeobiological, palaeobiogeographical and phylogenetic aspects. Records of the Queen Victoria Museum 111:1-94.

 

5 Responses to “Saltapotamus, meet Obesethocoelicaudia

  1. dale mcinnes Says:

    Always wondered about Opisthocoelocaudia !

  2. oliveunicorn Says:

    Obesethocoelicaudia Is pretty cute though. Even if they’re a bit thicc

  3. Jura Says:

    Are there any extant reptiles that show such girth? Yeah, there are plenty of examples of mammals getting tubby, but those are mammals. Mammalia is a clade that perfected fat storage. We store fat everywhere. As far as I know, all extant sauropsids store fat in specific regions of the body. The closest taxa that come to mind for girthiness are Sauromalus, which can look pretty beefy when they hold in air, potentially Uromastyx, and Chitra (though the latter mostly just looks wider).

    There are more examples of chubby reptiles in captive species, but those are all anomalies that often suffer from underlying problems due to overfeeding and lack of exercise.

  4. Mike Taylor Says:

    But there is no reason to think that dinosaur physiology more closely remembled that fof extant lizards than that of extant mammals. (And it likely wasn’t particularly close to either.)

    We do have this one piece of phylegenetic inference to work from: since their closest living relatives are birds, which have a very high BMR, we can infer (with only very weak confidence) that dinosaurs also had high BMRs. Which means they would have had good reason to store fat for lean times if they could.

  5. Jura Says:

    We are not necessarily talking physiology here, at least not metabolic physiology. This is strictly about evolutionary tool kits. Dinosaurs evolved from diapsid reptiles and as such would be saddled with the same evolutionary tool kit as other diapsids. No extant diapsid, or extant reptile (I’m still not sold on turtles as diapsids) shows extensive fat deposits around the body. That includes birds. Fat is stored in specific regions of the body (tail and base of hips in most reptiles, approximately 16 discrete “fat organs” on birds).

    You can’t just use birds as your phylogenetic inference. You need to weed out the features that are unique to birds vs. dinosaurs (or more accurately, Neornithes vs. the rest of Dinosauria). Dinosaurs may be more closely related to birds, but they are bracketed by crocodylians, then lizards, then (probably) turtles and then way at the base of Amniota, mammals. This bracket matters, as dinosaurs still shared a vast amount of features in common with other reptiles as opposed to birds and to the complete exclusion of mammals. Bracketing dinosaurs between these groups puts realistic limits on our inferences. It’s why the extant phylogenetic bracket works as well as it does. Using a properly bracketed EPB (crocodylians on one end, birds on the other, then expand the inference out to see how far back the trait can be traced), we obtain a fat distribution inference of level 1′ (shared anatomical structures with no osteological correlate). One can certainly argue that dinosaurs had novel fat storing abilities that allowed them to become as corpulent as mammals. It’s not out of the realm of possibility, but it is still “making stuff up” (level 3′ inference, novel anatomical feature with no osteological correlate and no presence in the extant bracket).

    I don’t think we have the phylogenetic evidence to argue for BMR in dinosaurs. Nonetheless, a comparison with birds still doesn’t work in this instance as birds are notorious for not storing a lot of fat. The only exceptions to this come from birds that are preparing for a long migration, which makes this a physiological trait limited to Neognaths, among birds, and even then it’s variable within the clade.

    A useful reference for this:

    Pond, C.M. 1978. Morphological Aspects and the Ecological and Mechanical Consequences of Fat Deposition in Wild Vertebrates. Ann Rev Ecol System. 9:519–570.


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