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Enter Sarmientosaurus

April 30, 2016

by Mike Taylor

Yesterday we got a treat: the description of a new titanosaur, Sarmientosaurus musacchioi, based on some decent cervical vertebrae and an almost absurdly nice skull from the Upper Cretaceous of Argentina (Martinez et al., 2016). It was published in PLOS ONE so it’s free to the world, including a 3D PDF of the skull and some awesome visualizations. Get all that good stuff here.

I had one day’s warning about this – Brian Switek contacted me on Monday to ask if I’d be willing to lend my thoughts on the new critter for his news article for National Geographic, which you can read here. As always, I sent more stuff than he could use, so I’m recycling the long form for the rest of this post.

Brian’s first question was about how Sarmientosaurus stands out. I wrote:

Sarmientosaurus has probably the most complete and best-preserved skull of any sauropod from South America to date. It’s kind of funny – for so long we had so few good skulls from brachiosaurs and titanosaurs, and now we’re getting them, but without much of the rest of the skeleton. In North America, unquestionably the nicest Cretaceous sauropod skull is that of the brachiosaurid Abydosaurus, but all we have with the skull is a bit of the neck. Same situation now with this new titanosaur, Sarmientosaurus. I’m not complaining – great skulls without bodies are still great skulls! – but it will be nice to someday connect heads and bodies.

Also, the authors are to be commended – I don’t think anyone has ever done such a thorough job describing the skull of a sauropod dinosaur. This paper will become the standard to which all others are compared going forward.

The next question from Brian was about the head posture and the inference drawn by Martinez et al. (2016) that Sarmientosaurus fed at ground level. My take:

It doesn’t seem unlikely to me that Sarmientosaurus had a downward-facing snout. As for being a low grazer, I am skeptical. The inner ear usually tells us something about the alert posture of an animal, not its feeding posture. Take rhinos – some of them graze from the ground, and some of them browse up higher, but they all carry their heads the same way. Most grazers have wide snouts, whereas that of Sarmientosaurus is pointed and even a little narrower than that of Giraffatitan. That’s a curious shape for a supposed grazer.

So there are three points to unpack here. First, I chose my words deliberately in saying that the inner ear tells us “something” about the alert posture, because in fact the horizontal semicircular canals (HSCCs) aren’t great even at that. As I wrote in this post seven years ago:

Where SCCs have really attracted attention in paleontology is the “more or less” horizontal orientation of the HSCCs in living animals. Some authors have argued that if you set the HSCCs level or close to level, you can figure out how the head was oriented in life.

Well, maybe. The problem is that there is a LOT of variation around level. In birds surveyed by Duijm (1951), HSCC orientation varied by 50 degrees among taxa, from 20 degrees below horizontal to 30 degrees above. Furthermore, in humans HSCC orientation varies by up to 20 degrees among individuals. Possibly humans are weirdly variable, but it seems at least equally likely that most critters are and we’ve only discovered that variation in humans because of the huge sample size.

However you slice it, those are darn big error bars around any given head posture. That doesn’t mean that HSCC orientations in dinosaurs and other extinct vertebrates are worthless for determining posture (they may also be a source of taxonomic information). Strictly speaking, it means that preserved HSCCs can get us in the 50-degree ballpark but can’t narrow things down any further. This is one of those areas in paleontology where we’re just going to have to live with a certain amount of uncertainty, at least for now.

As far as I know, that’s all still true. But I’d love to be wrong.

Second, there’s the difference between alert posture and feeding posture. Go watch horses graze – the skull is practically vertical while they’re feeding, but that’s not the orientation you get from the HSCCs. So if I’m skeptical about ignoring the error bars around HSCC orientation to determine alert posture, I’m even more skeptical about trying to infer feeding posture from them. Also, the rhino point – we have an extant group with closely related taxa where one is a grazer (white rhino, Ceratotherium) and one is a browser (black rhino, Diceros). They hold their heads about the same. So feeding preference will not necessarily be reflected in normal, non-feeding head posture.

Third, muzzle shape. Most grazers have wide mouths, but as I said in the email to Brian – and as this figure shows – the snout of Sarmientosaurus is narrower than that of Giraffatitan, and I don’t think anyone is seriously proposing that Giraffatitan was a grazer. So if Sarmientosaurus was more committed to low-level feeding than more basal titanosauriforms, its face was evolving in the wrong direction. Just sayin’.

(Incidentally, I am hugely in favor of figures like 33 and 34 in Martinez et al., 2016, which make it easy to compare the new critter to a selection of reference taxa. I wish everyone would do this all the time.)

Finally (final for the purposes of the interview), Brian noted that in the media sauropods are often depicted as all being pretty much the same, and he asked what made Sarmientosaurus stand out. My response:

Until now, the skulls we’ve found of basal titanosauriforms – brachiosaurs and relatives – and more derived titanosaurs haven’t looked much alike. To me Sarmientosaurus is cool because it bridges that gap. From the top and the front the skull looks a lot like those of Brachiosaurus and Giraffatitan – really wide, pretty big teeth, long toothrow. But from the side, the smaller nostrils and long snout have obvious similarities to more derived titanosaurs like Nemegtosaurus. And they phylogenetic analysis confirms that, which is nice. But you can take one look at this thing and say, “Yeah, cool, we’ve been waiting for someone like you.”

The lateral views of titanosauriform skulls in the above figure nicely illustrate my point. If you took the Giraffatitan skull in A and the Tapuiasaurus skull in F and did a 50% morph between them, you’d get something pretty darned close to Sarmientosaurus. And about halfway between Giraffatitan and the really derived saltasaurids is where the phylogenetic analysis puts Sarmientosaurus. The gestalt of the skull nicely reflects the animal’s relationships, which does not always happen.

Oh, there are cervical vertebrae, too, and a seriously weird ossified tendon that is apparently not a cervical rib, but those will keep for another post.

The take-home here is that although I disagree with the authors on a few points of paleobiological interpretation, the Sarmientosaurus fossils are spectacular and Martinez et al. (2016) have done a tremendous job of describing and illustrating them. And the paper is free to anyone who wants it, as it should be. One of the great delights of the last few years has been watching PLOS ONE and PeerJ become the preferred outlets for high-quality descriptive work on dinosaurs.

Now if we can just find more of this thing!

Martínez RDF, Lamanna MC, Novas FE, Ridgely RC, Casal GA, Martínez JE, et al. (2016) A Basal Lithostrotian Titanosaur (Dinosauria: Sauropoda) with a Complete Skull: Implications for the Evolution and Paleobiology of Titanosauria. PLoS ONE 11(4): e0151661. doi:10.1371/journal.pone.0151661



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