Supersaurus, Ultrasaurus and Dystylosaurus in 2019, part 7: at last, Dystylosaurus has its day!

July 8, 2019

Poor Dystylosaurus. Always the bridesmaid. No-one seems to care much about it, yet the one and only vertebra that bears that name is the single most diagnostic elements out of all the individual bones that have been assigned to Supersaurus over the years.

A nice drawing of the “Dystylosaurus” dorsal vertebra in anterior and right lateral views. It’s probably Tracey Ford’s work (awaiting confirmation), from the PaleoFile page on Supersaurus.

Unfortunately, we weren’t able to learn a whole ton about this vertebra on the Sauropocalypse visit. We did see it, but it was flat on its back on a shelf not much taller than the anteroposterior length of the bone itself, so we weren’t able to get a good look at it in anything but dorsal and ventral views. If we’d had more time to get things arranged, I’m sure the BYU people would have been happy to get it down from the shelf for us, but we simply had so much to do in their collections that time was never made for it.

BYU 4503, the holotype and only element of Dystylosaurus edwini, an anterior dorsal vertebra. here seen in approximately dorsal view with anterior to the top. Matt Wedel for scale.

Matt actually got some rather better photos a few years ago, though (based on his comment on that post), there are probably no more than the couple in that old blog-post. (By the way, notice how very different the colour of the bone appears in Matt’s old photos from how it appears in my more recent one above.)

Why do I say so confidently that the Dystylosaurus vertebra is diagnosable? Because it has a whole suite of characters that tell us it’s an anterior dorsal vertebra from a diplodocid (dual centroprezygapophyseal laminae, anteroposteriorly compressed spine composed primarily of spinozygapophyseal rather than spinodiapophyseal laminae, drooping transverse processes), yet two features of the spine are never seen in such vertebrae: the spine is wholly unsplit without even a hint of bifurcation, even featuring macronarian-like lateral apices; and it’s hollow inside rather than being constructed from intersecting plates of bone. (You can see the internal hollow in the photo above.)

So what happens to its genus name given the doubts about Supersaurus‘s diagnosability? The general trend of comments on these posts has been that Supersaurus should stand or fall on its holotype, and I am inclined to agree that parachuting in the Dystylosaurus vertebra or Jimbo as a neotype to save the name would be a mistake. For one thing, despite its numerous appearances in kids’ books, the name Supersaurus is not that important in the technical literature: for example, no-one has named a clade Supersaurinae or similar. For another, the holotypic scapulocoracoid BYU 9025 is only questionably undiagnosable. There would always be the possibility that if someone nominated a neotype and wrestled it through the ICZN petition process, someone else would find a good solid way to diagnose the original holotype. That would be embarrassing.

The rare ventral-ish view of the Dystylosaurus dorsal vertebra BYU 4503. Sorry it’s not better. I do have 93 photos of it in this shelf, all of them individually pretty terrible, which I took in the forlorn hope that one day we’ll get photogrammetry software simple enough and clever enough to make some kind of model out of them.

So I think we need to simply accept that the name Dystylosaurus, while perfectly diagnosable based on its holotype and only specimen, is destined to remain a junior synonym for as long as Supersaurus is considered taxonomically valid.

But it does leave Dystylosaurus in a bit of a quantum superposition. When Supersaurus is considered diagnosable, it ceases to exist, like a cat in a box. When Supersaurus is considered undiagnosable, it pops back into existence, like … well, a cat in a box. It’s an unsatisfactory kind of existence, but I think that’s the way it has to be.

So Dystylosaurus has its day — and it ends up being disappointing. Despite being perfectly diagnosable, it’s dependent for its validity on our assessment of other taxa. Some fossils just can’t catch a break.

11 Responses to “Supersaurus, Ultrasaurus and Dystylosaurus in 2019, part 7: at last, Dystylosaurus has its day!”

  1. Vahe David Demirjian Says:

    What were the characters used by Jensen (1985) to argue that Dystylosaurus might have belonged in a new family rather than Brachiosauridae?

  2. Matt Celeskey Says:

    The drawing of the vertebra looked to me like the work of Tracy Ford, and a little digging on his website found the source here:

    Enjoying this series of posts!

  3. It’s highly ironic that Dystylosaurus means “double beam lizard” when its most distinctive trait now is having a single neural spine, when its relatives all have double!

    It’s also strange that it’s the only one of the three to have a “technical” name rather than a superlative name. No wonder it’s the least-remembered of the three. Was Hypersaurus taken?

  4. Mike Taylor Says:

    Vahe, it doesn’t seem that Jensen even contemplated the notion that Dystylosaurus might have been brachiosaurid: that idea seems to have first come up in McIntosh’s (1990) encyclopaedic summary of sauropods. But Jensen’s own (1985:707) diagnosis does include one character to separate it from Brachiosaurus:

    A sauropod different from all described North American sauropod genera in having two parallel, diagonal infraprezygapophyseal laminae supporting each hypantrozygapophyseal arch in anterior dorsal vertebrae; lower half of neural arch massive, the neurocental sututre occupying nearly 7/8 the length of the centrum; neural spine fragile, being transversely broad but anteroposteriorly thin; supraprezygapohyseal laminae not convergent at midshaft as in Brachiosaurus; neural arches of dorsal vertebrae completely pneumatic, including spine, transverse processes, and zygapophyses.

  5. Mike Taylor Says:

    Thanks, Matt C., nice detective work.

    (Tracey, if you’re listening, it would be helpful if you could confirm that the image is yours.)

  6. TimW Says:

    Some nomenclatural trivia:

    1. Edwin (source of the Dystylosaurus species name edwini) and Vivian (source of the Supersaurus species name vivianae) were husband and wife, Eddie and Vivian Jones.

    2. Jensen’s etymology for Dystylosaurus expressly states the genus is from “di” for two and “stylos” for beam. So the spelling could (and arguably should) have been Distylosaurus. I assume from what follows in Jensen’s paper that it’s named after the “two parallel, diagonal infraprezygapophyseal laminae supporting each hypantrozygapophyseal arch” – I think this was mentioned in an earlier part of this series. As John mentioned, this name is not the least bit superlative.

  7. Neil Says:

    Bit late to discovering these, but a really interesting history!!! Been hooked!

    Quick question about the validity of Dystylosaurus relative to Supersaurus. If, although Supersaurus and Ultrasaurus were informally used long before, they were officially published in the same paper as Dystylosaurus, and if, unless I’m much mistaken, there are no rules on page priority, is there any reason why Supersaurus and Ultrasaurus could not be declared junior synonyms of Dystylosaurus as being the most diagnosable, rather than Ultrasaurus and Dystylosaurus being considered junior synonyms of Supersaurus which then has to be declared a nomen dubium?

  8. Mike Taylor Says:

    Hi, Neil. Your question is a good one. See the section Method 2. Nominate a neotype from the BYU material in the earlier post part 6: what happens to Supersaurus now?. In a nutshell:

    But that ship has now sailed. According to the principle of first reviser (Section 24.2.1), the pubished actions of Curtice and colleagues established a new status quo, and their choice of genus stands.

  9. Marja Erwin Says:

    So what is the situation?

    I take it the material comes from 3 different genera, but not as Jensen had assigned them?

    Some material appearing to come from Barosaurus, some from Dystylosaurus, and at least one Ultra-scapula distinct enough from the others? Other material, including the Ultrasauros holotype not diagnostic enough to assign to any genus?

  10. Mike Taylor Says:

    Marja, the only real answer at the moment is that we don’t know. Once we have establish for certain what is the holotype of Supersaurus, and thought through its differential diagnosis, we’ll have a better sense. There’s more in the subsequent posts in this series.

  11. […] the holotype dorsal vertebra of Dystylosaurus (now almost universally regarded as Supersaurus [but then…]), lurking with it in a corner of the collections […]

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