The hitherto hidden half of BMNH R46870

July 12, 2009

It’s no secret – at least, not if you’re a regular SV-POW! reader – that the Lower Cretaceous Wealden Supergroup of southern England includes more than its fair share of enigmatic sauropod remains (see Mystery sauropod dorsals of the Wealden part 1, part 2, part 3). Poor taxonomic decisions, a dearth of adequate descriptive literature, and (perhaps) the vague concept that sauropod diversity in the Lower Cretaceous of Europe must be low have combined to prevent adequate appraisal. Recent comments on Wealden sauropods have been provided by Naish et al. (2004), Naish & Martill (2007), Taylor & Naish (2007) and Mannion (2008).

Chondrosteosaurus_R46870_resized

One of the most interesting Wealden sauropods – and I mean ‘interesting’ in an entirely subjective, historiographical sense – is Chondrosteosaurus gigas. This taxon has a rather confusing history that I don’t want to repeat here. The type series consists of two cervical vertebrae: BMNH R46869 and BMNH R46870 (and it is BMNH R46870, despite the occasional use in the literature of ‘46780’). We’ve looked at BMNH R46869 before. This time round I want to briefly talk about BMNH R46870. Anyone familiar with the literature on Wealden sauropods will know that this specimen was sectioned and polished. However, to date, only half of BMNH R46870 has been published (Owen 1876, plate V; Naish & Martill 2001, text-fig. 8.4), on both occasions as a mirror-image of the actual specimen. Previously unreported is that both halves of the specimen were polished, and both are in the Natural History Museum’s collection today. And here they are, shown together for the first time ever. I screwed up on the lighting, so sorry for the poor image quality [images © Natural History Museum, London].

A little bit of science has been done on this specimen. Chondrosteosaurus has had a mildly controversial history: it’s been suggested at times to be a camarasaur, but its camellate interior show that it’s a titanosauriform. Because the exact ratio of bone to air can be measured, the specimen lends itself particularly well to an Air Space Proportion analysis of the sort invented by Matt. Indeed, Matt did some ASP work on the figured half of BMNH R46870 in his thesis, finding an ASP of 0.70 (Wedel, Phd thesis, 2007). The average ASP of sampled neosauropod vertebrae is 0.61, and an ASP of 0.70 for the mid-centrum (as opposed to the condyle or cotyle) is most similar to the values present in camarasaurs and brachiosaurs. Mid-centrum ASP values of titanosaurs seem to be lower (Wedel, Phd thesis, 2007).

Chondrosteosaurus_R46870_closeup_resized

Anyway, more on Wealden sauropods – hopefully, a lot more – in the future.

References

  • Mannion, P. 2008. A rebbachisaurid sauropod from the Lower Cretaceous of the Isle of Wight, England. Cretaceous Research 30, 521-526.
  • Naish, D. & Martill, D. M. 2001. Saurischian dinosaurs 1: Sauropods. In Martill, D. M. & Naish, D. (eds) Dinosaurs of the Isle of Wight. The Palaeontological Association (London), pp. 185-241.
  • Naish, D. & Martill, D. M. 2007. Dinosaurs of Great Britain and the role of the Geological Society of London in their discovery: basal Dinosauria and Saurischia. Journal of the Geological Society, London, 164, 493-510.
  • Naish, D., Martill, D. M., Cooper, D. & Stevens, K. A. 2004. Europe’s largest dinosaur? A giant brachiosaurid cervical vertebra from the Wessex Formation (Early Cretaceous) of southern England. Cretaceous Research 25, 787-795.
  • Owen, R. 1876. Monograph on the fossil Reptilia of the Wealden and Purbeck Formations. Supplement 7. Crocodilia (Poikilopleuron). Dinosauria (Chondrosteosaurus). Palaeontographical Society Monographs, 30, 1-7.
  • Taylor, M. P. & Naish, D. 2007. An unusual new neosauropod dinosaur from the Lower Cretaceous Hastings Beds Group of East Sussex, England. Palaeontology 50, 1547-1564.
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18 Responses to “The hitherto hidden half of BMNH R46870”

  1. Nathan Myers Says:

    Sorry, that last paragraph confuses me. How does an ASP more like that of a camarasaur than of a titanosaur imply it’s a titanosaur and not a camarasaur?

  2. Darren Naish Says:

    Hi Nathan.

    1) Internal morphology indicates that Chondrosteosaurus is a titanosauriform. Titanosauriformes incorporates brachiosaurs and titanosaurs, and not camarasaurs.

    2) ASP shows that Chondrosteosaurus is most like camarasaurs and brachiosaurs and it’s different from titanosaurs in this respect.

    3) Given that Chondrosteosaurus is most likely a titanosauriform – see (1) – then, overall, it’s brachiosaur-like.

    And now the big question: what, if anything, is a brachiosaur? There’s a clade there, but it really looks like some or many so-called brachiosaurs form a grade on the titanosaur stem (and, yeah yeah yeah, technically titanosaurs don’t have a stem because they don’t have a crown either. But you know what I mean).

  3. Mike Taylor Says:

    Nathan, it doesn’t. The ASP alone would suggest a camarasaur or brachiosaur rather than a titanosaur; and the internal morphology (may small spaces rather than a few large ones) suggests a titanosauriform (i.e. a brachiosaur or a titanosaur) rather than a camarasaur. Put it together and what we have here is quite possibly a brachiosaur. Which makes sense as there a quite a few putative brachiosaur genera knocking around the Wealden (Eucamerotus, Orthithopsis, Pelorosaurus, Oplosaurus) and no camarasaur or titanosaur genera.

    But …

    First of all, all these brachiosaur genera may be the same thing (and some have indeed treated them as synonyms). Second, it’s not 100% clear, or even much more than 50% clear in most cases, that they actually are brachiosaurs, despite the historical assumption. Third, the lack of Wealden titanosaur genera is misleading, since the misassigned species “Pelorosaurus” becklesii is a definite titanosaur and simply doesn’t have its own genus yet (we’re working on it). Fourth, the lack of camarasaur genera in the Wealden may also be misleading, as we have on this very blog shown at least two Wealden presacrals that a certain camarsauricious quality to them. And fifth (pant, pant, pant), the statement that “Mid-centrum ASP values of titanosaurs seem to be lower” is based on a very preliminary data-set of arbitrary sections based on what Matt happened to be able to get his hands on, and I would not be AT ALL surprised if, when a bigger data-set is one day built, the supposedly lower ASP of titanosaurs vanishes into the mist.

    In other words, we don’t really know ANYTHING, but we can take some good guesses.

  4. Mike Taylor Says:

    Darren,

    As of a couple of years back, the Phylocode guys have stopped talking about stem-based clades and switched to the terminology “branch-based” — largely for the very reason you highlight, that “stem-based” implies the outgroup to an extant crown. A good change, and one that I’ve been following. So you could say that “some or many so-called brachiosaurs form a grade on the titanosaur branch”.

    Ah, if only there were a paper in press that would really look closely at brachiosaurs, and figure out whether there actually is a clade there. Wouldn’t that be great?

  5. Darren Naish Says:

    My bad. I never talk (or write) of ‘stem-based clades’ anymore; however, I do have a problem when one needs to refer to those taxa that are outside the ‘crown’ of a given clade… Let’s say, for no reason at all, that we’re talking about early bats. We used to refer to these taxa as ‘stem-bats’. But what now? ‘Branch bats’ doesn’t make any sense, and ‘basal bats’ isn’t necessarily accurate (as some may be highly derived and more similar to the crown forms than the ones down at the base). Fact is, terms like ‘stem-bat’ are still being used and I haven’t yet seen a good alternative.

    Or.. is it that it’s ok to refer to ‘stem-bats’ (or stem-hummingbirds or stem-anurans or whatever) so long as there IS a stem and a crown, and that it’s only those clades that lack extant members that are the problem? (to those who don’t know: a crown, by definition, is the clade subtended by extant taxa. Extinct clades cannot, by definition, have crowns). If so, do we need a new convention?

  6. Mike Taylor Says:

    “Or.. is it that it’s ok to refer to ’stem-bats’ (or stem-hummingbirds or stem-anurans or whatever) so long as there IS a stem and a crown?”

    Exactly. The point of adopting the term “branch-based” was not to REPLACE “stem-based”, but to make it possible to differentiate the two cases. I admit, though, that I have not heard terms like “branch macronarians” — that’s because in order to nail down the boundaries of a paraphyletic group, you need to specify both the top and bottom, so you’d want to be clear about whether you meant (for example) non-titanosauriform macronarians or non-somphospondylian macronariants. For groups with an crown defined by extant members, of course, you can cheat by assuming that the crown is the group to be removed from the grade under discussion, which is why “stem bats” is unambiguous (at least compared to “branch macronarians”).

  7. Matt Wedel Says:

    the statement that “Mid-centrum ASP values of titanosaurs seem to be lower” is based on a very preliminary data-set of arbitrary sections based on what Matt happened to be able to get his hands on, and I would not be AT ALL surprised if, when a bigger data-set is one day built, the supposedly lower ASP of titanosaurs vanishes into the mist.

    Yes, to all of that. More on this coming in the next post. Stay tuned, true believers!

  8. Paul Barrett Says:

    Errm – you got the specimen numbers wrong. These are ‘old catalogue’ numbers and do not have the ‘R’ prefix…

  9. Paul Barrett Says:

    Oh, and these days our acronym is just NHM…

  10. Darren Naish Says:

    Ok, thanks. So, should the old numbers be ‘BMNH 46869’/’BMNH 46870′ and the new ones be ‘NHM R46869’/’NHM R46870’? I’d seen the new NHM prefix on a few specimens but didn’t realise that it’d been brought in across the board – when was this implemented? Thanks much.

  11. Mike Taylor Says:

    Yikes, this is news to me. Please tell me that the actual numbers are unchanged! (The Humboldt Museum recently took it into its head to rename HMN SII as MB.R.2181, and all their other specimens have horrible new hard-to-remember names. Here on SV-POW!, we’ll keep talking about SII, thanks!)

  12. 220mya Says:

    Darren – I believe they should be NHM 46869/NHM 46870 if I’m reading Paul’s message correctly.

  13. Paul Barrett Says:

    220mya is correct – there’s no R in these cases. The originals were part of the original undivided catalogue prior to the introduction of different prefixes (e.g. R for reptiles, P for fish) for the various parts of the collection.

  14. Mike Taylor Says:

    Thanks, Paul. So let me get this clear:
    1. ALL specimens with numbers that begin BMNH should now begin NHM instead.
    2. SOME of the BMNH/NHM specimens have an R in front of their number and some do not, and we have incorrectly introduced the R in specimens where it doesn’t belong.
    3. Nothing that did have an R now doesn’t, and nothing that didn’t have an R now does.
    Is that right? In which case the Xenoposeidon holotype is still R2095, but it’s now NHM R2095 rather than BMNH R2095.

    When did the change happen?

  15. Paul Barrett Says:

    Hi Mike,

    Yes, all correct. NHM has actually been our official acronym for sometime (it’s what various UK museum bodies have designated us as). However, very few people (including staff!) knew of the change. The same changes apply to many other UK museums, so, for example, the Sedgwick (formerly SMC) is now CAMSM.

    Cheers, Paul

  16. Mike Taylor Says:

    Thanks, Paul, that all makes sense. I appreciate the clarification.


  17. [...] of pneumatic sauropod vertebrae. Darren has covered the history of the specimen before, and in the last post he showed photographs of both this chunk and its other half. He also briefly discussed the Air [...]


  18. [...] is the mate of NHM 46870, a specimen that we have already given way too much coverage, and which has sometimes been considered the cotype along with 46869.  Unlike its [...]


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