Mid-Mesozoic Field Conference, Day 6: Sauroposeidon and kin

May 8, 2014

My camera had a possibly-fatal accident in the field at the end of the day on Saturday, so I didn’t take any photos on Sunday or Monday. From here on out, you’re either getting my slides, or photos taken by other people.

Powell Museum sauropod humerus

On Sunday we were at the John Wesley Powell River History Museum in Green River, Utah, for the Cretaceous talks. There were some fossils on display downstairs, including mounted skeletons of Falcarius and one or two ornithischians,* and this sauropod humerus from the Cedar Mountain Formation (many thanks to Marc Jones for the photo).

* A ceratopsian and Animantarx, maybe? They were in the same room as the sauropod humerus, so it’s no surprise that I passed them by with barely a glance.

There were loads of great talks in the Cretaceous symposium on Sunday, and I learned a lot, about everything from clam shrimp biostratigraphy to ankylosaur phylogeny to Canadian sauropod trackways. But I can’t show you any slides from those talks, so the rest of this post is the abstact from Darren’s and my talk, illustrated by a few select slides.

Wedel Naish 2014 Sauroposeidon and kin - slide 1 title

Sauroposeidon is a giant titanosauriform from the Early Cretaceous of North America. The holotype is OMNH 53062, a series of four articulated cervical vertebrae from the Antlers Formation (Aptian-Albian) of Oklahoma. According to recent analyses, Paluxysaurus from the Twin Mountain Formation of Texas is the sister taxon of OMNH 53062 and may be a junior synonym of Sauroposeidon. Titanosauriform material from the Cloverly Formation of Wyoming may also pertain to Paluxysaurus/Sauroposeidon. The proposed synonymy is based on referred material of both taxa, however, so it is not as secure as it might be.

Wedel Naish 2014 Sauroposeidon and kin - slide 34 Sauroposeidon characters

Top row, vertebrae of Paluxysaurus. From left to right, the centrum lengths of the vertebrae are 72cm, 65cm, and 45cm. Main image, the largest and most complete vertebra of the holotype of Sauroposeidon. Labels call out features that are present in every Sauroposeidon vertebra where they can be assessed, but consistently absent in Paluxysaurus. Evaluating the proposed synonymy of Paluxysaurus and Sauroposeidon is left as an exercise for the reader.

MIWG.7306 is a cervical vertebra of a large titanosauriform from the Wessex Formation (Barremian) of the Isle of Wight. The specimen shares several derived characters with the holotype of Sauroposeidon: an elongate cervical centrum, expanded lateral pneumatic fossae, and large, plate-like posterior centroparapophyseal laminae. In all of these characters, the morphology of MIWG.7306 is intermediate between Brachiosaurus and Giraffatitan on one hand, and Sauroposeidon on the other. MIWG.7306 also shares several previously unreported features of its internal morphology with Sauroposeidon: reduced lateral chambers (“pleurocoels”), camellate internal structure, ‘inflated’ laminae filled with pneumatic chambers rather than solid bone, and a high Air Space Proportion (ASP). ASPs for Sauroposeidon, MIWG.7306, and other isolated vertebrae from the Wessex Formation are all between 0.74 and 0.89, meaning that air spaces occupied 74-89% of the volume of the vertebrae in life. The vertebrae of these animals were therefore lighter than those of brachiosaurids (ASPs between 0.65 and 0.75) and other sauropods (average ASPs less than 0.65).

Wedel Naish 2014 Sauroposeidon and kin - slide 64 Mannion phylogeny

Check this out: according to at least some versions of the Mannion et al. (2013) tree, Sauroposeidon and Paluxysaurus are part of a global radiation of andesaurids in the Early and middle Cretaceous. Cool!

Sauroposeidon and MIWG.7306 were originally referred to Brachiosauridae. However, most recent phylogenetic analyses find Sauroposeidon to be a basal somphospondyl, whether Paluxysaurus and the Cloverly material are included or not. Given the large number of characters it shares with Sauroposeidon, MIWG.7306 is probably a basal somphospondyl as well. But genuine brachiosaurids also persisted and possibly even radiated in the Early Cretaceous of North America; these include Abydosaurus, Cedarosaurus, Venenosaurus, and possibly an as-yet-undescribed Cloverly form. The vertebrae of Abydosaurus have conservative proportions and solid laminae and the bony floor of the centrum is relatively thick. In these characters, Abydosaurus is more similar to Brachiosaurus and Giraffatitan than to Sauroposeidon or MIWG.7306. So not all Early Cretaceous titanosauriforms were alike, and whatever selective pressures led Sauroposeidon and MIWG.7306 to evolve longer and lighter necks, they didn’t prevent Giraffatitan-like brachiosaurs such as Abydosaurus and Cedarosaurus from persisting well into the Cretaceous.

Wedel Naish 2014 Sauroposeidon and kin - slide 65 Cloverly sauropods

The evolutionary dynamics of sauropods in the North American mid-Mesozoic are still mysterious. In the Morrison Formation, sauropods as a whole are both diverse and abundant, but Camarasaurus and an efflorescence of diplodocoids account for most of that abundance and diversity, and titanosauriforms, represented by Brachiosaurus, are comparatively scarce. During the Early Cretaceous, North American titanosauriforms seem to have radiated, possibly to fill some of the ecospace vacated by the regional extinction of basal macronarians (Camarasaurus) and diplodocoids. However, despite a flood of new discoveries in the past two decades, sauropods still do not seem to have been particularly abundant in the Early Cretaceous of North America, in contrast to sauropod-dominated faunas of the Morrison and of other continents during the Early Cretaceous.

Wedel Naish 2014 Sauroposeidon and kin - slide 66 acknowledgments

That final slide deserves some explanation. On the way back from the field on Saturday–the night before my talk–a group of us stopped at a burger joint in Hanksville. Sharon McMullen got a kid’s meal, and it came in this bag. We took it as a good omen that Sauroposeidon was the first dinosaur listed in the quiz.

For the full program and abstracts from both days of talks, please download the field conference guidebook here.

10 Responses to “Mid-Mesozoic Field Conference, Day 6: Sauroposeidon and kin”

  1. Andrew Thomas Says:

    That sauropod humerus up top looks awfully Wolf-Creeky, don’t you think?

  2. Allen Hazen Says:

    Trying to eyeball the, it looks as if the latest-surviving species on the cladogram has a range that ends about the beginning of the Campanian. (But “Titanosauroidea” seems to goof below the bottom of the portion of the chart reproduced here…)

    Really, really, embarrassingly naive question: do Sauropods (I have the impression that the Titanosaurs were the last survivors of Sauropoda) drop out then (ca. 18my before the end of the Mesozoic), or are there later survivors not on this chart?

  3. Matt Wedel Says:

    The big Wolf Creek humerus is the one from Unit V in the second-to-last slide. I think the CMF humerus is much more similar to Ostrom’s humerus from Unit VI; both have a fairly wide shaft (at least compared to the Unit V humerus) and the squared-off proximal end typical of titanosaurs. So my suspicion is a true brachiosaur in Unit V and a Sauroposeidon-like titanosaur in Units VI and VII. Maybe more than one of each–we are still in the very early days of understanding Cloverly sauropod diversity. Unlike the CMF, where there are probably at least nine genera* scattered over the four major units.

    *Cedarosaurus, Venenosaurus, Abydosaurus, Brontomerus, at least two at Dalton Wells, a similar but distinct form from another quarry, the Price River 2 brachiosaurid, and whatever left the tiny sauropod teeth in the Mussentuchit.

    You know, I really should do a series of posts on all of these things. And I should do it soon, while everything I learned at MMFC14 is still fresh in my head.

  4. Allen Hazen Says:

    (Allen should do at least minimal homework before bothering people with stupid questions.)

    O.k., checking I find that Alamosaurus is supposed to have made it into the late Maastrichtian. Is it outside the phylogenetic span shown in the diagram (maybe on the branch that seems to go off the bottom of the page), or just not included?

  5. Matt Wedel Says:

    Allen, I chopped out most of that figure to put the just the bit that was relevant to the conference on the slide at legible scale. Off the top should be the rest of the non-macronarian sauropods, and off the bottom should be the rest of Titanosauria, including a host of Late Cretaceous forms. Sorry for the confusion–this is something I went through step-by-step in talk, first putting up the whole tree and then selectively chopping down to just the bits I wanted to talk about. It deserves more discussion here, and I was already thinking about doing a follow-up post just on that phylogeny and its implications. I will put that post into the lineup for sure.

  6. Matt Wedel Says:

    :-) Our comments are passing in the ether, but you intuited what I had done. Like I said, full post on this to come soon.

  7. Andrew Thomas Says:

    Well, I’m positive that you know more about sauropod morphology than me, Matt, not that I needed to be reminded ;)
    The humerus up at the very top looks very similar to the sauropod elements on the cataloging table at the Sam Noble right now, all from Wolf Creek. May just be a coloring thing, plus the eye of an uninitiate :)

  8. Matt Wedel Says:

    Well, it is certainly possible that there is more than one sauropod present at Wolf Creek. I am most familiar with the big humerus that I put in my next-to-last slide. If the material just being catalogued doesn’t match that one in morphology, then the story just got more interesting. Guess that means I need to come back and visit the OMNH collections again, eh?

  9. Nima Says:

    Beautiful stuff. I would appreciate any more published info on these, including Dalton wells and Price River 2. I suspect there’s a whole fauna of brachiosaurs still undescribed in Cloverly too.

    BTW Matt, you might want to check this out:

    This is literally the biggest news I’ve seen for a long time. Even Puertasaurus may be in the shadows now.

  10. […] cuentan el congreso día por día, y con numerosas fotografías: Day 1, Day 2, Day 3, Day 4, Day 5, Day 6, Day […]

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