Diamantinasaurs, and being or not being a titanosaur

It's not all about new taxa with titanosaurs here. There are always publications on other topics; it might be fun to do a roundup feature every so often. Anyway, I thought I'd catch up on the Australian branch of the tree, as well as the eternal question of just what is a titanosaur, after all.

Item 1: Diamantinasaurs

A couple of months back, a truly titanic* monograph (Beeston et al. 2024) on the sauropods of the Winton Formation was published. If you don't recall who those are, it's Australotitan cooperensis, Diamantinasaurus matildae, Savannasaurus elliottorum, and Wintonotitan wattsi. The first three were introduced in "Your Friends The Titanosaurs" entries, and the last was waved at as a sauropod that sometimes was a titanosaur and sometimes wasn't. Over the past few years, a consensus has been growing that these sauropods (plus or minus W. wattsi) and Sarmientosaurus musacchioi form a small clade, Diamantinasauria (published in Poropat et al. 2021; discussed to some extent in the Australotitan entry and another on the skull of Diamantinasaurus). An interesting situation is developing: Diamantinasauria may or may not be within Titanosauria. Granted, this falls under neither "tragic" nor "unprecedented" (the base of Titanosauria being a permeable membrane), but, like I wrote, "interesting".

*I apologize for nothing!

Anyway, back to Beeston et al. (2024). Over the past 15 or so years, a sizeable crop of sauropod specimens has come out of the Winton Formation, include several partial skeletons and a couple of partial skulls. Table 1 in the article by itself is worth the price of admission just to be able to see what all of the specimens comprise (and what their nicknames are; any respectable Winton Formation sauropod specimen has a nickname like "Matilda" or "Wade"). All of the Winton Formation specimens come in for analysis and assessment in something that's like the grand Morrison diplodocid specimen-level study (surprisingly, almost ten years old this year). There are fewer species, fewer specimens, and fewer fossils as a whole to contend with, but the principle is the same. Beeston et al. also look at the proposed autapomorphies for the four species. Several observations and conclusions can be drawn from their work, including:

The Winton Formation sauropod sample, at least the specimens complete enough to reliably analyze, can all be ascribed to Diamantinasauria. This includes W. wattsi. In the phylogenetic analysis, the sauropods fall into two clusters: A. cooperensis + D. matildae and S. elliottorum + W. wattsi, each cluster with a small clutch of unassigned specimens. Sarmientosaurus is their next closest cousin and rounds out Diamantinasauria, and Diamantinasauria is the sister group to Titanosauria.

Looking at autapomorphies and character states, it turns out that two of the Winton sauropods are distinctly drawn: D. matildae and S. elliottorum. S. elliottorum is least like the other Winton sauropods. W. wattsi is more ambiguous; by character scores, it is distinct from S. elliottorum and not too different from the other two, but in the phylogenetic analysis, it does the opposite, keeping close company with S. elliottorum. Beeston et al. elected to keep it as a separate genus and species. A. cooperensis, though, turns out to be basically D. matildae. Beeston et al. did not draw the figurative trigger on a synonymization, instead provisionally regarding the holotype as indeterminate within Diamantinasauria and reassigning other specimens that had been assigned to the species.

Finally, there is some possibility that a new specimen, AODF 0032 ("Mick"), may represent something new based primarily on pelvic differences, but Beeston et al. opted to wait for more material rather than coin a new name.

Given that N through U represent AODF 0032, it looks like a good idea to wait for a little more before getting too excited. Figure 14 from Beeston et al. (2024). CC BY 4.0.

So, where does all that leave us (and The Compact Thescelosaurus)? The specimen-level assessments all seem sound; I have no qualms listing A. cooperensis as dubious and a potential synonym of D. matildae, nor moving W. wattsi to join the others in Diamantinasauria. It's what to do with Diamantinasauria that's the tricky part. Given it might be inside or outside Titanosauria, I split the difference and placed it at Titanosauria?.

Item 2: Sibirotitan a titanosaur?

Meanwhile, there's a sauropod that's looking to make the opposite move, from outside to inside Titanosauria. Sibirotitan astrosacralis Averianov et al. 2018 (or 2017, if you go by first online appearance) is a sauropod that was found, logically enough, in Siberia, specifically the Lower Cretaceous Ilek Formation of Kemerovo Oblast. S. astrosacralis is based on a dorsal vertebra, with a handful of other compatible bones from the same location assigned to it (cervicals, sacrum and ilium, foot elements, and teeth). It was initially described as a somphospondyl nearish but not within Titanosauria.

More recently, Averianov et al. (2023) described caudal vertebrae and chevrons they attributed to S. astrosacralis. The caudals are distinctly procoelous, of the form that practically any sauropod researcher would have confidently placed in Titanosauria throughout the 20th century. Fortified by these caudals, S. astrosacralis phylogenetically becomes a titanosaur. Let's write up another entry, right?

For now, I'm not convinced, for a couple of reasons. I *will* state up-front that I agree they've got some very titanosaurian-like caudals; it's the other parts of the argument I'm concerned about. First, the taxonomic assignment of the material: Of the caudals, only one was from the type locality; the others came from another site several kilometers away. There is some positional overlap (the topotype caudal is anterior, and the other material include an anterior caudal), and the two specimens are reasonably compatible, but comparison is limited, more or less, to the centra. So, they could represent the same species, but it's not guaranteed. For that matter, although the postcranial specimens at the type locality have been interpreted as one scattered skeleton (Averianov et al. 2018; Averianov and Lopatin 2022), we have seen plenty of examples of multiple sauropod species in a site (to say nothing of formations!), so I think it's reasonable to be cautious for the moment.

In addition, the phylogenetic results are not clear-cut. When treated separately, S. astrosacralis comes out as a mamenchisaurid and the caudals come out as turiasaurian or technically titanosaurian (the data set is one of those that results in Andesaurus delgadoi inviting practically every somphospondyl into Titanosauria). When the two are put together, the result is a technical titanosaurian that appears to owe a lot to the dysfunction of Andesaurus. For some context, within Titanosauria the augmented S. astrosacralis is found in the neighborhood of Daxiatitan binglingi and Erketu elllisoni, both of which show euhelopodid tendencies (and the latter almost never turning up in Titanosauria). Other company may include Gobititan shenzhouensis (almost never a titanosaur), Padillasaurus leivaensis (almost never a titanosaur), and, oddly enough, Savannasaurus elliottorum. In short, even in the best-case scenario of all the bones belonging to the same species, S. astrosacralis ends up as a titanosaur more or less because Andesaurus doesn't know what the heck it's doing. With friends like that, a non-titanosaurian placement is at least as likely as the opposite.

References

Averianov, A. O., and A. V. Lopatin. 2022. New data on Sibirotitan, a titanosauriform sauropod from the Early Cretaceous of Western Siberia. Doklady Earth Sciences 506(1):650–653.

Averianov, A., S. Ivanstov, P. Skutschas, A. Faingertz, and S. Leschinskiy. 2018. A new sauropod dinosaur from the Lower Cretaceous Ilek Formation, Western Siberia, Russia. Geobios 51:1–14. doi: https://doi.org/10.1016/j.geobios.2017.12.004.

Averianov, A., A. Podlesnov, D. Slobodin, P. P. Skutschas, O. Feofanova, and O. Vladimirova. 2023. First sauropod dinosaur remains from the Early Cretaceous Shestakovo 3 locality, Western Siberia, Russia. Biological Communications 68(4):236–252. doi: https://doi.org/10.21638/spbu03.2023.404.

Beeston, S. L., S. F. Poropat, P. D. Mannion, A. H. Pentland, M. J. Enchelmaier, T. Sloan, and D. A. Elliott. 2024. Reappraisal of sauropod dinosaur diversity in the Upper Cretaceous Winton Formation of Queensland, Australia, through 3D digitisation and description of new specimens. PeerJ 12:e17180.
doi: https://doi.org/10.7717/peerj.17180.

Poropat, S. F., M. Kundrát, P. D. Mannion, P. Upchurch, T. R. Tischler, and D. A. Elliott. 2021. Second specimen of the Late Cretaceous Australian sauropod dinosaur Diamantinasaurus matildae provides new anatomical information on the skull and neck of early titanosaurs. Zoological Journal of the Linnean Society 192(2):610–674. doi:https://doi.org/10.1093/zoolinnean/zlaa173.