Your Friends The Titanosaurs: Chucarosaurus and (maybe) Ruixinia

We check in again with titanosaurs with one new genus and species that is definitely titanosaurian, and another that falls into the hazy uncertainty of potential Early Cretaceous titanosaurs.

Chucarosaurus diripienda

It's been a thing the past couple of years for dinosaurs to take the winter off, with few new names in December, January, and February, and 2023 has continued the tradition. Also, oddly enough, titanosaurs seem to strike during the winter; not that any time of the year is safe from them, but you can count on them taking up the slack left by tyrannosaurs and duckbills and so on.

Where was I? Anyway, the first publicized non-avian dinosaur of 2023 is our first guest, the titanosaur Chucarosaurus diripienda.

Genus and Species: "Chucaro" is a Quechua word for a "hard and indomitable animal", while "diripienda" is Latin for "scrambled" (Agnolin et al. 2023), giving us something like "scrambled indomitable lizard".

Citation: Agnolin, F. L., B. J. Gonzalez Riga, A. M. Aranciaga Rolando, S. Rozadilla, M. J. Motta, N. R. Chimento, and F. E. Novas. 2023. A new gigant titanosaur (Dinosauria: Sauropoda) from the Upper Cretaceous of northwestern Patagonia, Argentina. Cretaceous Research (preprint). doi:https://doi.org/10.1016/j.cretres.2023.105487.

(Thank you to Matias Motta for providing a pdf!)

Stratigraphy and Geography: C. diripienda is to date known only from the lower Upper Cretaceous Huincul Formation of the Pueblo Blanco Natural Reserve, northwestern Río Negro Province, Argentina (Agnolin et al. 2023). The site is rather better known for a variety of theropods representing several lineages (Tralkasaurus, Aoniraptor, Gualicho, Taurovenator, Overoraptor) (Agnolin et al. 2023).

Holotype: MPCA (Museo Provincial "Carlos Ameghino", Cipolletti, Río Negro, Argentina) PV 820, consisting of a left humerus, most of a left radius, left metacarpal II, left ischium, somewhat eroded left femur, the shaft of the left fibula, the proximal end of the right tibia, and the distal end of an anonymous metapodial, found disarticulated but associated and interpreted as representing one individual (Agnolin et al. 2023). A left femur and tibia (MPCA PV 821) represent another individual.

You might notice right away that, unlike most titanosaurs, we're not dealing with vertebrae here. This is a little inconvenient for comparative purposes, but on the other hand it does focus attention on areas of titanosaurian anatomy that do not receive as much attention. The limb bones indicate a large but relatively gracile form, with an estimated femur length of 200 cm (78.7 inches) (Agnolin et al. 2023).

Two other titanosaurs have been named from the Huincul Formation, the redoubtable Argentinosaurus huinculensis and the somewhat less famous Choconsaurus baileywilsoni. Limb bones of A. huinculensis and C. diripienda can be distinguished by several details, as well as A. huinculensis being more robust overall (although I suppose that shouldn't be too surprising). There is very little overlap with C. baileywilsoni, only metacarpal II, which is much more elongate in C. baileywilsoni. The phylogenetic analysis places C. diripienda as a colosssaurian but not quite a lognkosaurian, hanging out in the area of Bonitasaura salgadoi and Notocolossus gonzalezparejasi (Agnolin et al. 2023).

Ruixinia zhangi

Oddly enough again, the last non-avian dinosaur of 2022 was a potential titanosaur, Ruixinia zhangi. It falls into the grand tradition of Early Cretaceous East Asian titanosaur-ish sauropods, and as such is well worth an entry.

Genus and Species: The entire name is an allusion to Ruixin Zhang, a benefactor of the Erlianhaote Dinosaur Museum (Mo et al. 2023). As such, a translation isn't really applicable.

Citation: Mo, J., F. Ma, Y. Yu., and X. Xu. 2023 (preprint 2022). A new titanosauriform sauropod with an unusual tail from the Lower Cretaceous of northeastern China. Cretaceous Research 144:article 105449. doi:https://doi.org/10.1016/j.cretres.2022.105449.

(Cretaceous Research could almost be sub-headed as "The Journal of Titanosaurian Research" these days. They're all over it.)

Stratigraphy and Geography: R. zhangi hails from the famous Yixian Formation and was found at Batuyingzi, Beipiao, western Liaoning Province, northeastern China (Mo et al. 2023).

Holotype: ELDM (Erlianhaote Dinosaur Museum, Inner Mongolia, China) EL-J009, an articulated partial skeleton featuring most of the vertebrae (14 cervicals, dorsals, several sacrals, and 52 caudals), several dorsal ribs, 36 chevrons, left hip minus the ischium, left femur, left tibia, left astragalus, left metatarsal V, and a possible pedal phalanx (Mo et al. 2023).

Going from the description of the holotype, you'll note that we're dealing with a nearly complete vertebral column. Unfortunately, the other side of the coin in this case is that the precaudals are rather smooshed (Mo et al. 2023). From a visual inspection the cervicals seem kind of chunky, suggesting a relatively thick neck. R. zhangi has a rather odd tail; some of the highlights are as follows. The anterior caudals are strongly procoelous. The last six caudals are fused into a rod, simple in structure unlike the club of Shunosaurus and the cockscombs of Mamenchisaurus hochuanensis (Mo et al. 2023). (If it's just a rod, I wonder if it's just a one-off pathology.) The caudal neural spines are notably low and mildly split at the tops in the anterior caudals; the supporting neural arches are not cheated anteriorly in the first part of the tail like most titanosaurs but do have an anterior bias later on (Mo et al. 2023).

R. zhangi is not particularly large, with a femur 137 cm (53.9 inches) long and an overall body length on the order of 12 m (39 ft), but it is the current champion of the Yixian Formation sauropods (followed by Dongbeititan with a femur length of about 114 cm [44.9 inches] and Liaoningotitan slightly smaller at 108 cm [42.5 inches]). The phylogenetic analysis finds it well within Titanosauria (Mo et al. 2023), which is promising, but it's also closely associated with two taxa that are questionably titanosaurian per other authors (Daxiatitan and Xianshanosaurus), which is not so promising.

References

Agnolin, F. L., B. J. Gonzalez Riga, A. M. Aranciaga Rolando, S. Rozadilla, M. J. Motta, N. R. Chimento, and F. E. Novas. 2023. A new gigant titanosaur (Dinosauria: Sauropoda) from the Upper Cretaceous of northwestern Patagonia, Argentina. Cretaceous Research (preprint). doi:https://doi.org/10.1016/j.cretres.2023.105487.

Mo, J., F. Ma, Y. Yu., and X. Xu. 2023 (preprint 2022). A new titanosauriform sauropod with an unusual tail from the Lower Cretaceous of northeastern China. Cretaceous Research 144:article 105449. doi:https://doi.org/10.1016/j.cretres.2022.105449.

Hunting the wild stromatolite at Vermillion Falls

The gorge at Vermillion Falls, as seen before, is cut through quite a bit of the Prairie du Chien Group. Therefore, it is entirely reasonable to suppose that somewhere in all of those oodles of outcrop, there are some stromatolites. It should be just a matter of walking down and having a look, right? We-e-ll, easier said than done. A few factors are pushing against going from "predicted" to "established":

1) The classic booby prize of vertical exposures—sure, there's a lot of surface, but you don't get to see most of it up-close;

1b) Furthermore, for many of the places where you *can* examine the walls, you can't change your vantage point because backing up a foot or two puts you a foot or two lower or in open space (and that never helps). What makes this annoying with stromatolites is they can be expressed in various scales, and what you can't see with your nose on the rocks may be perfectly apparent from a couple of arm's lengths away except for that whole "absence of footing" thing, or a promising feature may disappear as you clamber up the slope to inspect it;

2) The surfaces have been fried to a crisp by weathering, which for our purposes disguises the fine layering and other subtle features.

The existing literature is not especially illustrative. Stauffer and Thiel (1941) included a section taken at the railroad bridge (now a footbridge), which describes the walls as 58.5 ft (17.8 m) of Shakopee Dolomite over 1 ft (0.3 m) of Root Valley Sandstone over 47 ft (14 m) of Oneota Dolomite, with nary a stromatolite mentioned (to be fair, the lithological descriptions are also very slim). The site has made it into a few theses and other student papers (Shea 1960; Squillace 1979; Robins 2005), but that seems to be about it. Maybe this limited documentation is a recognition of the above limitations by wiser heads than mine, but I have the faith of a gambler that something will turn up if I look long enough, so down we go back into the gorge.

Besides, why would I want to sit around inside when I could see things like this?

My strategy involves *not* looking directly for stromatolites, at least not the classic laminated features. I would never see them here except on fresh breaks. Instead, I'm looking for a couple of other features: bedding planes that are undulating rather than horizontal, and anomalously recessive intervals (I've seen elsewhere that stromatolitic intervals may be relatively erosion-prone compared to non-stromatolitic intervals). These two features should be visible even through the stain of weathering that began before the most recent Ice Age.

Down near the falls, in some of the lowest accessible beds, something very promising is apparent.

Left

Center

Right

We've definitely got an undulating surface that is appropriately stromatiform* in appearance, as well as being within an interval that is recessed. (In fact, the interval above is recessed in comparison to the interval above that, so this is not for people who don't like rocks poised above their heads [or, for that matter, people who have a phobia of birds pooping on them, because there are pigeons roosting in cavities above].)

*Google says it can come up with about 328,000 hits for "stromatiform". What it doesn't tell you is how many are useful and how many are just dictionaries, rhyming words, anagrams, pronunciations, and SEO things seeking to boost traffic with lists from dictionaries.

A little closer, between two apparent mounds

So stromatiform...

Obviously the next thing to do is to is to get close and look for finer details. That's where things become less clear. There are, conveniently, some fresh surfaces (not made by me!), and they do show alternating compositions. They are, however, rather thick layers.

Layers in one fresh surface.

Not quite as fresh, but you get the idea.

Layers on a spalled piece, frozen to the underlying moss.

I'd been hoping to find nice stacks of small columns beneath the undulating layers, but they aren't appearing (yet, at least). If we compare to Logan et al. (1964) and May et al. (2012), the latter also dealing with large Prairie du Chien stromatolites, it's as if we've skipped the columnar "Cryptozoon" stage and gone straight to broad "Collenia". This may say something about the local environment, or it may say that I'm just getting excited about some inorganic stromatiform-producing process.

Then there's whatever the heck is this. Bedded sedimentary rocks don't do things like this without a good reason.

If the issue is not immediately apparent, check the annotated version below and click to expand as necessary.

Too bad it was on the other side of the gorge. There's probably a way down somewhere...

References

Logan, B.W., R. Rezak, and R. N. Ginsburg. 1964. Classification and environmental significance of algal stromatolites. The Journal of Geology 72(1):68-83.

May, S. L., L. E. Davis, and D. G. Brown. 2012. Algal stromatolites in the Willow River Member of the Lower Ordovician Shakopee Formation near Chatfield, Minnesota, USA. The Compass: Earth Science Journal of Sigma Gamma Epsilon 84(1, Article 6):42–48.

Robins, C. 2005. The geology of the New Richmond Sandstone. Senior Integrative Exercise. Carleton College, Northfield, Minnesota.

Shea, J. H. 1960. Stratigraphy of the Lower Ordovician New Richmond Sandstone in the Upper Mississippi Valley. Thesis. University of Wisconsin, Madison, Wisconsin.

Squillace, P. J. 1979. The geology of the New Richmond Member of the Shakopee Formation (Lower Ordovician), Upper Mississippi Valley. Thesis. University of Minnesota, Minneapolis, Minnesota.

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Bulletin 29. Minnesota Geological Survey, St. Paul, Minnesota.