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.