Our latest guest in the series is Inawentu oslatus from the middle Late Cretaceous of Argentina. I. oslatus is more than your average new titanosaur; it is one of the two major unnamed titanosaurs discussed a couple of years back, MAU-Pv-LI-595. This is the more recently discovered of the two skull-bearing titanosaurs from Rincón de los Sauces (the other is MAU-Pv-AC-01).
Genus and Species: Inawentu oslatus. "Inawentu" is a Mapundung word meaning "imitator" or "mimic", and alludes to the strong similarity to rebbachisaurids. "Oslatus" combines Latin words for "mouth" ("os") and "broad" ("latus"), because the business end of the skull is broad (Filippi et al. 2023). Combined, we get something like "broad-mouthed imitator".
Citation: Filippi, F. S., R. D. Juárez Valieri, P. A. Gallina, A. H. Méndez, F. A.
Gianechini, and A. C. Garrido. 2023. A rebbachisaurid-mimicking
titanosaur and evidence of a Late Cretaceous faunal disturbance event in
South-West Gondwana. Cretaceous Research 105754 (online pre-proof). doi: https://doi.org/10.1016/j.cretres.2023.105754. (Thanks to Skye McDavid for sending me a copy!)
Geography and Stratigraphy: The type and only known specimen was found near the top of the Upper Cretaceous (Santonian) Bajo de la Carpa Formation, 11.1 m (36.4 ft) below the Anacleto Formation, in floodplain mudstones. The location is in the La Invernada area, southwest of Rincón de los Sauces in the Neuquén Basin of Patagonia, Neuquén, Argentina (Filippi et al. 2023).
Holotype: MAU-Pv-LI-595 (Museo Municipal Argentino Urquiza, Rincón de los Sauces, Neuquén, Argentina), an articulated partial skeleton including a nearly complete skull and lower jaws, a complete vertebral column from the atlas to the end of the sacrum, cervical and sacral ribs, and both ilia (Filippi et al. 2023).
MAU-PV-LI-595 is an articulated specimen, a rarity among titanosaurs, which have historically preferred bonebeds (with bonus points for multiple species). A largely complete skull plus complete cervical, dorsal, and sacral series vaults I. oslatus into the inner circle of titanosaur completeness and will no doubt make it an important point for comparison, given how important vertebrae are for differentiating species. Preservation quality is mixed, and it is thought that other bones were lost to erosion (Filippi et al. 2023). (Drat.)
The skull as preserved looks a bit like someone pulled the muzzle down and away from the rest of the skull. The anterior margin of the jaw is more or less squared off. The maxillae flare laterally, but the lower jaw does not. The actual tooth-bearing portion of the upper jaw appears to be absent, but I count 14 tooth positions in the figured dentary, with simple peg-like teeth. Compared to Bonitasaura salgadoi, the other squarish-jawed titanosaur known from fairly complete skull material, there are a number of differences, such as: the antorbital fenestrae are long skinny slots rather than broad ellipsoids; the lateral temporal fenestrae are constricted into two small holes each; the external naris has at least part of a midline bar (as opposed to being a simple hole); and the foramen magnum, through which the spinal cord passes, is oriented facing the bottom of the skull rather than the posterior. This last point is notable because it means the skull, when held neutrally, made almost a right angle with the neck. Hold the neck near horizontal, and the snout is pointed straight down.
The neck is proportionally short for a sauropod. There are only 12 cervicals (the fewest of any titanosaur to date) and they are relatively short (Filippi et al. 2023). Most of them look rather similar except for size, with simple triangular neural spines and short cervical ribs generally about the length of their verts or slightly longer. Things change around the base of the neck, with the neural spines becoming shorter fore-aft with C11, then C12 being quite short lengthwise with a distinctly angled profile, rising up anteriorly. The first dorsal also is stepped up anteriorly, indicating an anatomically enforced rise of the neck at its base. The dorsals mostly have low neural spines, generally swept posteriorly back to the sacrum, where the angle becomes close to vertical. The last sacral indicates a procoelous first caudal. The ilia are long and rather low but not notably broad (Filippi et al. 2023). The neural arches are more or less all there (i.e., missing bits are due to erosion, not lack of fusion), indicating we aren't looking at a juvenile. However, the type specimen was not a particularly large sauropod, on the order of 10 m (33 ft) long judging from Filippi et al.'s restoration.
As its name hints, I. oslatus has a lot in common with rebbachisaurs, including a broad squared-off snout with small slender teeth packed into the front, a skull oriented steeply downward with respect to the neck, a relatively short neck, short cervical ribs, and smallish body size. The jaw characteristics are known from several other titanosaurs (Antarctosaurus wichmannianus, Baalsaurus mansillai, Bo. salgadoi, Brasilotitan nemophagus), so we should expect them to share at least some of these other general features as well. There is rather little anatomical overlap with most of them except for the mandibles, though. (A. wichmannianus is overdue for a redescription, which could be done in comparison to I. oslatus and Bo. salgadoi to see what of the associated postcranial elements might most likely pertain to it.) Rebbachisaurs appear to have shuffled off their mortal coil around the end of the Cenomanian, 8 million years or so before I. oslatus's appearance in the Santonian. Whatever had caused the rebbachisaurs' ecological role to go sour must not have persisted, if within 8 million years a branch of titanosaurs had waltzed in and made it their own. (And you haven't lived until you have seen waltzing sauropods.)
Unsurprisingly, Filippi et al.'s phylogenetic analysis places I. oslatus close to several of the other square-jawed titanosaurs: A. wichmannianus, Ba. mansillai, and Bo. salgadoi. Br. nemophagus is in the neighborhood but more remote, which is interesting. In fact, it is actually found to be closer to a clade consisting of Rinconsauria and Aeolosaurini. The Rinconsauria+Aeolosaurini clade plus the unnamed square-jawed clade make up Filippi et al.'s "Clade A", distinct from groups such as saltasaurs and lognkosaurs. It is not unheard of for rinconsaurs to be found with aeolosaurs or with square-jaws (Bo. salgadoi), but this particular configuration appears to be unique to date in the literature.
The description did not go into it, but the Bajo de la Carpa Formation has several other named titanosaurs, and it's worth having a quick look at them. Bonitasaura salgadoi we've already mentioned, with several significant differences from I. oslatus in the skull alone. Next is Microcoelus patagonicus, based on a partial anterior dorsal. This doesn't seem to be a good match with I. oslatus, but that's just based on eyeballing figures. Next is the aeolosaur Overosaurus paradasorum, which overlaps quite nicely from the posterior cervicals to the sacrum and ilia. There are some similarities, such as the abrupt angle at the base of the neck, but the two can be distinguished by features such as the form and angle of the neural spines and the general shape of the ilia. Rinconsaurus caudamirus does not overlap quite as well, but can also be distinguished by the vertebrae and ilia. "Titanosaurus" nanus includes both a cervical and a dorsal, neither of which are complete; neither look much like their corresponding number in I. oslatus. Finally, there is Traukutitan eocaudata, which has no anatomical overlap with I. oslatus, but is unlikely to be the same thing simply based on size: T. eocaudata includes a femur 1.85 m [6.07 ft] long and caudals with unfused neural arches. The Bajo de la Carpa Formation just seems to represent a good place for titanosaur diversity, with aeolosaurs, a couple square-jaws, rinconsaurs, saltasaurs, and potential lognkosaurs.
Filippi, F. S., R. D. Juárez Valieri, P. A. Gallina, A. H. Méndez, F. A. Gianechini, and A. C. Garrido. 2023. A rebbachisaurid-mimicking titanosaur and evidence of a Late Cretaceous faunal disturbance event in South-West Gondwana. Cretaceous Research 105754 (online pre-proof). doi: https://doi.org/10.1016/j.cretres.2023.105754.