A non-saltasaurine titanosaur tibia from Cerro Overo (Allen Formation), Upper Cretaceous, northern Patagonia, Argentina

FILIPPI, LEONARDO S.; BELLARDINI, F.; MÉNDEZ, ARIEL H.; PAULINA-CARABAJAL, ARIANA; GIANECHINI FEDERICO A.; CRUZADO-CABALLERO, P.; ULLOA-GUAIQUIN, K.S.; GONZÁLEZ-DIONIS, J.; GARRIDO, ALBERTO C.; MANIEL, I.J.; LEE, Y.-N.; DO-KWON, K.

Congreso; IV Jornadas de Paleontología de la Cuenca Neuquina; 2023

In the northeastern Neuquén Basin, the Cerro Overo-La Invernada area comprises extensive, fluvialoutcrops of the Upper Cretaceous, ranging from the Plottier Formation (Coniacian–Santonian) to the Allen Formation (Campanian–Maastrichtian). Within this thick sequence, the most abundant and diverse vertebrate fossil record comes from the Bajo de la Carpa and Anacleto formations, while the record from the Allen and Plottier formations are poorly known, at least in this sector of the basin, limited to an undetermined hadrosauridae cervical centrum (MAU-Pv-CO-667) from Cerro Overo (Allen Formation). Here, we present an almost complete sauropod tibia (MAU-Pv-CO-723), the first record for this locality to the Allen Formation, discovered in the yellowish-fine sandstones, which outcrop at the top of the Cerro Overo. The tibia is well-preserved (missing parts of the cnemial crest and the lateral distal condyle) and the proximal and distal articular ends exhibit rough surfaces indicating cartilaginous insertions. The tibia is a slender bone (robustness index, RI= 0.19), thinner than other titanosaurians such as Laplatasaurus (RI= 0.22), Lirainosaurus (RI= 0.23), Aeolosaurus sp. (RI= 0.25), Bonatitan (RI= 0.26), and Bonitasaura (RI= 0.27), distinguishing it from saltasaurines like Neuquensaurus, which have extremely robust tibiae (RI> 0.31). The proximal end of MAU-Pv-CO-723 exhibits a subcircular morphology, as in Laplatasaurus, but differing from the subelliptic contour observed in Bonitasaura, Rapetosaurus, Lirainosaurus, and Petrobrasaurus, and from the subsquared contour seen in Kaijutitan and Dreadnoughtus. The distal end is slightly anteroposteriorly and lateromedially expanded, resembling Laplatasaurus, in contrast to the strongly expanded distal ends of saltasaurines. The cnemial crest appears triangular in lateral view, and the cnemial fossa is deep, extending distally for over one-third of the total length of the bone, similar to Laplatasaurus. The malleoli of the distal end are posterolaterally prominent and separated by a deep and narrow medial notch, representing the articular surface for the ascending process of the astragalus. On this surface, two small fossae with small foramina are observed. Various characters in MAU-Pv-CO-723, such as the subcircular proximal articular surface and the distally extended cnemial fossa, resemble Laplatasaurus more closely than any known titanosaurs. This new finding provides additional information on the sauropod faunal diversification in the Cerro Overo-La Invernada area, enhancing our knowledge of titanosaurian dispersion and evolution in the northeastern Neuquén Basin during the Campanian–Maastrichtian transition.