New ankylosaurian trackways (cf. Tetrapodosaurus) from an uppermost Cretaceous level of the El Molino Formation of Bolivia

Abstract

Since the second half of the 20th Century, the study of fossil tracks in South America has steadily increased. A large number of tetrapod ichnogenera is currently known mainly from Argentina, Bolivia and Brazil. In this study, we present a new record consisting of several trackways that we refer to cf. Tetrapodosaurus, representing the first explicit mention of the ichnogenus in South America. These tracks come from the lacustrine to transitional El Molino Formation, deposited in Maastrichtian-Danian times, at the locality of Niñu-Mayu, near Sucre, Bolivia. These tracks preserve a quite similar manual morphology to that of Tetrapodosaurus borealis and minor differences in its pedal prints. Tetrapodosaurus is a typical ichnogenus of the northern hemisphere and commonly assigned to an ankylosaurian trackmaker. Tracks assigned to ankylosaurs were already reported from other sites of Bolivia, Brazil and tentatively Argentina; the new finding further improves our understanding of the ankylosaurian record from South America. A detailed morphological analysis allows us to make an accurate trackmaker identification and footprints are attributed to a member of the ankylosaurian family, the Nodosauridae. The trackways from Niñu-Mayu also have paleobiological implications. Manus prints are complete, pes prints generally lack the proximal region of the pes, the sole pad, expulsion rims are proximally placed in pes prints and the trackway parameters are highly variable, suggesting formation influenced by certain buoyancy of the trackmakers.