Southernmost Spiny Backs and Whiplash Tails: Flagellicaudatans from South America

Abstract

Flagellicaudatan diplodocoids include the two families Dicraeosauridae and Diplodocidae. Although different in sizes and relative proportions (e.g. neural arches height, neck length, tail length), they share several features, both cranial and postcranial, that recover them as a monophyletic group in updated phylogenies. The record of the group in South America was particularly scarce during the twentieth century, but their number and taxonomical diversity noticeably increased in the last decade. Up to now, five dicraeosaurid taxa (Amargasaurus cazaui, Amargatitanis macni, Bajadasaurus pronuspinax, Brachytrachelopan mesai, and Pilmatueia faundezi) and one diplodocid (Leinkupal laticauda) were recognized. Additionally, two presumably dicraeosaurid and three diplodocid records are known from fragmentary materials. Jurassic strata have provided both Brachytrachelopan and two of the indeterminate diplodocids, whereas the remaining five taxa, the third indeterminate diplodocid and the indeterminate dicraeosaurids come from the Early Cretaceous. Curiously, they are the only Cretaceous flagellicaudatan diplodocoids in the world, together with fragmentary records from South Africa, since the Jurassic–Cretaceous boundary marks a global extinction event for numerous species within the group. All these occurrences come from the only two countries of Patagonia: Argentina and Chile. The currently rich record of South American flagellicaudatans demonstrates that they were a key component of the Late Jurassic to the earliest Cretaceous sauropod fauna, the Bajadan tetrapod assemblage, occupying the niches of narrow-crowned megaherbivores by a time when macronarian neosauropods only attained broad-crown forms.