Southernmost Spiny Backs and Whiplash Tails: Flagellicaudatans from South America

Abstract

Flagellicaudatan diplodocoids include the two families Dicraeosauridaeand Diplodocidae. Although different in sizes and relative proportions (e.g. neuralarches height, neck length, tail length), they share several features, both cranial andpostcranial, that recover them as a monophyletic group in updated phylogenies. Therecord of the group in South America was particularly scarce during the twentiethcentury, but their number and taxonomical diversity noticeably increased in the lastdecade. Up to now, five dicraeosaurid taxa (Amargasaurus cazaui, Amargatitanismacni, Bajadasaurus pronuspinax, Brachytrachelopan mesai, and Pilmatueia faundezi)and one diplodocid (Leinkupal laticauda) were recognized. Additionally, twopresumably dicraeosaurid and three diplodocid records are known from fragmentarymaterials. Jurassic strata have provided both Brachytrachelopan and two of theindeterminate diplodocids, whereas the remaining five taxa, the third indeterminatediplodocid 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?Cretaceousboundary 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 demonstratesthat they were a key component of the Late Jurassic to the earliest Cretaceoussauropod fauna, the Bajadan tetrapod assemblage, occupying the niches of narrowcrownedmegaherbivores by a time when macronarian neosauropods only attainedbroad-crown forms.