Amargasaurus

Amargasaurus cazaui was among the last of the dicraeosaurids. It lived in what is now Argentina. The remains of A. cazaui were found in the lowest, Puestal Antigual member of the La Amarga formation. This unit is thought to be Barremian in age c 123 Ma. Amargasaurus was a rather small sauropod, about 9-10m in length. It shared its habitat with theropods, the bigger dicraeosaur Amargatitanis, the rebbachisarid Zapalasaurus, unnamed titanosaurs and the only ornithiscian known from the La Amarga, a stegosaur informally named Amargastegos.
Amargasaurus is best known for its spiny neck, a feature unique among sauropods. The spines may have evolved to attract mates, or as a means of defense. An abelisaur, Ligabueino, lived alongside A. cazaui as did another theropod, possibly a carcharodontosaur.
Like a stegosaur's plates, cervical spines made a defender appear more formidable. No doubt, theropods often targeted the neck of a sauropod, so spines may have deterred such attacks. They may  even have been used as horns, to impale predators. By lowering its head, Amargasaurus could point its spines at an attacker. But the uniqueness of this defense, and lack of any later dicraeosaur, suggests it was not particularly effective.