Reappraisal of the Early Cretaceous sauropod dinosaur Amargatitanis macni (Apesteguía, 2007), from northwestern Patagonia, Argentina

• Amargatitanis macni is a valid taxon of sauropod dinosaur from the Early Cretaceous.
• Amargatitanis macni is reassigned as a new member of the family Dicraeosauridae.
• Unpublished material enhances the knowledge on the anatomy of Amargatitanis.

Amargatitanis macni Apesteguía, 2007 was described as a purported titanosaur sauropod. That referral is significant because Amargatitanis would represent one of the oldest known members of Titanosauria. However, this referral and even the systematic validity of the taxon were questioned. Here, all the available remains of the taxon are evaluated including a description of unpublished elements. The identity of the type material is discussed based on a first-hand examination of the specimens and evaluation of the original field notes as reliable evidence for bone association. The original holotype of Amargatitanis is a chimaera, as pointed out by previous authors. Herein a new, modified holotype for Amargatitanis is proposed. This analysis presents a revised diagnosis for Amargatitanis macni as a valid taxon, and description of unpublished material clarifies the anatomy of this sauropod dinosaur. The inclusion of Amargatitanis macni in an updated phylogenetic analysis revealed the presence of a second species of the family Dicraeosauridae in the La Amarga Formation, suggesting that, at present, there is no record of titanosaur body fossils in Patagonia prior to Cenomanian times.