Those look to have a striking resemblance to Snow Leopards in their juvenile state, however this species is also known as Potamotrygon "sp" Xingu as it hails from that river, maybe it is found further a field so to speak?
Its also worth mentioning that Snow Leopards have multiple spines running down the tail as seen in leopoldi (P13) . If there is just one single row then they could be one of the orbignyi complex, there is a lot of work to be done regarding positive id on this species imo as the patterns are just too variable not to mention found throughout many South American countries.