Final answer:
The discovery of juvenile Rioarribasaurus bones inside adult specimens indicates cannibalism instead of post-depositional mixing or live birth.
Step-by-step explanation:
The presence of juvenile Rioarribasaurus bones inside the abdomens of adult Rioarribasaurus indicates cannibalism. This observation suggests that the adults may have preyed on the juveniles. This behavior has been noted in other prehistoric species as well and is often revealed through careful examination of fossil evidence. This activity differs from post-depositional mixing, which would imply the bones got mixed after the animals died due to geological processes, and it is not indicative of live birth, as in that case there would not be fully developed juvenile bones within adults.