The significance of "disciples AND Peter" in Mark 16:7

At the end of the Gospel of Mark, the boy in the tomb tells the women to give a message to "[Jesus's] disciples AND Peter" (Mark 16:7). Nowhere else in Mark is any apostle "separated" from the word "disciples" and Peter, the "leader" of the disciples, is given second billing. What, if any, are the official church explanations for this unusual language?