Try contracting Moriah Moore at the Coastal Education and Marine Academy in Florida, USA. She works with elasmobranchs and may be able to put you in touch with Myliobatis researchers. Contact her at [email protected].
Have you considered using a DNA sequencing/barcode of life approach? I worked on a similar project for a while in Humboldt squid, and it seems like it might be a productive direction to consider (particularly for highly-digested samples).