No one knows (at this time); whatever they used may not have survived all this time. Seals could have been speared on the beach. Dolphins would require hook/line, nets, or boats and spears. Could also be the general friendliness of the dolphin was exploited, in which case any hand-held blade might have done the trick.
“No one knows...”
From where I stand watching N theories come and go over some 60 years, I think we’ve got the Ns all wrong. But what “right” is, I don’t know - just that what we have now is not BS, but something like that.