I wonder is 'rigido rostro' here in dative or ablative? Under "Dative and verbs compounded with prepositions" (Gildersleeve & Lodge) it is said, that " Many verbs compounded with the prepositions ad, ante, con, in, inter, ob, (post), prae, sub, and super, take the Dative, especially in moral relations.
The Dative is found, as a rule, only when these verbs are used in transferred sense. In poetry and later prose the Dative is extended even to the local signification."
..., alteraque alterius rigido concrescere rostro ora videt ...<
Ovid Metamorphoses V 673
each saw another's face stiffening into a hard beak,..(F.J. Miller)
Altera (each; f.nom.) alterius ( of other, f.gen) rigido (stiff,hard adj. dat?) concrescere (to grow together, stiffen harden, inf.) rostro (rostrum - a beak;n.dat.?) ora (os - the mouth; n.acc.) videt (pres.ind.act.)
Or could it be ablative after all?