The functional morphology of mating in the Silurian eurypterid, Baltoeurypterus tetragonophthalmus (Fischer, 1839)

The functional morphology of the genital appendages, horn organs and modified prosomal limb III of the Silurian eurypterid, Baltoeurypterus tetragonophthalmus (Fischer, 1839) (Chelicerata: Eurypterida) is investigated. The longer type A genital appendage, interpreted as a female structure, is associated with a pair of horn organs, regarded as spermathecae. The shorter type B appendage, interpreted as male, is associated with scimitar lobes on prosomal limb III, regarded as clasping structures. A mechanism is proposed by which the type A appendage was lowered through flexure of the genital operculum acting along sutures in the cuticle. A model of mating is suggested, involving the male depositing an unstalked spermatophore on the substrate and the female retrieving it for storage in the spermathecae. This model is more arachnid-like than mating in Limutus (an extant aquatic chclicerate), and suggests that sperm transfer using spermatophores and storage of sperm in spermathecae, allowed the timing of egg production in eurypterids to be controlled.