Considering taxonomic diversity, extant xyelid sawflies represent a very small subset of the past diversity of the group. A complementary approach to investigate evolutionary changes in this family is to consider ovipositor morphology, an ecologically important organ. New, well-preserved specimens from the UpperTriassic Ominé locality (Momonoki Formation, Mine Group; Carnian; Japan) and belonging to the †Madygellinae, one of the earliest Xyelidae subfamilies, are newly described and assigned to four new species, belonging to three genera (one being new).Two of the specimens expose body parts and wings, providing important information regarding ovipositor morphology. To further appreciate the evolution of this organ, we surveyed both fossil and extant Xyelidae. Extant forms display both extremes of the range of ovipositor types, namely the high aspect ratio “dagger-like” and “needle-like” types (in Xyelinae) and the low aspect ratio “short and wide” type (in Macroxyelinae). All known Triassic representatives display high aspect ratio ovipositor types. In contrast, Middle Jurassic to Lower Cretaceous sawflies show the highest diversity of ovipositor types for the family. Afterwards, the observed range declined and concentrated on the two, very distinct types known nowadays. Decline in ovipositor disparity was not contemporaneous in Xyelinae and Macroxyelinae: specialization occurred during the Jurassic-Cretaceous transition for the Xyelinae, while Macroxyelinae displayed their peak of diversity during the Lower Cretaceous, with a postponed decline. Ovipositor shape transition was probably accompanied with a larval diet change in Macroxyelinae. Ovipositor disparity therefore further contributes depicting extant Xyelidae as a subset on a once more diverse group.