Insects exhibit remarkable morphological extremes, including elongate appendages and prominent outgrowths. However, exaggerated morphologies in insects are exceptionally rare in the fossil record, leaving their origins and functional significance largely unexplored. Here, we describe a new short-winged flower beetle (Kateretidae) from mid-Cretaceous amber (ca. 99 Ma), Protokateretes ensifer Zhao, Engel & Cai, sp. nov., characterized by an extremely elongate and highly modified antennal scape in males, a striking morphological specialization unparalleled in extant insects. Our morphological disparity analysis demonstrates the diagnostic reliability of the scape for species differentiation and provides evidence of surprising negative allometry in antennal scape size. We show that the exaggerated scape likely functioned as claspers for male positioning on females or as a female receptor-interacting signal. Abundant aggregations of Asteropollis pollen grains preserved with Protokateretes ensifer suggest a putatively beetle-mediated pollination of chloranthaceous angiosperms during late Mesozoic. Together with previous findings of Cretaceous kateretids and their associated angiosperm hosts, our discovery highlights that diverse flowering plant lineages were likely pollinated by beetles during the mid-Cretaceous angiosperm radiation. This study sheds light on the functional significance of exaggerated morphological traits and illuminates possible early beetle-angiosperm coevolution that predates the dominance of butterflies and bees.