Penion proavitus is an extinct species of marine snail or whelk, belonging to the true whelk family Austrosiphonidae.

[1][2][3][4][5] Penion proavitus fossils are the earliest known specimens of Penion siphon whelks.

[2][5] It is possible that fossils of Penion proavitus represent a stem lineage that was the common ancestor of Penion, Kelletia and Antarctoneptunea, instead of a species of Penion itself.

[4] Fossils of Penion proavitus are found in New Zealand's South Island.

[2] The species lived in the seas surrounding Zealandia between 56 and 66 million million years ago (Teurian stage).