Porosothyone

Porosothyone is an extinct sea cucumber genus from Australia’s Silurian period, containing only one species, Porosothyone picketti. It is considered the earliest known body fossil of the group.

Its fossils come from upper Silurian deposits in New South Wales’ Yass Basin, specifically the Pridoli-aged Elmside Formation (where the holotype was found) and the upper Ludlow-aged Black Bog Shale. The genus name combines the Latin porosus (“full of holes”)—referring to its highly perforated skin ossicles—with thyone, a common suffix for holothurian genera. The species name honors John Picket of the Geological Survey of New South Wales, who first identified the specimens.

The animal was cigar-shaped, reaching up to 40 mm long and 7 mm wide, with slightly narrower ends. Overlapping, highly perforated sieve-plate ossicles covered its body; these were wider than long, had broadly curved front edges, and lacked spires. A calcareous ring around its ventral mouth consisted of five radial and five interradial elements.

Among the oldest holothurians with articulated fossils, Porosothyone has been placed within the crown group Holothuria as a stem-group apodid in one analysis, but its exact relationships within the holothurian crown group remain unclear.