Xenusion is a possible very primitive
arthropod/
onychophore known from two specimens found in glacial debris in Germany. Probably they originated in the Kalmarsund Sandstone of Southern Sweden (Jaeger and Martinsson 1966). They are probably late
Neoproterozoic. The specimens are not especially well preserved. The older specimen is 10cm or so in length with a narrow, weakly segmented body. A depression runs up the bottom on all but the rearmost segments. There is a slightly bulbous tail and each segment beyond that seems to have a single pair of tapering annulated legs similar to the modern
Onychophore, but without claws. Nine segments are present. There is a spine on each body bump and faint transverse parallel striations on the annulations on the legs. The legs of the foremost (?) segments are either missing or not preserved. The head(?) is missing or poorly preserved. If
Xenusion is an
arthropod/
onychophore it is one of the oldest currently known fossils of a mobile, modern animal.
Xenusion has been reinterpreted as a Vendian frond animal by Tarlo and a convincing drawing of that interpretation has been presented by McMenamin. If the creature is actually an onychophore, the original specimen would appear to be part of an onychophore about 20cm in length which is much larger than most modern
species.
The photograph in The Treatise of Invertebrate Paleontology Volume O looks more like the original interpretation. The presence of 'worm tubes' Skolithes in the Kalmarsund Sandstone would seem to make
Xenusion rather younger than most Vendian fossils. A likely onychophore, Aysheaia, is known from the slightly younger
Burgess Shale of British Columbia.
There is one species of Xenusion -- Xenusion Auerswaldae.
A picture of Xenusion can be found at http://www.ucmp.berkeley.edu/onychoph/onychophorafr.html
