A second possible species "S." sundbergi is known from the Kaili Formation of China,[1] but its placement within the genus has been questioned.

[2] Specimens of S. fragillis range in length from 7.5 to 13.5 millimetres (0.30 to 0.53 in) The cordiform dorsal shield/carapace covers the entire body, with its midline having a raised keel.

In contrast, the trunk appendages are shorter (not extending beyond the shield) and are less curved, and the segments are not preserved.

There was a diverticular (digestive) network running through the carapace, along with a central gut tract.

[1][2] While previously enigmatic, it is now thought to be a marrellomorph, with both species, regardless of generic placement, belonging to Acercostraca.