Sarotrocercus is a small Cambrian organism known from Burgess shale

Burgess Shale

The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...

-type localities, reaching a centimetre or two in length. It was pelagic and swam on its back.

Morphology

It had a head shield followed by a body of nine segments and a caudal tip featuring a series of spines on the end. A pair of big eyes at the end of stalks emerged from the front of the shield. The head bore a pair of sturdy appendages that ended in a bidentate segment. Ten pairs of appendages with an elongated structure of comb-shaped, probably gill branches, are attached to the at the head segment (one pair) and the body segments (nine pairs).

Ecology

This small animal was a creature that swam freely on its back, moving perhaps through movements of the gills and the action of its the long tail tuft. The Burgess Shale contains few swimming organisms; the submarine landslides that buried organisms mainly smothered benthic and nektobenthic organisms. Organisms living in the water column above this habitat were able to escape and above the 'danger zone' and are therefore much rarer. Among these are the notable Amiskwia

Amiskwia

Amiskwia is a large, soft-bodied invertebrate of unknown affinity known from fossils of the Middle Cambrian Lagerstätten both in the Burgess shale formation in British Columbia and the Maotianshan shales of Yunnan Province, China....

, Nectocaris

Nectocaris

Nectocaris pteryx is a species of possible cephalopod or arthropod affinity, known from the Middle Cambrian Burgess Shale.Nectocaris was a free-swimming, predatory or scavenging organism, possibly occupying a niche similar to the arrow worms...

and Anomalocaris

Anomalocaris

Anomalocaris is an extinct genus of anomalocaridid, which are, in turn, thought to be closely related to the arthropods. The first fossils of Anomalocaris were discovered in the Ogygopsis Shale by Joseph Frederick Whiteaves, with more examples found by Charles Doolittle Walcott in the famed...

.