We have identified fossil cyclomedusid impressions from the Antelope Mountain Quartzite (AMQ) of the Yreka terrane (YT) of northern California which resemble previously described Ediacaran forms. The AMQ is a previously undated unit found in several thrust sheets of the YT; other units of the YT are considered to be Ordovician through Early Devonian in age. The fossils have been found only in red phyllitic argillite interbedded with the dominant quartzite; minor chert is also included in the unit. The collection includes some 200 well-preserved individuals from several localities within the unit. The disc-shaped impressions occur on bedding surfaces and are bounded by thin raised rims, inside of which are weakly developed concentric raised rings. The impressions range in diameter from 2-5 cm, most are about 4 cm. Cross sections reveal that they are not dewatering, or related compactional features.

The presence of cyclomedusids in the Antelope Mountain Quartzite suggests the previously undated AMQ is late Neoproterozoic in age. The presence of an Ediacaran-like biota in this accreted terrane also supports the interpretation of Dalziel (1991), who suggested that Australia and East Antarctica (AEA) may have been located west of Laurentia in 570 Ma/late Proterozoic (SWEAT Hypothesis). These YT sediments may have been deposited in an oceanic basin lying between AEA and Laurentia. Sediment provenance studies suggest that the YT was closely associated with the 580-469 Ma Trinity ophiolitic terrane (TT) during the early Paleozoic. The two terranes were juxtaposed about 400 Ma during the collision of the Central Metamorphic terrane, thrusting the YT over the TT to form the Yreka-Trinity composite terrane.