Cambrian ‘Orsten’-type arthropods and the phylogeny of Crustacea

Abstract

Orsten-type preservation refers to the finds of three-dimensional, phosphatized Cam- brian microscopic arthropod fossils with soft parts, most of which specimens are smaller than one millimeter. Such unique material permits the reconstruction of the early phylog- eny of the Crustacea and thereby the recognition of progressive modification of the ceph- alic locomotory and feeding system as one of the major evolutionary strategies of this group. Orsten fossils also help to re-evaluate traditional hypotheses of arthropod phy- logeny. Since the record of Orsten-type fossils assignable to the late stem lineage of the Eucrustacea ranges down to the Early Cambrian, it is proposed that the stem species of Crustacea existed in the late Pre-Cambrian and therefore that all prior branchings down the metazoan lineage had occurred even earlier. The record of exceptionally preserved Cambrian fossils is still scattered and limited to a , more or less, exclusively of the Chengjiang, Burgess-shale or Orsten type. few We predict that the most promising fossils to hunt for in the future to improve our still fragmentary knowledge of the early phylogeny of Arthropoda and, particularly, of the evolutionary paths to the major arthropod groups with living derivatives, Chelicerata, Atelocerata and Crustacea, are those of the Orsten-type of preservation