Cambrian phosphatized fossils from southern China and their bearing on early metazoan evolution

Abstract

Richly fossiliferous nodular and thin-bedded limestones of Lower and Middle Cambrian age are widespread in southern China, and contain abundant, diverse, and exceptionally well preserved fossils. By acid digestion of the limestones, numerous phosphatized shells displaying detailed three-dimensional characters have been isolated. The univalved ostracode larvae, displaying a univalved-bivalved transition, argue that univalved form may be a primitive larval character for shell-secreting crustaceans. The arthropod (possibly eodiscid trilobite) embryos with preserved blastomeres belong to two embryonic stages; the basic embryonic development of arthropods seems unchangeable with time. Cuticles of three palaeoscolecid worms display unique ornaments and body structures; their discovery reveals that wormlike organisms have achieved a high taxonomic diversity and a wide geographic distribution during the Early Cambrian. The description of redlichiid trilobite larvae and ontogenetic stages of Ichangia ichangensis Chang is of potential interests in recognizing the relationships among the oldest trilobite faunas. Morphological and functional analysis on especially long genal and axial spines of some rodiscids provodes key evidence to deduce their locomotion and life mode. Early instars of bradoriid ostracodes and eodiscid trilobites exhibit apparent morphological diversity, which seems to contrast with the traditional view that regards early development as conservative. However, a complex silicified sponge spicule assemblage associated with the Early Cambrian phosphatized fauna in Zhenping demonstrates that their conservative styles of body architecture were established quickly in the earliest Phanerozoic. These fossil Lagerstatten, as representatives of early skeletal metazoans, have displayed their primitive features without 'modification' relevant to adaptive radiation's of later times. Their morphological and ontogenetic data are therefore essential in evaluating the rapid diversification of early skeletal organisms and the phylogenetic relationships among major metazoan groups.