G. Palyi, C. Zucchi, & L. Caglioti (eds) Advances in Biochirality 153-190. Elsevier Science S.A. (1999)
Evolutionary origin of asymmetry in early metazoan animals

Jerzy Dzik
Instytut Paleobiologii PAN, Twarda 51/55, 00-818 Warszawa, Poland. e-mail:

Abstract. Anatomical asymmetry developed in several Vendian Dipleurozoa, organisms which preceded the alleged basal Cambrian evolutionary radiation of the Metazoa. It was expressed in an alternate arrangement of the fluid-filled paired metameric chambers (Dickinsonia) or in lateral bending of the anteriormost unpaired chamber (Vendia, Valdainia, Marywadea, Spriggina, Praecambridium). Both sinistral and dextral asymmetry in trochoidal conch coiling developed in the basal Cambrian within the same clade of earliest gastropods, there was thus no preference of polarity in asymmetry. Subsequent extinction of the antistrophic mimospirine gastropods was not necessarily related to the direction of coiling of their conches. Strongly asymmetric were the Early Cambrian helicoplacoids, the oldest echinoderms represented by articulated fossil specimens. They probably departed from the main lineage of the earliest echinoderms at the stage preceding the evolutionary development of pentaradial symmetry of the ambulacra, as did the Middle Cambrian virtually bilaterally symmetrical ctenocystoids. Another kind of asymmetry characterises the carpoids, echinoderms ranging from the late Early Cambrian to the Devonian, frequently referred to as ‘calcichordates’, allegedly ancestral to the chordates. There is no evidence that chordates truly inherited their anatomical asymmetry after the echinoderms and the oldest Early Cambrian chordates (Yunnanozoon) show strictly bilateral symmetry. A special case of the evolution of asymmetry was the transformation of the mouth apparatus elements in conodont chordates, leading to their strictly axial symmetry in pairs. Evolution of behaviour, resulting in construction of asymmetric colony skeleton structures, is well documented in several lineages of the Silurian pelagic hemichordates (graptolites).