|
G.
Palyi, C. Zucchi, & L. Caglioti (eds) Advances
in Biochirality 153-190.
Elsevier Science S.A. (1999) 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). |