The cambrian evolutionary ‘explosion’ recalibrated

The sudden appearance in the fossil record of the major animal phyla apparently records a phase of unparalleled, rapid evolution at the base of the Cambrian period, 545 Myr ago. This has become known as the Cambrian evolutionary ‘explosion’, and has fuelled speculation about unique evolutionary processes operating at that time. The acceptance of the palaeontological evidence as a true reflection of the evolutionary narrative has been criticised in two ways: from a reappraisal of the phylogenetic relationships of the early fossils, and from predicitions of molecular divergence times, based on six appropriate metazoan genes. Phylogenetic analysis of the arthropods implies an earlier, Precambrian history for most clades, and hence an extensive period of cladogenesis unrecorded by fossils. A similar argument can be applied to molluscs, lophophorates and deuterostomes. Molecular evidence implies divergence between clades to at least 1000 Myr ago. The apparent paradox between the sudden appearance of recognisable metazoans and their extended evolutionary history might be explained by a sudden Cambrian increase in body size, which was accompanied by skeletisation. A new paradigm suggests that the ‘explosion’ in the record may have been decoupled from the evolutionary innovation.

[1]  H. Gee.  Heads and Tails. (Book Reviews: Before the Backbone. Views on the Origin of the Vertebrates.) , 1996 .

[2]  B. Waggoner.  Phylogenetic Hypotheses of the Relationships of Arthropods to Precambrian and Cambrian Problematic Fossil Taxa , 1996 .

[3]  Sudhir Kumar,et al.  Continental breakup and the ordinal diversification of birds and mammals , 1996, Nature.

[4]  Kenneth S. Johnson.  Going to Sea , 1996, Science.

[5]  D. Shu,et al.  Reinterpretation of Yunnanozoon as the earliest known hemichordate , 1996, Nature.

[6]  T. Lacalli,et al.  DORSOVENTRAL AXIS INVERSION : A PHYLOGENETIC PERSPECTIVE , 1996 .

[7]  J. W. Valentine,et al.  Developmental evolution of metazoan bodyplans: the fossil evidence. , 1996, Developmental biology.

[8]  R. Doolittle,et al.  Determining Divergence Times of the Major Kingdoms of Living Organisms with a Protein Clock , 1996, Science.

[9]  E. Davidson,et al.  Origin of Bilaterian Body Plans: Evolution of Developmental Regulatory Mechanisms , 1995, Science.

[10]  J. Bacon,et al.  Arthropod evolution: great brains, beautiful bodies. , 1995, Trends in ecology & evolution.

[11]  A. J. Kaufman,et al.  Biostratigraphic and Geochronologic Constraints on Early Animal Evolution , 1995, Science.

[12]  G. Edgecombe,et al.  A possible Early Cambrian chordate , 1995, Nature.

[13]  D. Tautz,et al.  Ribosomal DNA phylogeny of the major extant arthropod classes and the evolution of myriapods , 1995, Nature.

[14]  C. Nielsen.  Animal Evolution: Interrelationships of the Living Phyla , 1995 .

[15]  X. Hou,et al.  CAMBRIAN LOBOPODIANS-ANCESTORS OF EXTANT ONYCHOPHORANS? , 1995 .

[16]  S. Morris,et al.  Articulated Halkieriids from the Lower Cambrian of North Greenland and their Role in Early Protostome Evolution , 1995 .

[17]  M. Wills,et al.  Disparity as an evolutionary index: a comparison of Cambrian and Recent arthropods , 1994, Paleobiology.

[18]  G. Budd.  A Cambrian gilled lobopod from Greenland , 1993, Nature.

[19]  W. Wheeler,et al.  ARTHROPOD PHYLOGENY: A COMBINED APPROACH , 1993, Cladistics : the international journal of the Willi Hennig Society.

[20]  J. Laurie,et al.  Phylogenetic analysis of higher taxa of Brachiopoda , 1993 .

[21]  S. Morris.  Burgess Shale-type faunas in the context of the ‘Cambrian explosion’: a review , 1992 .

[22]  Katharine,et al.  The phylogenetic status of arthropods, as inferred from 18S rRNA sequences. , 1991, Molecular biology and evolution.

[23]  Kenneth J. McNamara,et al.  Heterochrony: The Evolution of Ontogeny , 1991 .

[24]  M. McMenamin,et al.  The Emergence of Animals the Cambrian Breakthrough , 1990 .

[25]  P. Willmer.  Invertebrate Relationships: Patterns in Animal Evolution , 1990 .

[26]  P. Willmer.  Invertebrate Relationships: Frontmatter , 1990 .

[27]  R. Fortey,et al.  The Early Radiation and Relationships of the Major Arthropod Groups , 1989, Science.

[28]  P. Boaden.  Meiofauna and the origins of the Metazoa , 1989 .

[29]  G. Boxshall,et al.  Kabatarina pattersoni; a fossil parasitic copepod (Dichelesthiidae) from a Lower Cretaceous fish , 1989 .

[30]  R. Raff,et al.  Molecular phylogeny of the animal kingdom. , 1988, Science.

[31]  K. Porter,et al.  Pellet microfossils: Possible evidence for metazoan life in Early Proterozoic time. , 1985, Proceedings of the National Academy of Sciences of the United States of America.

[32]  B. Runnegar.  A molecular‐clock date for the origin of the animal phyla , 1982 .

[33]  R. A. Dewel,et al.  The brain of Echiniscus viridissimus Peterfi, 1956 (Heterotardigrada): a key to understanding the phylogenetic position of tardigrades and the evolution of the arthropod head , 1996 .

[34]  I. Dalziel.  Earth before Pangea , 1995 .

[35]  Ian M. Kinchin,et al.  The biology of tardigrades , 1994 .

[36]  A. Hallam.  An Outline Of Phanerozoic Biogeography , 1994 .

[37]  L. Buss,et al.  The Phylum Vendobionta: a sister group of the Eumetazoa? , 1994, Paleobiology.

[38]  J. W. Valentine,et al.  The biological explosion at the precambrian-cambrian boundary , 1991 .

[39]  J. Dzik,et al.  The oldest arthropods of the East European Platform , 1988 .

[40]  J. Gehling.  Earliest known echinoderm — a new Ediacaran fossil from the Pound Subgroup of South Australia , 1987 .

[41]  K. Müller,et al.  A remarkable arthropod fauna from the Upper Cambrian “Orsten” of Sweden , 1985, Earth and Environmental Science Transactions of the Royal Society of Edinburgh.