• DOI:10.1134/S2079086415050084
• Corpus ID: 17791423

The early history of the metazoa—a paleontologist’s viewpoint

@article{Zhuravlev2015TheEH,
  title={The early history of the metazoa—a paleontologist’s viewpoint},
  author={Andrey Yu. Zhuravlev},
  journal={Biology Bulletin Reviews},
  year={2015},
  volume={5},
  pages={415-461}
}

• A. Zhuravlev
• Published 14 October 2015
• Geography, Environmental Science, Biology
• Biology Bulletin Reviews

Successful molecular biology, which led to the revision of fundamental views on the relationships and evolutionary pathways of major groups (“phyla”) of multicellular animals, has been much more appreciated by paleontologists than by zoologists. This is not surprising, because it is the fossil record that provides evidence for the hypotheses of molecular biology. The fossil record suggests that the different “phyla” now united in the Ecdysozoa, which comprises arthropods, onychophorans, tardigrades, priapulids, and nematomorphs, include a number of transitional forms that became extinct in the early Palaeozoic. The morphology of these organisms agrees entirely with that of the hypothetical ancestral forms reconstructed based on ontogenetic studies. No intermediates, even tentative ones, between arthropods and annelids are found in the fossil record. The study of the earliest Deuterostomia, the only branch of the Bilateria agreed on by all biological disciplines, gives insight into their early evolutionary history, suggesting the existence of motile bilaterally symmetrical forms at the dawn of chordates, hemichordates, and echinoderms. Interpretation of the early history of the Lophotrochozoa is even more difficult because, in contrast to other bilaterians, their oldest fossils are preserved only as mineralized skeletons. However, the unity of the microstructures of mollusks, brachiopods, and bryozoans, which is absent in other metazoans, is indicative of the presence of close relatives among the various earliest lophotrochozoans, some of which were sedentary suspension-feeders while others were mobile epibenthic detritophages. In the aggregate, modern data from molecular biology, palaeontology, and comparative embryology/morphology, having been revitalized by the introduction of new microscopy techniques, imply that the hypothesized planktotrophic gastrae-like common ancestor is the least likely of the diverse suggestions on the origins of the Metazoa. The common ancestor of the Bilateria had to be a motile epibenthic animal, and the explosive metazoan diversification embracing the Late Ediacaran–Early Cambrian interval (c. 40 Ma) was probably a real event, which was predated by a long (ca. a billion years) period of the assembly of the metazoan genome within the unicellular and colonial common ancestors of the Opisthokonta, and possibly even the entire Unikonta.