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

Abstract

The molecular biology success, which became influential for a revision of principal views on the relationships and evolutionary pathways of major groups ("phyla") of multicellular animals, were much more appreciated by palaeontologists rather than by neontologists. This is not surprising because this is the fossil record that serves as a firm test for molecular biological hypotheses. The fossil record indicates that the different "phyla" united nowadays into the Ecdysozoa, namely, arthropods, onychophorans, tardigrades, priapulids, nematomorphs, comprise a number of transitional forms having become extinct already in the early Palaeozoic. The very appearance of those organisms fits entirely to hypothetical ancestral forms which have to exist in accordance with some data on ontogeny. There are no even tentative intermediates between arthropods and annelids in the fossil record. The oldest Deuterostomia, which is the only branch of the Bilateria being agreed upon by all the biological disciplines, allow us, although in less details, to present their early history and point to agile bilaterally-symmetrical forms at the dawn of chordates, hemichordates, and echinoderms. The interpretation of the early history of the Lophotrochozoa is even more difficult because in comparison to other bilaterians, their oldest fossils are preserved as mineralized skeletons only. However, a unity of microstructures of molluscs, brachiopods, and bryozoans, absent in other metazoans, is indicative of the presence of close relatives among different earliest lophotrochozoans some of which are sedentary filterers while others are motile epibenthic detritophages. In the aggregate, modern data of molecular biology, palaeontology, and comparative embryology/morphology, having got a second wind with an introduction of new microscopy techniques, imply that the suggestion of a planktotrophic gastraea-like common ancestor is the least possible among diverse suggestions on the Metazoa origins. 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 (c. a billion years) assembly of the metazoan genome within unicellular and colonial common ancestors of the Opisthokonta and even the Unikonta as a whole.