Phylochronology of early metazoans: combined evidence from molecular and fossil data

Abstract

AbstractTiming the evolutionary appearances of early Metazoa on Earth may rely on the combined information of fossil occurrences, molecular divergences and the geological background. Recent advances in the studies of Neoproterozoic and earliest Cambrian strata in South China and elsewhere, together with numerous works on molecular dating, provide an opportunity to draw a more precise timescale for the early metazoan evolution. This study presents a synthesis of available molecular and fossil dating results, placed in the palaeoenvironmental background, which leads to the recognition of the following time events for early metazoan cladogenesis: (1) Animal multicellularity occurred most probably during early to middle Neoproterozoic times (between ∼1000 and ∼660 Ma) during the geological period of gradual increase in oxygen content; (2) Eumetazoans and sponges (or sponge‐like organisms) parted from each other during the Neoproterozoic glaciation interval (ca. 750–580 Ma), but both experienced adaptive radiation after the glaciations as evidenced by the Doushantuo Biota (ca. 635–551 Ma); (3) A great metazoan radiation event (MRE) occurred during the late Ediacaran and earliest Cambrian periods (ca. 550–530 Ma), interpreted here as a metazoan radiation at higher taxonomic levels, as compared with the early Cambrian radiation representing one at lower taxonomic levels (lower than subphylum or class). In order to better constrain the phylochronology of early metazoans, we present evidence for the divergences of arthropod subgroups on the basis of molecular dating results and the fossil record. It is shown that the arthropod stem group diverged from its sister taxon at between ∼700 and ∼565 Ma and major arthropod subgroups originated coevally with MRE. The MRE is probably correlatable with the δ13C negative anomaly in the late Ediacaran Period.Although the phylochronology of early Metazoa is gradually emerging, a great deal of research still will be needed in the following directions in order to reduce uncertainties in, and discrepancies among, dating results: (1) Evaluate different molecular dating techniques so as to establish a proper set of standard procedures for dating early metazoan events; (2) Set up a standard set of fossil calibration points suitable for dating early metazoan divergences; (3) Use both molecular and fossil constraints as far as possible; (4) Place the time sequence in the geological and palaeoecological background so that one can visualize a logical evolutionary scenario. We suggest that the fossil record plus the geological background should serve to construct a general timeframe for evolution, while molecular dating will supplement and probably increase the precision of the chronology. Copyright © 2007 John Wiley & Sons, Ltd.