Uncertainty in the Timing of Origin of Animals and the Limits of Precision in Molecular Timescales
Authors:
dos Reis et al
Abstract:
The timing of divergences among metazoan lineages is integral to understanding the processes of animal evolution, placing the biological events of species divergences into the correct geological timeframe. Recent fossil discoveries and molecular clock dating studies have suggested a divergence of bilaterian phyla greater than 100 million years before the Cambrian, when the first definite crown-bilaterian fossils occur. Most previous molecular clock dating studies, however, have suffered from limited data and biases in methodologies, and virtually all have failed to acknowledge the large uncertainties associated with the fossil record of early animals, leading to inconsistent estimates among studies. Here we use an unprecedented amount of molecular data, combined with four fossil calibration strategies (reflecting disparate and controversial interpretations of the metazoan fossil record) to obtain Bayesian estimates of metazoan divergence times. Our results indicate that the uncertain nature of ancient fossils and violations of the molecular clock impose a limit on the precision that can be achieved in estimates of ancient molecular timescales. For example, although we can assert that crown Metazoa originated during the Cryogenian (with most crown-bilaterian phyla diversifying during the Ediacaran), it is not possible with current data to pinpoint the divergence events with sufficient accuracy to test for correlations between geological and biological events in the history of animals. Although a Cryogenian origin of crown Metazoa agrees with current geological interpretations, the divergence dates of the bilaterians remain controversial. Thus, attempts to build evolutionary narratives of early animal evolution based on molecular clock timescales appear to be premature.
Taphonomy of the Ediacaran Podolimirus and associated dipleurozoans from the Vendian of Ukraine
Authors:
Dzik et al
Abstract:
Fossiliferous strata of the late Ediacaran Lomoziv Member of the Mohyliv Formation, once known from now flooded outcrops along the Dniester River, have been recently exposed in a large quarry near the Novodnistrovs’k electric plant dam in Podolia, Ukraine. Finely bedded arkosic sandstone with claystone/siltstone intercalations is locally rich in ‘elephant skin’ surfaces indicating original presence of microbial mats. Imprints of soft-bodied organisms frequently occur on the sole surfaces of the sandstones. There are no signs of early diagenetic cementation with iron sulfides and the fossils are usually strongly compacted, with low relief. Specimens of bilaterally symmetrical latest Precambrian animals newly collected at the quarry offer additional evidence that their bodies had a complex internal anatomy. This relates especially to Podolimirus mirus, previously known from only fragmentary specimens representing only marginal parts of its chambered organ (‘quilt’). It appears that there is a large region of the body in front of the ‘quilt’, with a lobate organ resembling bifurcating anterior structures earlier thought to represent intestinal diverticula in other dickinsoniids. The ‘quilt’ (presumably a dorsal muscular organ) of Podolimirus has a deep medial sinus that may have hosted a cylindrical axial organ analogous to that reported in the dickinsoniids from northern Russia and Australia.
A New Enigmatic, Tubular Organism from the Ediacara Member, Rawnsley Quartzite, South Australia
Authors:
Joel et al
Abstract:
Here we reconstruct a new tubular, serially divided organism with a bilateral morphology from the Ediacaran of South Australia. The organism, Plexus ricei new genus new species, was a broadly curving tube that resided on the Ediacaran seafloor. Plexus ricei individuals range in size from 5 to 80 cm long and 5 to 20 mm wide, and are comprised of two main components: a rigid median tubular structure and a fragile outer tubular wall. Plexus ricei is preserved as an external mold on bed soles, and as a counterpart cast on bed tops in sandstones interpreted to represent deposition between storm and fairweather wave-base. The phylogenetic affinities of P. ricei are uncertain; P. ricei symmetry implies a bilaterian origin, but a lack of defined anterior and posterior ends precludes definitive assignment.
Patterns of Evolution of the Ediacaran Soft-Bodied Biota
Author:
Grazhdankin
Abstract:
When each of the Avalon-, Ediacara-, and Nama-type fossil assemblages are tracked through geological time, there appear to be changes in species composition and diversity, almost synchronized between different sedimentary environments, allowing a subdivision of the late Ediacaran into the Redkinian, Belomorian and Kotlinian geological time intervals. The Redkinian (580–559 Ma) is characterized by first appearance of both eumetazoan traces and macroscopic organisms (frondomorphs and vendobionts) in a form of Avalon-type communities in the inner shelf environment, whereas coeval Ediacara-type communities remained depauperate. The Belomorian (559–550 Ma) is marked by the advent of eumetazoan burrowing activity in the inner shelf, diversification of frondomorphs, migration of vendobionts from the inner shelf into higher energy environments, and appearance of tribrachiomorphs and bilateralomorphs. Ediacaran organisms formed distinctive ecological associations that coexisted in the low-energy inner shelf (Avalon-type communities), in the wave- and current-agitated shoreface (Ediacara-type communities), and in the high-energy distributary systems (Nama-type communities). The Kotlinian (550–540 Ma) witnessed an expansion of the burrowing activity into wave- and current-agitated shoreface, disappearance of vendobionts, tribrachiomorphs and bilateralomorphs in wave- and current-agitated shoreface, together with a drop in frondomorph diversity. High-energy distributary channel systems of prodeltas served as refugia for Nama-type communities that survived until the end of the Ediacaran and disappeared when the burrowing activity reached high-energy environments. This pattern is interpreted as an expression of ecosystem engineering by eumetazoans, with the Ediacaran organisms being progressively outcompeted by bilaterians.
