This is the final version of the article. Available from Elsevier via the DOI in this record.In the decade since the first pan-European testate amoeba-based transfer function for peatland palaeohydrological reconstruction was published, a vast amount of additional data collection has been undertaken by the research community. Here, we expand the pan-European dataset from 128 to 1799 samples, spanning 35° of latitude and 55° of longitude. After the development of a new taxonomic scheme to permit compilation of data from a wide range of contributors and the removal of samples with high pH values, we developed ecological transfer functions using a range of model types and a dataset of ∼1300 samples. We rigorously tested the efficacy of these models using both statistical validation and independent test sets with associated instrumental data. Model performance measured by statistical indicators was comparable to other published models. Comparison to test sets showed that taxonomic resolution did not impair model performance and that the new pan-European model can therefore be used as an effective tool for palaeohydrological reconstruction. Our results question the efficacy of relying on statistical validation of transfer functions alone and support a multi-faceted approach to the assessment of new models. We substantiated recent advice that model outputs should be standardised and presented as residual values in order to focus interpretation on secure directional shifts, avoiding potentially inaccurate conclusions relating to specific water-table depths. The extent and diversity of the dataset highlighted that, at the taxonomic resolution applied, a majority of taxa had broad geographic distributions, though some morphotypes appeared to have restricted ranges.GTS was supported by the Worldwide University Network (‘Arctic Environments, Vulnerabilities and Opportunities’) and a Department of Employment and Learning (Northern Ireland) PhD studentship. ET was supported by UK NERC-funded Doctoral Training Grant NE/G52398X/1. ML was supported by EU grant PSPB-013/2010 (CLIMPEAT, www.climpeat.pl) and grant 2015/17/B/ST10/01656 from the National Science Center (Poland). RJP was supported by the Leverhulme Trust (RPG 2015-162). RJP and YM were supported by the Russian Science Foundation (grant 14-14-00891 for field work and grant 14-50-00029 for taxonomic analysis). AB was supported by the RFBR Scientific Fund (grants 16-04-00451А and 15-29-02518) and by the German Ministry of Education and Research (CARBOPERM-Project, BMBF Grant No. 03G0836). EM was supported by the European Community funded BERI project (Bog Ecosystem Research Initiative; ENV4-CT95–0028) and the Swiss Federal Office for Education and Science (95.0415). DJC was supported by the EU ACCROTELM project (EVK2-CT-2002-00166)

Background: The origin of animals from their unicellular ancestor was one of the most important events in evolutionary history, but the nature and the order of events leading up to the emergence of multicellular animals are still highly uncertain. The diversity and biology of unicellular relatives of animals have strongly informed our understanding of the transition from single-celled organisms to the multicellular Metazoa. Here, we analyze the cellular structures and complex life cycles of the novel unicellular holozoansPigoraptorandSyssomonas(Opisthokonta), and their implications for the origin of animals.Results: SyssomonasandPigoraptorare characterized by complex life cycles with a variety of cell types including flagellates, amoeboflagellates, amoeboid non-flagellar cells, and spherical cysts. The life cycles also include the formation of multicellular aggregations and syncytium-like structures, and an unusual diet for single-celled opisthokonts (partial cell fusion and joint sucking of a large eukaryotic prey), all of which provide new insights into the origin of multicellularity in Metazoa. Several existing models explaining the origin of multicellular animals have been put forward, but these data are interestingly consistent with one, the “synzoospore hypothesis.”Conclusions: The feeding modes of the ancestral metazoan may have been more complex than previously thought, including not only bacterial prey, but also larger eukaryotic cells and organic structures. The ability to feed on large eukaryotic prey could have been a powerful trigger in the formation and development of both aggregative (e.g., joint feeding, which also implies signaling) and clonal (e.g., hypertrophic growth followed by palintomy) multicellular stages that played important roles in the emergence of multicellular animals.