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The taxonomy of synurophytes, which relies extensively on the morphology of the siliceous components, is largely congruent with molecular analyses. Scales of extinct synurophytes were significantly larger than those of modern taxa and may have played a role in their demise. In contrast, many fossil species linked to modern lineages were smaller in the middle Eocene, possibly reflecting growth in the greenhouse climatic state that characterized this geologic interval.

Despite the fact that the genus A~~lncoseirn Thwaites is a common component of phytoplankton communities in many North American water bodies, there are relatively few taxonomic based surveys utilizing the scanning electron microscope (SEM). In addition, based on the current literature, some species are difficult to identify or distinguish from other taxa. We present taxonomic and morphological observations on 19 taxa based primarily on collections made in Connecticut, U.S.A., and from central and northern Canada. Arllaco.seirn nttlbigrln (Grunow) Simonsen, A~rkc~coseit-el srlbarctica ( 0 . Miiller) Haworth, and Aulr~coseirn tet~elln (Nygaard) Simonsen were the most common species encountered in Connecticut, while Arllnco.seira islntlclica ( 0 . Miiller) Simonsen, Arllncoseirn grc~tzr~lnta (Ehrenberg) Simonsen, A. subarcticel, and A. clmbigrla were the dominant species from many central Canada localities. In the Canadian Shield region, specimens of the Aulncoseirn cli,stc~t~.s (Ehrenberg) Simonsen group, thc A~ilncoseirn lirnta (Ehrenberg) Ross in Hartley group, and Aulncoseirn perglabra (Oestrup) Haworth were more common. We had the most difficulty identifying taxa in the A. clistnns and A. pc7rglnbrc1 complexes and distinguishing A. lirnta from Aulncoseirn nlpigetla (Grunow) Krammer. The primary objectives of the paper were to provide an additional source of high-resolution SEM micrographs for many of the more common species of Aulncoseirc~, comment on the morphological detail of each taxon, and to discuss the difficulties we encountered using the current literature to identify some of the species. A few ecological observations are also provided. Lastly, we propose two new combinations, Aulncoseira p.seudontt~ericc~na (Camburn) comb.nov. and Arllncoseirn clistarls var. tzivaloides (Camburn) comb.nov. RCsurnC: Bien que le genre Ar~lncoseirr~ Thwaites soit une composante commune des communautCs de phytoplancton dans plusieurs milieux aquatiques de I'AmCrique du Nord, il existe peu d'Ctudes basks sur la taxonomie qui utilisent la microscopic electronique par balayage (SEM). De plus, sur la base de la IittCrature courante, certaines espkces sont difficiles B identifier ou B distinguer de d'autres taxons. Les auteurs prkscntent les observations morphologiques ct taxonomiques de 19 taxons, b a s k s prioritairement sur des collections cffectuCes au Connecticut, U.S.A., et au centre ainsi qu'au nord du Canada. Les Au/c~co.seirc~ ntt~bigrm (Grunow) Simonsen, A~~le~coseirc~ sllbarcrica ( 0 . Miiller) Haworth et Arrlacoseirn tetlellc~ (Nygaard) Simonsen sont les espkces lcs plus friquentes au Connecticut, alors quc Ics Arrlacoseirn islatrclicc~ ( 0 . Miiller) Simonsen, A~llr~coseirn gmtlrtlnm (Ehrenberg) Simonsen, A. srlbarcticrr ct A. nttrbigrla sont les espkces doniinantes dans plusieurs IocalitCs du Canada central. Dans la rCgion du bouclier canadien, les specimens des groupes Aulc~coseira clistatls (Ehrenberg) Simonsen et Aulacoseirc~ lirnta (Ehrenberg) Ross in Hartley group, et du A~llncoseirn perglnbrn (Oestrup) Haw...

The global distribution, abundance, and diversity of microscopic freshwater algae demonstrate an ability to overcome significant barriers such as dry land and oceans by exploiting a range of biotic and abiotic colonization vectors. If these vectors are considered unlimited and colonization occurs in proportion to population size, then globally ubiquitous distributions are predicted to arise. This model contrasts with observations that many freshwater microalgal taxa possess true biogeographies. Here, using a concatenated multigene data set, we study the phylogeography of the freshwater heterokont alga Synura petersenii sensu lato. Our results suggest that this Synura morphotaxon contains both cosmopolitan and regionally endemic cryptic species, co-occurring in some cases, and masked by a common ultrastructural morphology. Phylogenies based on both proteins (seven protein-coding plastid and mitochondrial genes) and DNA (nine genes including ITS and 18S rDNA) reveal pronounced biogeographic delineations within phylotypes of this cryptic species complex while retaining one clade that is globally distributed. Relaxed molecular clock calculations, constrained by fossil records, suggest that the genus Synura is considerably older than currently proposed. The availability of tectonically relevant geological time (10⁷-10⁸ years) has enabled the development of the observed, complex biogeographic patterns. Our comprehensive analysis of freshwater algal biogeography suggests that neither ubiquity nor endemism wholly explains global patterns of microbial eukaryote distribution and that processes of dispersal remain poorly understood.

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