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Background and aims - Diatoms began to inhabit freshwater by at least the Late Cretaceous, becoming well established by the early to middle Eocene. Aulacoseira, an important diatom in numerous ponds, lakes and rivers today, was one of the earliest known genera to colonize freshwater ecosystems. Members of this genus with characteristics familiar to those found on modern species became increasingly more abundant by the Eocene, and continued to thrive throughout the Miocene to the present. We describe a new species of Aulacoseira from an early to middle Eocene site near the Arctic Circle in northern Canada. Methods - Twelve samples taken from the Giraffe Pipe core were analysed in this study. Light and scanning electron microscopy were used to document morphological characters. Morphometric measurements were made from 200 specimens per sample (n = 1200), and used to investigate changes in valve size over time. Key results - The new species, Aulacoseira giraffensis, has valves with a length:width ratio close to 1, a hyaline valve face, straight mantle striae, a shallow ringleiste, branched linking spines, concave-convex complementarity on adjacent valve faces, and rimoportulae with simple papillae-like structure. The suite of characters, especially the highly branched spines, concave-convex valves and simple rimoportulae, is unique for this species. Large numbers of A. giraffensis specimens were found over a ten-metre section of the core, representing thousands of years. These high concentrations are indicative of abundant, bloomlike, growth. Conclusions - The locality represents one of the earliest known records of Aulacoseira dominating a freshwater community. Findings confirm that the morphological body plan for the genus was well established by the Eocene. Although findings indicate evolutionary stasis in morphological structure for A. giraffensis over a time scale of thousands of years, oscillations in valve morphometrics could potentially be used to trace changes in the environment of this ancient Arctic waterbody. [ABSTRACT FROM AUTHOR]

Abstract: Chrysophyte algae are common in the plankton of oligotrophic lakes and produce a rich microfossil record of siliceous cysts and scales. Paleolimnological investigations and phytoplankton records suggest that chrysophyte populations are increasing in a wide range of boreal and arctic lakes, ultimately representing one component of the limnological response to contemporary global changes. However, the exact mechanisms responsible for widespread increases of chrysophyte populations remain elusive. We hypothesize that recent increases in chrysophytes are related to rising pCO2 in lakes, in part because these algae lack carbon concentrating mechanisms and therefore rely on diffusive entry of CO2 to Rubisco during photosynthesis. We assessed the abundance of modern sediment chrysophyte microfossils in relation to summer CO2 relative saturation in 46 New England (USA) lakes, revealing significant positive relationships for both cysts and scales. These observations imply that correlations between chrysophytes and limnological conditions including low pH, oligotrophy, and elevated dissolved organic matter are ultimately underscored by the high pCO2 associated with these conditions. In lakes where chrysophyte populations have expanded over recent decades, we infer that increasingly heterotrophic conditions with respect to CO2 have stimulated production by these organisms. This linkage is supported by the remarkable abundance and diversity of chrysophytes from middle Eocene lake sediments, deposited under atmospheric CO2 concentrations significantly higher than present. The Eocene assemblages suggest that any chrysophyte-CO2 connection borne out of results from modern and sub-recent sediments also operated on evolutionary time scales, and thus the absence of carbon concentrating mechanisms appears to be an ancient feature within the group. Chrysophyte microfossils may potentially provide important insights concerning the temporal dynamics of carbon cycling in aquatic ecosystems. [Copyright &y& Elsevier]

Complex bristle types formed by species in the genus Mallomonas include those with helmet or lance-shaped apices. The ornamentation on each side of the helmet has been thought to be equivalent or symmetrical, whereas on a lance-shaped bristle an expanded portion folds over one side of the shaft to form an asymmetrical structure. We describe, for the first time, helmet bristles with a distinctly asymmetrical design, also formed by the folding of a siliceous membrane over one side of the helmet. We postulate that the asymmetrical helmet represents a structure that combines the formation of a symmetrical helmet and a lance-shaped design on the same bristle. Further, we report structurally similar asymmetrical helmet bristles, lance-shaped bristles and scales that are unambiguously assigned to Mallomonas asmundiae in Middle Eocene sediments from a maar lake in northern Canada, supporting the hypothesis that scale and bristle morphology in the Synurophyceae has undergone extensive prolonged evolutionary stasis. Given differences in scale morphology and the presence of asymmetrical helmet bristles, we transfer the North American endemic Mallomonas acaroides var. muskokana to the rank of species. Further, we formally describe Mallomonas dispar and M. lancea, fossil species with asymmetrical helmet bristles and lance-shaped bristles, respectively. The taxonomic and biogeographic significance of asymmetrical and lance-bearing bristles is discussed. [ABSTRACT FROM AUTHOR]

The evolutionary history of diatoms is only constrained partially by the fossil record. The timing of several key events, such as initial colonization of freshwater habitats by marine taxa, remains poorly resolved. Numerous specimens of the genera Cyclotella, Discostella, and Puncticulata (Ochrophyta: Thalassiosirales) have been recovered in Middle Eocene lacustrine sediments from the Giraffe Pipe locality in the Northwest Territories, Canada. These diatoms extend the fossil record of the family Stephanodiscaceae to at least 40 million years before present (Ma) and thus provide a new evolutionary milepost for the thalassiosiroid diatoms, an important dade of centric diatoms with global representation in both marine and freshwater environments. The quality of the fossil material enables detailed investigations of areolae, fultoportulae, and rimoportulae, revealing direct morphological affinities with a number of extant taxa. These observations extend the antiquity of several characters of phylogenetic importance within the thalassiosiroid diatoms, including the fultoportula, and imply that the entire lineage is considerably older than prior constraints from the fossil record, as suggested independently by several recent molecular phylogenies. [ABSTRACT FROM AUTHOR]

Freshwater diatoms belonging to the genus Eunotia Ehrenberg 1837 are well preserved in Middle Eocene lake sediments from the Northwest Territories, Canada. Although two distinct valve morphologies are recognized, both have features that are entirely compatible with modern congeneric taxa, including dorsiventral valve morphology, striae spanning the valve face and extending onto the mantle, raphe structures on the mantle that extend distally onto the valve face, two helictoglossae per valve, and one apical rimoportula per valve. These findings establish that the genus Eunotia, and likely freshwater raphid diatoms as a whole, are more ancient than previously believed, dating back to at least 40 Ma. One of the observed morphotypes has a raphe spanning more than 80% of the valve length as well as a narrow hyaline area extending between proximal raphe endings and interrupting mantle striae. These features imply that the raphe system among certain ancient representatives of Eunotia was more developed relative to modern counterparts. The notion of progressive degeneration of the raphe within Eunotia is supported by several extant species. Collectively, these observations confirm molecular phylogenetic arguments that position Eunotia as a basal raphid diatom. [ABSTRACT FROM AUTHOR]

Abstract: Pinnularia is an ecologically important and species-rich genus of freshwater diatoms (Bacillariophyceae) showing considerable variation in frustule morphology. Interspecific evolutionary relationships were inferred for 36 Pinnularia taxa using a five-locus dataset. A range of fossil taxa, including newly discovered Middle Eocene forms of Pinnularia, was used to calibrate a relaxed molecular clock analysis and investigate temporal aspects of the genus’ diversification. The multi-gene approach resulted in a well-resolved phylogeny of three major clades and several subclades that were frequently, but not universally, delimited by valve morphology. The genus Caloneis was not recovered as monophyletic, confirming that, as currently delimited, this genus is not evolutionarily meaningful and should be merged with Pinnularia. The Pinnularia–Caloneis complex is estimated to have diverged between the Upper Cretaceous and the early Eocene, implying a ghost range of at least 10million year (Ma) in the fossil record. [Copyright &y& Elsevier]

The transition from the extreme greenhouse of the early Paleogene (~52 Ma) to the present-day icehouse is the most prominent change in Earth's Cenozoic climate history. During the late Middle Eocene climate transition (42-38 Ma), which preceded the onset of long-lived, continental-scale ice sheets, there is concordant evidence for brief pulses (<1 m.y. in length) of global warmth and ice sheet growth but few constraints on atmospheric CO2. Here we estimate the concentration of atmospheric CO2 during this critical interval using stomatal indices of fossil Metasequoia needles from ten levels in an exceptionally well-preserved core from the Giraffe kimberlite locality in northwestern Canada. Reconstructed CO2 concentrations are mainly between700 to 1000 ppm, but include a secular decline to 450 ppm towards the top of the investigated section. Because the CO2 threshold for nucleating continental ice sheets at this time was ~500 to 750 ppm, the CO2 decline is compatible with a rapid (<104 yrs) transition from warm, largely ice-free conditions to cooler climates with ice sheets. These fossils provide direct evidence that high-latitude deciduous forests thrived in thegeological past under CO2 concentrations that will likely be reached within the 21st century (500-1000 ppm). [ABSTRACT FROM AUTHOR]