Anne Marie Lott

Taxonomic significance of asymmetrical helmet and lance bristles in the genus Mallomonas (Synurophyceae) and their discovery in Eocene lake sediments

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.

Assessing the evolutionary history of the class Synurophyceae (Heterokonta) using molecular, morphometric, and paleobiological approaches

PREMISE OF THE STUDY Heterokont algae of the class Synurophyceae, characterized by distinctive siliceous scales that cover the surface of the cell, are ecologically important in inland waters, yet their evolutionary history remains enigmatic. We explore phylogenetic relationships within this group of algae relative to geologic time, with a focus on evolution of siliceous components. METHODS We combined an expansive five-gene and time-calibrated molecular phylogeny of synurophyte algae with an extensive array of fossil specimens from the middle Eocene to infer evolutionary trends within the group. KEY RESULTS The group originated in the Jurassic approximately 157 million years ago (Ma), with the keystone genera Mallomonas and Synura diverging during the Early Cretaceous at 130 Ma. Mallomonas further splits into two major subclades, signaling the evolution of the V-rib believed to aid in the spacing and organization of scales on the cell covering. Synura also diverges into two primary subclades, separating taxa with forward-projecting spines on the scale from those with a keel positioned on the scale proper. Approximately one third of the fossil species are extinct, whereas the remaining taxa are linked to modern congeners. CONCLUSIONS 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.

Fossil species of Mallomonas from an Eocene Maar Lake with recessed dome structures: Early attempts at securing bristles to the cell covering?

The genus Mallomonas, the largest within the Class Synurophyceae, consists of motile, unicellular, freshwater heterokonts that are covered with an outer layer of overlapping siliceous scales. The scales are flat, circular to oval structures with taxonomically distinct designs. In addition to scales, the cell coverings of most Mallomonas species contain a second siliceous component known as a bristle. Bristles are composed of a long thin shaft and a flattened proximal foot. The foot of the bristle is fastened under the distal end of a scale in such a way as to allow the shaft to radiate out from the cell and rotate on its axis. For many species, the distal scale margin is distinctly raised, forming a hollow space that secures the bristle foot, and an inverted U-shaped opening from which the shaft emerges. This structure is called the dome. We describe three fossil species of Mallomonas from an Arctic Eocene maar lake that contain domes that are recessed to varying degrees back from the distal scale margin. This design is not known in the modern flora and we hypothesize that it represents an early attempt at securing bristles to the scale coat.

The Scaled Chrysophyte Flora in Freshwater Ponds and Lakes from Newfoundland, Canada, and their Relationship to Environmental Variables

Abstract The distributions of scaled chrysophytes in 22 waterbodies from the eastern portion of Newfoundland, Canada, are described relative to environmental gradients. A rich and diverse flora of scaled chrysophytes was recorded that included 47 species from six genera, Mallomonas (n = 29), Synura (n = 7), Neotessella (n = 1), Chrysosphaerella (n = 2), Spiniferomonas (n = 7) and Paraphysomonas (n = 1). Combined with previous works, 57 species of scaled chrysophytes are now known from Newfoundland. On average, 15 species were recorded from each site, and 34 of the taxa were found in more than 10% of the sites. The most widely distributed species were Mallomonas duerrschmidtiae, M. galeiformis, Synura echinulata, S. sphagnicola, and S. petersenii sensu lato, each found in over 75% of the study sites. Seven additional species were found in over 50% of the localities indicating the rich diversity of the flora. One site is of particular significance as it represents the type locality for three species, including Mallomonas newfoundlandicus sp. nov. described in this communication, and represents the northern-most site harboring the marker taxon, M. binocularis. Expanded descriptions of M. maculata and Spiniferomonas abei are given, and evidence is presented that Synura synuroidea represents two distinct species. Overall, the flora reflects habitats that are acidic, dilute and contain moderate to high concentrations of humic substances. Since many of the species are diagnostic bioindicators, these organisms would provide a valuable resource for future paleolimnological studies aimed at understanding impacts from environmental stressors on Newfoundland lakes.

Heteropolar eunotioid diatoms (Bacillariophyceae) were common in the North American Arctic during the middle Eocene

Eunotioid diatoms that express asymmetry in both the apical and transapical axes, forming heteropolar valves, are generally placed in the genus Actinella. The degree of heteropolarity varies between species, ranging from subtle differences between poles to highly differentiated head poles bearing an apical protuberance. Actinella species with less difference between the poles and lacking an apical protuberance are gradational with Eunotia. With over 100 known species reported globally, primarily in the tropics, only Actinella punctata Lewis, 1864 is currently known from North America. As part of a biotic survey and inventory project focused on the middle Eocene Giraffe crater locality near the Arctic Circle in northern Canada, we have uncovered a wealth of eunotioid diatoms including at least five heteropolar species attributed to Actinella, three of which are described formally herein as A. hickeyi sp. nov., A. goodwinii sp. nov. and A. kimberlitica sp. nov.. These diatoms all lack apical protuberances and bear resemblance to modern heteropolar counterparts within Eunotia. The objectives of this contribution are to report the findings from the Giraffe locality relative to modern and fossil eunotiophycid taxa, discuss the use of heteropolarity as a distinguishing character for the genus Actinella, and consider the palaeoclimatic and biogeographical implications of these observations.