Fossil Unicellular Eukaryotes
Micropaleontology:Includes study of microscopic remains of single-celled organisms. We have touched on the difficult record of early life forms. Starting with the 1.6 Ga Ruyang Group, however, we encounter eukaryotic cell fossils of increasing diversity (Yin, 1997).
Schematic of typical eukaryote from The Gates of Academe
Eukaryota:The key concept here is the stepwise acquisition, by endosymbiosis of the characters that we associate with the "typical" eukaryotic cell, Including:
- Flagella (whip-like organelles of locomotion - also arguably the products of endosymbiosis.)
- Pelobionta: Lacking mitochondria, flagella, and other typical eukaryotic organelles; and able to tolerate a low[O2]. Trumpeted during the late 20th century as representing an early stage of endosymbiosis, but now considered close to amoebas and thought to have lost these organelles secondarily.
- Diplomonada (including Giardia) possess organelles that seem to represent degenerate mitochondria that have lost their respiratory function (Van der Giezen and Tovar, 2005.)
Note: The phylogram to which this cladogram links superimposes the phylogeny of fossil-forming groups on to the stratigraphic record. Typically we could infer minimum divergence ages with some confidence. In the case of Eukaryotes, however, this is perilous as so few yield fossils. El Albani et al. 2014, for instance describe "colonial" organisms with features associated with colonies of eukaryotic cells in the 2.1 ga Francevillan B Formation of Gabon. These colonies were not organized into bodies in the manner of multicellular fungi, plants, and animals, but were akin to congregations of independent cells like slime molds.
Hypotheses of the phylogeny or eukaryotes are subject to frequent revision, so we focus on descriptions of major fossil-forming groups, rather than the broad patterns. And yet a few words on large groups:
- Archaeplastida: Includes Rhodophyta (red algae) and Viridiplantae (green algae and plants). Potential synapomorphies:
- Chlorophyll a
- Presence of cellulose cell walls
- Opisthokonta: Includes Fungi and Metazoa (animals). Potential synapomorphy:
- Collagen as structural protein
- When a flagellum is present it is in a posterior position and pushes the cell. (E.G. sperm cell.)
- Chromalveolata: Includes Foraminiferida, Radiolaria, Coccolithophorida, and Alveolata. Potential synapomorphies:
- Chlorophyll c
- Chloroplasts with three or more cell membranes, indicating that they arose from a secondary episode of endosymbiosis, probably of a unicellular rhodophyte.
- Rhizaria. Nested within Chromalveolata. Includes Foraminiferida and Radiolaria. Potential synapomorphies:
- Slender pseudopodia
- Morphology of mitochondria
Major microfossil-forming groups:
Acritarch from Digermulen
- Exclusively marine
- Probably not a monophyletic group
- Organic walled cysts of... something
- Spheroids with various degrees of ornamentation
- Taxonomy is based strictly on general similarity and conveys no intentional evolutionary information.
- At least some from Doushantou have been argued to be metazoan eggs, though that obviously can't be the case for all of them.
- Most nearly similar to resting stage of dinoflagellates (see below), so a relationship to these is possible.
Chitinozoan from Wikipedia
- Chitinous organic-walled chambers with "neck", often arranged in chains
- Speculated by Kozlowski, 1963 to be eggs of graptolites and/or polychaet annelids. In fact, their abundance seems to track that of the former, lending strength to the argument, however no further clarification has been achieved.
- Lack flagella.
- Chlorophyll a and chlorophyll b present.
- Uniquely possess phycoerythrin, a photosynthetic pigment that absorbs blue light (hence functioning at greater depths than other photosynthesizers.)
- Complex life cycle involves up to three stages of alternating generations. In the case of nori:
- Gametes fuse to form a spore bearing form
- Spores mature into a conchocelis phase that bores into the shells of marine animals.
- These release conchospores which germinate into the edible phase, which produces gametes.
Red alga from Univeristá di Catania
Evolution: Red algae including the 1.2 Ga Bangiamorpha (probably) represent the first multicellular eukaryotes (Butterfield, 2000.)
- Mineralized tests - internal skeletons formed from:
- opal (hydrated silica)
- Strontium sulfate
- Distinct morphology in which: by a perforate membrane into:
- Cytoplasm is separated into inner and outer components by the capsule, a perforated membrane
- Cytoplasm inside the capsule is termed endoplasm
- Peripheral extracapsular cytoplasm called ectoplasm [sic] characterized by gelatinous bubbles called the calymna.
- The skeleton lies within the ectoplasm.
- Thick, sticky axopodia and thin filipodia protrude from the cell for feeding.
- Diploid = containing a full complement of chromosomes, which are grouped in homologous pairs.
- Haploid = containing half of a full complement of chromosomes, where each homologous pair is represented by one chromosome. (In animals, body cells are typically diploid and gametes (sperm and ova) are haploid.)
Chert nodule in limestone
- Major fossil forming groups are:
- Geological significance:
- As index fossils
- As source of marine silica cements, chert, etc. (right)
- Test morphology:
- Typically with multichambered tests, although some are unilocular.
- All tests have an aperture - an opening or group of openings at the most recent chamber. Tests may, in addition, be perforate or imperforate.
- Basal forms are agglutinated, with calcareous forms becoming common in Devonian.
Life cycle involves alternation of generations:
- Diploid microspheric forms (in which the proloculus or original chamber is small) grow by serial addition of chambers or by marginal accretion.
- Asexual reproduction occurs when nucleus and cytoplasm is divided into small packages that exit the test.
- Each grows into a haploid megalosphere (i.e. big proloculus - macrosphere of some authors). Ironically, the megalosphere is typically smaller, in total than the microsphere.
- Microspheric cytoplasm and nucleus divided into many flagellated gametes.
- Gametes fuse forming new microsphere.
- Shallow-marine benthic ancestrally. Planktonic forms appear in Jurassic, become common in Cretaceous (although suffering in K-P extinction.)
Groups of special paleontological importance:
- Fusulinida: (Late Carboniferous-Permian), up to 10 cm long
- Globogeriina: (Jurassic - Quaternary) Very common from Cretaceous onward, planktonic
- Nummulitida: (Eocene epoch of Paleogene), extremely large lentil-shaped tests.
Significance to geologists:
- As biostratigraphic index fossils (planktonic for late Mesozoic and Cenozoic, fusulinids for Late Paleozoic.)
- As reliable source of untransported carbonate shell material for stable-isotope analyses.
- Alveoli develop into organic walled tests
- Active stages characterized by a circumferential groove, the cingulum which houses a transverse flagellum. An additional longitudinal flagellum is at the trailing edge.
- Several different components to life cycle:
- Haploid, motile schizonts reproduce by fission.
- Schizonts act as gametes, fusing to form diploid zygotes
- Zygotes are motile for a while, but eventually encyst and enter resting phase as hypnozygotes (The wall of the hypnozygote, termed a "dinocyst" what's usually preserved in fossil record).
- Hypnozygote excysts (i.e. cytoplasm exits the dinocyst) and fissions into new schizonts. The fossil record of dinoflagellates consists mostly of hypnozgote cysts.
- Single chambered tests made of two silicaceous frustules. One frustrule encloses the other like the gold wrapping of Chanukkah geld or like a petrie dish and lid. They are barely motile, lacking pseudopodia or flagella.
- Diploid cells reproduce asexually. With each division, each offspring receives one frustrule, and secretes another nested inside it. Thus, with each generation, final size decreases.
- Cells divide into haploid gametes.
- Gametes fuse for form a diploid auxospore, which quickly achieves maximum size before forming frustrules.
- Centrales: (Jurassic? - Quaternary) primitive, probably paraphyletic, radial symmetry
- Pennales: (Eocene epoch of Paleogene - Quaternary) derived, likely monophyletic, bilaterally symmetrical
- Useful index fossils, esp. in silica rich marine environments (below CCD)
- Source of deep marine silica cement and chert
- Diatomite - sediment from diatom tests - has numerous commercial applications.
- Environmental sensativity makes them good paleoenvironmental indicators.
- Small (approx. 15-100 microns across) for eukaryotes.
- Covered with calcareous plates (coccoliths) 2 - 25 microns across, arranged into coccospheres. Small size of coccoliths limited study prior to SEM.
- Highly motile. During active stages, bear three flagella, one each at top and bottom, and a specialized haptonema at the "equator"
- Different coccolith morphs may be present in the same coccosphere. Because coccospheres invariably fall apart after death, this leads to a situation similar to that facing biostratigraphers who must distinguish sexual or life-stage morphs of whole organism body fossils.
Life cycle not well studied, however like other chromalveolates, they alternate between haploid and diploid phases. They are distinctive, however in that they can reproduce asexually through mitosis during either life stage. Different coccolith morphs may be secreted by the same species depending on life stage.
- Chalk is limestone made of coccoliths. Hence "Cretaceous" - the age of chalk, from Latin "creta" - "chalk."
- Environmental sensitivity makes some taxa good paleoenvironmental indicators. Some ecological opportunist coccolithophorids mark intervals of global environmental stress.
- Despite potential caveats above, useful as index fossils.
Radiolarian from U C Berkeley MicroGallery
Foraminiferan from Wikipedia
Dinoflagelalte from College of DuPage BIO1151
Diatom from Nicerweb.com
Coccolithophorid from Cocco Express
Choanoflagellates from M. Carr, 2008, PNAS
Choanoflagellates, the putative sister taxon to Metazoa, characterized by a collar of cilia surrouding a single flagellum. These free-living organisms are outwardly indistinguishable from the specialized choanocytes of sponges.
- Sina M. Adl, Alastair G. B. Simpson, Mark A. Farmer, et al.. 2005. The New Higher Level Classification of Eukaryotes with Emphasis on the Taxonomy of Protists. Eukaryotic Microbiology. 52(5), 399–451.
- Nicholas J. Butterfield. 2000. Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes. Paleobiology 26(3):386-404.
- Abderrazak El Albani , Stefan Bengtson, Donald E. Canfield, Armelle Riboulleau, Claire Rollion Bard, Roberto Macchiarelli, Lauriss Ngombi Pemba, Emma Hammarlund, Alain Meunier, Idalina Moubiya Mouele, Karim Benzerara, Sylvain Bernard, Philippe Boulvais, Marc Chaussidon, Christian Cesari, Claude Fontaine, Ernest Chi-Fru, Juan Manuel Garcia Ruiz, Franćois Gauthier-Lafaye, Arnaud Mazurier, Anne Catherine Pierson-Wickmann, Olivier Rouxel, Alain Trentesaux, Marco Vecoli, Gerard J. M. Versteegh, Lee White, Martin Whitehouse, Andrey Bekker. 2014. The 2.1 Ga Old Francevillian Biota: Biogenicity, Taphonomy and Biodiversity. PlosOne, June 2014.
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