Abstract Detail

Paleobotanical Section

Strother, Paul K [1], Wellman, Charles H. [2].

Non-marine fossils from the Torridonian Sequence: A first look at Precambrian terrestrial ecosystems.

We have recovered abundant organic-walled microfossils and small organic thalli from over 50 horizons throughout the Torridonian Sequence in the northwest Scottish Highlands. The ca 1.2 to 1.0 Ga Torridonian is a thick sequence of largely arkosic sandstones that includes minor lacustrine deposits along with phosphatic nodules which have long been known to harbor microfossils. Lacustrine deposits include desiccation cracks, raindrop impressions and microbially induced sedimentary structures (MISS). Fossil assemblages are dominated by sphaeromorph acritarchs, but a variety of less common fossils indicate that at 1 Ga, terrestrial habitats were occupied by organisms that were different from simple chlorophycean green algae. Many cysts over 200µm wide are recovered; some have complex internal wall structure (similar to Dictyosphaera) and others possess distinctive asymmetric structure. Dorsiventral lobate thalli up to nearly 1 mm in size appear to be of parenchymatous rather than filamentous construction. Multicellular balls of cells are found enclosed in both simple-walled spherical sacs and in well-differentiated walls. Spine-like structures and a range of acellular cuticles give the impression that multicellular or larger coenocytic organisms were present in the Torridonian ecosystem. Neither cryptospores nor modern-looking freshwater phytoplankton have been recognized. Sediments filling desiccation cracks contain populations of ellipsoidal cells embedded in organic groundmass (mat rip-ups), similar to the extant Halothece/Euhalothece group of halotolerant cyanobacteria. Although individual filamentous sheaths are persistent in low levels throughout the entire sequence, cyanobacterial remains do not dominate any assemblages. This is somewhat surprising, because the Precambrian surface has long thought to have either been barren or populated largely by cyanobacteria. So much for speculation. The Neoproterozoic Era began with a diverse freshwater/terrestrial biota that included cyanobacteria but also a surprisingly substantial eukaryotic component. This may help explain why geochemical studies seem to indicate increased surface weathering intensity beginning around 850 Ma. It also means that palynological assemblages from Neoproterozoic near-shore, siliciclastic marine deposits will contain non-marine elements.

Broader Impacts:
The study of Precambrian life is based almost entirely on sediments formed originally under marine conditions. We have spent two field seasons collecting samples from a sequence of ancient lake deposits, formed between 1.2 and just under 1.0 billion years ago, from the northwest Scottish Highlands - the Torridonian Sequence. Sporadic efforts at microfossil recovery from these rocks over the past 100 years has left the impression that they contain a depauperate assemblage of simple microfossils, however, our more comprehensive survey has recovered a surprising array of far more complicated evidence of life in living in terrestrial environments that previously thought. These include large cells, up to about 500µm in diameter, multicellular balls of cells enclosed in complex walls, spline-like structures, cuticles or what appear to be extra-celular coverings (normally associated with multicellular organisms), and small lobate thalli (that appear to have been adapted to living directly on substrate). Cyanobacteria are present, but they do not dominate these assemblages, which is surprising because the only prior evidence for life on land during the first 7/8ths of earth history has been attributed solely to cyanobacteria. This is the first time we have had a good look at what actually lived in Precambrian terrestrial habitats: freshwater lakes, streams, stream banks and probably soils. The results are proving very different from what we would have predicted and seem to indicate that life was for more extensive in terrestrial habitats than previously thought. The basal cyanobacteria have recently been shown to have had a non-marine origin. Our research opens up the possibility that early eukaryotes may have evolved not just in marine settings, only to migrate later into terrestrial settings, but also in primary terrestrial environments. Habitat heterogeneity in terrestrial settings exceeds that of the oceans in various ways - evolution may have progressed more effectively in terrestrial habitats than in the ocean.

Related Links:
Palaeobotany Laboratory at Weston Observatory

1 - Boston College, Geology & Geophysics, Weston Observatory, 381 Concord Road, Weston, Massachusetts, 02493, USA
2 - University of Sheffield, Animal and Plant Sciences, Alfred Denny Building, Western Bank, Sheffield, S10 2TN, UK

Keywords:
Precambrian
Torridonian
Terrestrial
palynology
Scotland
Neoproterozoic
multicellular
thalloid
lacustrine
evolution.

Presentation Type: Oral Paper:Papers for Sections
Session: 8
Location: 555B/Convention Center
Date: Monday, August 2nd, 2010
Time: 10:45 AM
Number: 8003
Abstract ID:209