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Colonization of brackish-water systems through time; evidence from the trace-fossil record

Luis A. Buatois, Murray K. Gingras, James MacEachern, M. Gabriela Mangano, John-Paul Zonneveld, S. George Pemberton, Renata G. Netto and Anthony Martin
Colonization of brackish-water systems through time; evidence from the trace-fossil record
Palaios (August 2005) 20 (4): 321-347


Trace fossils in estuarine deposits of different ages have been compared to evaluate colonization history of brackish-water ecosystems and to calibrate trace-fossil, brackish-water models with respect to geologic time. This comparative analysis reveals that, although the colonization of marginal-marine, brackish-water environments was a long-term process that spanned most of the Phanerozoic, this process of invasion of fully marine organisms into restricted, marginal-marine habitats did not occur at a constant rate. Five major colonization phases can be distinguished. The first phase (Ediacaran-Ordovician) represents a prelude to the major invasion that occurred during the rest of the Paleozoic. While Ediacaran-Cambrian ichnofaunas seem to be restricted to the outermost zones of marginal-marine depositional systems, Ordovician assemblages show some degree of landward expansion within brackish-water ecosystems. Intensity of bioturbation and ichnodiversity levels were relatively low during this phase. The second phase (Silurian-Carboniferous) is marked by the appearance of more varied morphologic patterns and behavioral strategies, resulting in a slight increase in ichnodiversity. While previous assemblages were arthropod dominated, brackish-water Silurian-Carboniferous ichnofaunas include structures produced by bivalves, ophiuroids, and polychaetes. Ichnofaunas from the third phase (Permian-Triassic) seem to be characterized by the presence of crustacean burrows, reflecting the late Paleozoic crustacean radiation and adaptation of some groups to brackish-water conditions. The fourth phase (Jurassic-Paleogene) is typified by a remarkable increase in ichnodiversity and intensity of bioturbation of estuarine facies. Colonization occurred not only in softgrounds and firmgrounds, but also in hardgrounds and xylic substrates. The fifth phase (Neogene-Recent) records the onset of modern brackish-water benthos. Although still impoverished with respect to their fully marine counterparts, brackish-water ichnofaunas may reach moderately high diversities, particularly in middle- and outer-estuarine regions, and degree of bioturbation may be high in certain estuarine subenvironments. Comparative analysis of brackish-water ichnofaunas through geologic time provides valuable evidence to understand colonization of marginal-marine environments through the Phanerozoic, and allows for calibration of ichnologic models that may aid in the recognition of estuarine valley-fill deposits in the stratigraphic record.

ISSN: 0883-1351
Serial Title: Palaios
Serial Volume: 20
Serial Issue: 4
Title: Colonization of brackish-water systems through time; evidence from the trace-fossil record
Affiliation: University of Saskatchewan, Department of Geological Sciences, Saskatoon, SK, Canada
Pages: 321-347
Published: 200508
Text Language: English
Publisher: Society of Economic Paleontologists and Mineralogists, Tulsa, OK, United States
References: 217
Accession Number: 2005-055742
Categories: Invertebrate paleontology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 plates
Secondary Affiliation: University of Alberta, CAN, CanadaSimon Fraser University, CAN, CanadaGeological Survey of Canada, CAN, CanadaUniversidade do Vale do Rio dos Sinos, BRA, BrazilEmory University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by SEPM (Society for Sedimentary Geology), Tulsa, OK, United States
Update Code: 200519
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