MP9: Chapter 13: The Trace-fossil Record of Tidal Flats Through the Phanerozoic: Evolutionary Innovations and Faunal Turnover
by M.G. Mángano and L.A. Buatois.
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This chapter is from GAC's Miscellaneous Publication 9: "Ichnology: Papers from Ichnia III" editted by Dr. Duncan McIlroy. To purchase the entire book on disc, please see the Miscellaneous Publications section of our bookstore at http://www.gac.ca/publications/view_pub.php?id=262.
A comparison of tidal-flat ichnofaunas through geologic time allows distinction of five main evolutionary phases: 1) the initial invasion by animals (earliest Cambrian), 2) the arthropod expansion (CambrianOrdovician), 3) diversification and faunal turnover (SilurianDevonian), 4) setting the stage for the Modern Fauna (CarboniferousPermian), and 5) infaunalization and the role of crustaceans (MesozoicCenozoic). This framework allows understanding of the importance of tidal flats as sites of evolutionary innovations and the associated faunal turnover that took place in the intertidal realm through the Phanerozoic. Tidalflat colonization started during the Fortunian, as revealed by the presence of monospecific suites of Treptichnus pedum in extremely shallow water. Later in the early Paleozoic, ichnologic data reveal the establishment of an intertidal fauna dominated by mollusc-like animals, euthycarcinoids and trilobites. Early Paleozoic tidal flats, and particularly in the Cambrian, were anactualistic, reflecting reduced predation pressure, absence of terrestrially derived food and microbial binding. The rest of the Paleozoic witnessed faunal turnover, with the appearance of new tracemakers, and a consequent increase in ichnodiversity. In particular, bivalves became dominant in late Paleozoic tidal flats, including relatively deep-tier siphonate representatives. MesozoicCenozoic tidal-flat ichnofaunas are dominated by deep-tier bioturbators, particularly crustaceans, as well as polychaetes and bivalves. The evolutionary history of tidal-flat ichnofaunas reflects an increasing role of ecosystem engineers, culminating in the MesozoicCenozoic phase.Global and alpha ichnodiversity both steadily increased through the Paleozoic. However, ichnodiversity of MesozoicCenozoic tidal flats decreased with respect to Paleozoic levels. This pattern is regarded as a preservational bias, resulting from increased shallow-tier destruction by deep-tier key bioturbators in post-Paleozoic times.
Geological Association of Canada
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