Furongian

London Lecture: The Cambrian Alum Shales of Scandinavia and their Remarkable Trilobites

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• published: 11 May 2015
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This London Lecture was delivered by Professor Euan Clarkson of the University of Edinburgh on 10 March 2015. Description: During the Cambrian the large continent of Baltica (including modern Scandinavia) lay isolated in the southern hemisphere. The Lower Cambrian of Baltica consists mainly of sandstones, but in the Middle and Upper Cambrian (Furongian) a muddy sequence known as the Alum Shales were deposited. Whereas the Middle Cambrian carries a rich and diverse trilobite fauna, oxygen levels dramatically decreased thereafter, and the uppermost Middle Cambrian bears a fauna consisting only of a single species of agnostoid, Agnostus pisiformis. In the following Furongian, the faunas were dominated by the olenid trilobites, adapted to dysoxic conditions. The Furongian of southern Sweden forms a superb natural laboratory for studying processes and patterns of evolution in the olenids. The rapid turnover of species and superb preservation of the fossils, both in shales and in carbonate concretions, allows evolutionary changes to be assessed both at the micro - and the microevolutionary scale. Also, the dynamics of the evolving faunas can be assessed and their relations with environmental fluctuations established by bed-by-bed collecting and analysis. Moreover since all trilobite growth stages often occur along with the adults, it is possible to establish the complete or partial ontogeny (individual development from the larval stages onwards) of many species, and to explore the relationships between ontogeny and phylogeny. All these separate dimensions will be considered here and particular attention will be given to adaptations of various olenids, and the functions, for example of the extreme spinosity which characterises some genera. Information gained from various lines of evidence from the faunas can be used, along with geochemical approaches to build up a coherent picture of an extinct muddy environment and its inhabitants (which include brachiopods at certain levels, and superbly preserved agnostoids and small phosphatised arthropods). The olenids persisted to the end of the Ordovician but lost their dysoxic adaptations and became part of the normal trilobite fauna.