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The Cambrian Alum Shales of Scandinavia and their Remarkable Trilobites

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.

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Euan Clarkson

University of Edinburgh

Professor Emeritus Euan Clarkson graduated from Cambridge University in 1960, and subsequently continued as a PhD student on trilobite functional morphology, graduating in 1964. Meanwhile he had obtained a lectureship in palaeontology at Edinburgh University in 1963,  rising to Reader in 1981 and Professor in 1998, retiring in 2002. His textbook ‘Invertebrate Palaeontology and Evolution’ ran to four editions, the last being 1998. He continued to work on trilobites, especially vision, functional morphology, ontogeny and taxonomy throughout his career, but also on the Lower Palaeozoic faunas and environments of the Midland Valley of Scotland and Ireland, and two crustacean-bearing Carboniferous Lagerstätten near Edinburgh (in one of which the first conodont animal was found in 1982) . 

As well as several field guides on Scottish geology, he produced two books with his igneous colleague Brian Upton ‘Edinburgh Rock. The Geology of Lothian’ (2006) and ‘Death of an Ocean; A Geological Borders Ballad’ (2010) both published by Dunedin Press. His main work at present is on Cambrian faunas and environments in Scandinavia, with colleagues from Lund University, Sweden and the visual systems of early arthropods, with Cologne-based physiologist Brigitte Schoenemann. He holds a DSc from Edinburgh and is a Fellow of the Royal Society of Edinburgh.


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Year of Mud

Find out about the Year of Mud, and other themed events in 2015

Remarkable Trilobites - March

Event Details

Date: 10 March 2015

Venue: The Geological Society, Burlington House, London

Speaker: Euan Clarkson