Morphology, ontogeny and distribution of the Cambrian Series 2 ellipsocephalid trilobite Strenuaeva spinosa from Scandinavia

Abstract

The ellipsocephalid trilobite Strenuaeva spinosa occurs widely in Scandinavia in the lower part of the Ornamentaspis? linnarssoni Zone (Cambrian Series 2). Its first appearance is a readily recognisable, widespread biohorizon in Scandinavia. During ontogeny, intergenal spines are lost and the librigenae develop from a generative zone in the genal region. The palpebro-ocular ridges are distinct and connected to the posterior part of the frontal glabellar lobe in the early meraspid period, but eye ridges become separate from the palpebral lobes and the glabella in the late meraspid period. S. spinosa is characterised by large genal spines, distinct axial spines and falcate pleural tips. The spines may have functioned in predation resistance. S. spinosa also has a small pointed projection at the anterolateral corners of some thoracic pleurae, the function of which is unknown. The presence of five furrows in the cephalic axial lobe of S. spinosa, best expressed in meraspides, suggests that the head of this ellipsocephalid trilobite was composed of six fused primary segments.

Keywords:

• trilobite
• ellipsocephalid
• biostratigraphy
• morphology
• ontogeny
• Cambrian

Acknowledgements

Niklas Andersson is thanked for permission to work in the Hardeberga quarry, and Birgitta and Bengt Fredriksson are thanked for allowing us to excavate and sample the Högland locality in the Lake Storuman area. Leopold Eriksson kindly gave permission to sample the Tallträsk locality on the northern shore of Lake Storuman. Anna Żylińska and the journal reviewers, Euan N. K. Clarkson and Jan Ove R. Ebbestad, critically read the manuscript and provided useful comments that helped to improve the text. Arne T. Nielsen kindly read and commented upon an earlier draft of the manuscript. Financial support for this work was provided by Magnus Bergvalls Stiftelse (MBS) to Per Ahlberg.