Transactions of the Royal Society of Edinburgh: Earth Sciences

Research Article

The first Silurian chasmataspid, Loganamaraspis dunlopi gen. et sp. nov. (Chelicerata: Chasmataspidida) from Lesmahagow, Scotland, and its implications for eurypterid phylogeny

O. Erik Tetliea1 and Simon J. Braddya2

a1 Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, UK e-mail: o.e.tetlie@bristol.ac.uk

a2 Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, UK e-mail: s.j.braddy@bristol.ac.uk

ABSTRACT

A new chasmataspid (Chelicerata: Chasmataspidida) is described from the Early Silurian (Late Llandovery–Early Wenlock) of Lesmahagow, Scotland, as Loganamaraspis dunlopi gen. et sp. nov. It is distinguished from related forms by the low tapering ratio of the postabdomen, pediform sixth prosomal appendage and a heart-shaped metastoma. Additionally, a genital operculum with a genital appendage is preserved. The recognisable morphology of L. dunlopi gen. et sp. nov. bridges some of the gap between the Ordovician Chasmataspididae and the Devonian Diploaspididae. Traces of the gut are reported for the first time from a chasmataspid. Chasmataspids are regarded as sister group to the eurypterids: an anterior opercular plate anterior to the genital appendage in L. dunlopi gen. et sp. nov. is regarded as homologous to the anterior opercular plate of the three-segmented genital operculae of Dolichopterus and Stylonurina. This is considered a plesiomorphic character within Eurypterida, whilst the two-segmented genital operculum (with deltoid plates) of Eurypterina is considered apomorphic.

(Received January 20 2003)

(Accepted January 07 2004)

KEY WORDS

  • Arthropod;
  • Diploaspididae;
  • Eurypterida;
  • genital operculum;
  • metastoma