Hettangian, Early Jurassic coleoids from West Somerset, SW England—filling a gap in the coleoid record of NW Europe

Abstract

The semi-articulated remains of two specimens of the putative diplobelid coleoid species of Clarkeiteuthis and a coleoid of uncertain affinity (possibly a phragmoteuthid) are described from a single bed in the Blue Lias Formation (early Hettangian, Planorbis Subchronozone) of the Somerset coast (United Kingdom). Remains of Hettangian, (Early Jurassic) coleoids are generally rare worldwide and consist largely of disassociated hard parts. Previously, the oldest recorded putative diplobelids (Clarkeiteuthis) were known from the Sinemurian of Dorset. The Somerset specimens extend their range into the earliest Jurassic, shortly after the end Triassic mass extinction. Combined with the coleoid of uncertain affinity and a previously described belemnitid, this assemblage is significant in the light of the Mesozoic radiation of coleoids. Several isolated coleoid ink sacs are also known from the same bed, suggesting that this unit may have some of the properties of a Lagerstätte. The relatively frequent occurrence of coleoids in this bed combined with a record of an ichthyosaur stomach containing numerous arm hooks attributable to Clarkeiteuthis (Planorbis Chronozone of Dorset) suggest that coleoids may have been relatively abundant and diverse during the earliest Jurassic.

Keywords

• cephalopoda
• coleoidea
• diplobelida
• early jurassic
• hettangian
• blue lias formation
• somerset
• united kingdom