A probable thecate hydroid from the upper cretaceous of southern England preserved by bioimmuration

An inferred thecate hydroid, moulded on the attachment scar of the bivalvePycnodonte vesiculare, is described from the Santonian Chalk of Kent. The living hydroid was evidently overgrown by theP. vesiculare shell and is preserved as a bioimmuration.Eisenackiella thanetensis gen. et sp. nov. is characterized by a hydrorhiza whose stolons bear hydrothecae alternately to the left and right, and a hydrocaulus consisting of several unbranched stems with alternating hydrothecae. The widely spaced stolonal hydrothecae have adnate proximal parts and erect distal parts. Assignment to an extant hydroid family is difficult: whereas the hydrorhiza resembles that of the Family Lafoeidae, the hydrocaulus is reminiscent of the Family Sertulariidae.