Stem chondrichthyan microfossils from the Lower Old Red Sandstone of the Welsh Borderland

摘要

Placoid and polyodontode scales of stem chondrichthyans have been found in the early Lochkovian “Ditton?Group” of the Brown Clee Hill district, Shropshire, England and at Talgarth, south Wales. One of the forms is?assigned to a new species of Altholepis Karatajūt?-Talimaa, 1997, a genus already recognised from Lochkovian?shallow marine deposits in Celtiberia, Spain and the Northwest Territories, Canada as well as the type locality in?Podolia, Ukraine. Altholepis salopensis sp. nov. is based on small polyodontode scales with typically three to eight?high odontodes; the scale form was previously considered to belong to acanthodian “ Nostolepis ”? ? robusta (Brotzen,?1934). The structure of other scales formerly assigned to “ Nostolepis ” robusta has led us to erect a new genus? Jolepis for this scale form, which differs from Altholepis in lacking an ordered layout of odontodes . Jolepis robusta? (Brotzen, 1934), originally (and possibly still) considered to be an acanthodian, is also known from the Baltic?countries, Russia, and northern Germany (ex erratic limestones). Scales of acanthodian Parexus recurvus Agassiz,?1845, and/or possibly from the stem chondrichthyan Seretolepis elegans Karatajūt?-Talimaa, 1968 (scales of these?two taxa are barely distinguishable), and of stem chondrichthyan Polymerolepis whitei Karatajūt?-Talimaa, 1968?are also present. Altholepis , Jolepis gen. nov., Seretolepis Karatajūt?-Talimaa, 1968 and Polymerolepis Karatajūt?-Talimaa, 1968 are found in marine deposits elsewhere; the British occurrence of these taxa adds to the debate on?the sedimentological origins of the Lower Old Red Sandstone deposits in the Welsh Borderland. The geographic?range of several early sharks is now known to extend around the Old Red Sandstone continent and beyond.