Microvertebrates from a Rhaetian neptunian dyke at Holwell, Somerset: Dating the fissures

Dating the Mesozoic-aged fissure deposits around Bristol and South Wales has been problematic, with ages of the older examples disputed as either Carnian or Rhaetian, a 30-million-year difference. The deposits filling fissures at Holwell, Somerset offer a chance to establish a date for at least one system of fissures because they are on the coast of the Mendip Palaeoisland, close to bedded Rhaetian marine deposits. The Holwell fissures have been known since the 1850s when they yielded some of the first ever reported Mesozoic mammals, and they are the type locality for several fossil shark species, Duffinselache holwellensis , Pseudocetorhinus pickfordi and Palaeobates reticulatus . Bone-rich fissure fills from a neptunian dyke at Holwell yielded over 3000 identifiable specimens, comprising mainly marine fishes but also coastal-dwelling placodonts and terrestrially derived lepidosaurs that lived on the palaeoisland. Over 95 % of the fauna comprises four fish taxa that are typical of the bedded Westbury Formation. The less common Rhomphaiodon minor associated with abundant Synechodus rhaeticus indicates that the deposits are likely not basal but are within the upper half of the Westbury Formation. We hereby confirm that these Holwell fissure fill faunas, including the mammals, are of Rhaetian age.