Belemnitella mucronata Zone s.l.
Base: The base of the zone is marked by the Farlington Marls in southern England, by the Whitehead Flint Band in Northern Ireland, and is coincident with the base of the Pre-Weybourne Chalk in East Anglia (Wood, 1988, 1995; Wilson & Manning, 1978). This zone is not exposed in northern England.
Top: In East Anglia, the top of the zone is marked by the Overstrand Pyramidata Hardground, at the top of the Paramoudra Chalk (Johansen & Surlyk, 1990; Pitchford, 1990). In Northern Ireland, the top of the zone is marked by the Long Gilbert Flint Band (Wilson & Manning, 1978).
Range of the index: Belemnitella mucronata s.s ranges from the top of the G. quadrata Zone into the lower part of the nominate zone in East Anglia (Christensen, 1995). In Northern Ireland, the various species concepts of Belemnitella referred to in Wilson & Manning (1978) are probably in need of re-evaluation, so that it is not certain what the range of the index is there. However, the genus Belemnitella is stated to range from the upper G. quadrata Zone, through the B. mucronata Zone, to the lower part of the Belemnella lanceolata Zone (Wilson & Manning, 1978).
|Porifera:||sponges (locally abundant)|
|Bivalvia:||Inoceramus (locally abundant shell fragments)|
|oysters (including Pycnodonte vesiculare & Hyotissa semiplana; locally very common)|
|Nostoceras (Bostrychoceras) sp.|
|E. gibba oviformis|
|Galeola spp. (including giant forms)|
|M. (Isomicraster) stolleyi|
|Offaster pilula planata*|
|Zoophycos (locally abundant)|
(*: found in the zone in Northern Ireland)
Faunal abundance & preservation: The fauna of this zone is more abundant and diverse than in the G. quadrata Zone, and generally the zone can be described as fossiliferous, locally, abundantly so. The extreme hardness of the Northern Ireland Chalk Group means that many fossils are difficult to collect, and some have been reworked at hardground surfaces (Wilson & Manning, 1978).
Bio-markers: The Catton Sponge Bed of East Anglia and the correlative North Antrim Hardground of Northern Ireland are convenient lihostratigraphical markers, at which faunal changes also occur, for subdividing the B. mucronata Zone into lower and upper parts. This informal subdivision is used in the following description.
'lower' B. mucronata Zone: In southern England, and probably also in East Anglia, Echinocorys subconicula occurs at the base of the zone, followed in all regions by an horizon of giant Galeola and a broader interval with Echinocorys conica, which locally becomes more frequent and larger in size towards the top of its range (Wood, 1988; Wilson & Manning, 1978; Wood, 1995). In East Anglia, the bryozoan Volviflustrellaria taverensis is particularly characteristic of the basal part of the zone (Wood, 1988), and the trace fossil Zoophycos, typically preserved as orange-skinned flints, occurs widely at this level in southern England. Higher in this interval, the brachiopods Cretirhynchia lentiformis and C. woodwardi have acmes in all regions, and in southern England and East Anglia there is an abundance of pink-shelled oysters and the brachiopod Magas chitoniformis (Wood, 1988, 1995). In the higher part of the 'lower' mucronata Zone in Northern Ireland there is a further acme of Galeola; an ammonite bed characterised by Patagiosites? stobaei; a unique anomalous occurrence of Offaster pilula planata, and a further echinoid bed containing Echinocorys cf. ovata and E. gibba oviformis (Wilson & Manning, 1978).
'upper' B. mucronata Zone: The 'upper' B. mucronata Zone, as defined herein, only occurs in East Anglia and Northern Ireland. There are many faunal features in common with both, such as the local abundance of Inoceramus shell; the development of the trace fossil Bathichnus paramoudrae, and the occurrence of the echinoids Micraster grimmensis, Echinocorys belgica and E. pyramidata (Wood, 1988; Wilson & Manning, 1978). In East Anglia, the basal part of this interval has a locally developed Baculites-rich bed, and the characteristic rhynchonellids are Cretirhynchia arcuata and C. norvicensis (Wood, 1988). In Northern Ireland, the basal part of the interval is rich in small fossils, especially bryozoans, and this is overlain higher in the succession by a bed with Echinocorys conoidea (Wilson & Manning, 1978). Near the top of the interval in East Anglia, specimens of Pycnodonte vesiculare become large and gryphaeate, and resemble forms from the Maastrichtian (Wood, 1988).