Marsupites testudinarius Zone
Base: In southern England, the base of the zone is marked by an un-named marl just above the Hawks Brow Flint (Young & Lake, 1988), and it is placed immediately below the first record of the index species in northern England, c. 0.7 m below the lower of the two Maidlands Upper Marls (Mitchell, 1994). In East Anglia, faunal records of the index are rather sparse (Woods, WH/94/90R; Jukes-Browne & Hill, 1904; Pattison et al., 1993; Peake & Hancock, 1970), making precise definition problematic; it was grouped with the underlying U. socialis Zone in the BGS Trunch Borehole [TG 2933 3455] (Wood & Morter, 1994). In Northern Ireland, the base of the zone is coincident with the base of the Cloghastucan Chalk (Wilson & Manning, 1978).
Top: In southern England, the top of the zone is placed at Friar's Bay Marl 1, and in northern England, a little above marl SY 17 of Whitham (1993). In the BGS Trunch Borehole [TG 2933 3455], the top of the zone is defined by the base of a coarse, shell-rich chalk facies - the so-called 'grobkreide' facies of german successions (Wood & Morter, 1994). However, in northern England, similar coarse-grained chalk occurs within the M. testudinarius Zone (Mitchell, 1995). In Northern Ireland, the top of the zone is coincident with the top of the Cloghastucan Chalk (Wilson & Manning, 1978).
Range of index species: The index ranges through the greater part of the nominate zone in southern and northern England (Young & Lake, 1988; Mitchell, 1994), but clear data for East Anglia is lacking. In Northern Ireland, the index ranges from the base of the zone to just below the Oweynamuck Flint, a short distance below the top of the zone (Wilson & Manning, 1978).
|Inoceramus (comminuted shell abundant in Northern Ireland)|
|Sphenoceramus (Inoceramus) lingua*|
(*:occur in the zone in northern England)
Faunal abundance & preservation: In northern and southern England the zone is quite richly fossiliferous with a variety of fauna. However, in East Anglia, the sparsity of outcrops means that faunal records from this zone are rather limited. The index crinoid is typically preserved as dissociated calyx plates, but complete or semi-complete calices can occasionally be found. In Northern Ireland, the extreme hardness of the Chalk Group makes it difficult to collect fossils.
Bio-markers: In southern England there is a trend in the size and ormentation of M. testudinarius calyx plates through the zone.They are small and unornamented below the Brighton Marl, becoming large and ornamented just above this marl, and then becoming less ornamented but remaining large in the interval up to the Kemp Town Marl; higher in the zone they decrease in size but remain ornamented (Mitchell, 1995). A less clear trend is discernible in northern England, where the calyx plates in the lower part of the zone are either smooth or weakly ornamented, and those in the upper part of the zone generally more strongly ornamented (Mitchell, 1995). On this basis, Mitchell (1995) suggested a tentative correlation of the Brighton Marl and Danes Dyke Lower Marls. Young & Lake (1988) recorded that oysters, particularly Pseudoperna boucheroni, characterise the middle and higher parts of the zone in southern England, but they in fact range through the greater part of the zone and also occur in some adjacent zones. In northern England, Pseudoperna boucheroni has numerous acmes at and above the U. socialis Zone (Whitham, 1993). An acme of Echinocorys elevata characterises the lower to middle parts of the zone in both northern and southern England, and the latter region has Cretirhynchia exsculpta ranging through the middle and upper parts of the zone (Young & Lake, 1988; Whitham, 1993). Two important bio-markers in the upper part of the zone in northern England are the restricted occurrence of the asteroid Crateraster rotundus between marls SY11 and SY 20 of Whitham (1993), and the first appearance of the long-ranging Sphenoceramus (Inoceramus) lingua (Mitchell, 1995).