Uintacrinus socialis Zone

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Base: In southern England, the base of the zone is marked by Buckle Marl 1, and in northern England, it is defined by the first appearance of the index species, c. 9.5 m above Enthorpe Marl 1 of Mitchell (1994), between marls DD15 and DD16 of Whitham (1993; NB marls not shown on published log of succession). In the BGS Trunch Borehole [TG 2933 3455], in East Anglia, there is no lithological marker for the base of the zone, which is placed at the first record of Uintacrinus socialis (Wood & Morter, 1994). In Northern Ireland, the base of the Chalk Group belongs to this zone (Wilson & Manning, 1978), although it is not clear if this is coincident with the base of the zone.

Top: In southern England, the top 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 Marsupites testudinarius in northern England, c. 0.7 m below the lower of the two Maidlands Upper Marls (Mitchell, 1994). In East Anglia, the U. socialis Zone is better defined on its typically flintless lithology than on fauna (Peake & Hancock, 1970), and in the BGS Trunch Borehole [TG 2933 3455] the zone was grouped with the overlying M. testudinarius Zone (Wood & Morter, 1994). In Northern Ireland, the top of the zone is coincident with the top of the Galboly Chalk (Wilson & Manning, 1978).

Range of index: In Kent and northern England, the index ranges through the whole of the socialis Zone and into the lower part of the overlaying M. testudinarius Zone (Bailey et al., 1983; Mitchell, 1994), but elsewhere, the range of the index appears to be approximately coextensive with the nominate zone (Mortimore, 1986; Wilson & Manning, 1978).

Key Fauna:

Brachiopoda: Cretirhynchia plicatilis
Bivalvia: Acutostrea incurva
Inoceramus (comminuted shell fragments abundant in Northern Ireland)
Pseudoperna boucheroni
Pycnodonte vesiculare
Sphenoceramus pinniformis*
Belemnoidea: Gonioteuthis*
Crinoidea: Bourgueticrinus granulosus
Uintacrinus socialis
Echinoidea: Echinocorys scutata (flat-topped forms)
E. tectiformis (early forms)*
Hagenowia anterior*
Micraster coranguinum
M. rostrata

(*: more characteristic of the zone in northern England)

Faunal abundance & preservation: In southern and northern England the zone is quite fossiliferous, but the fauna tends to be dominated by small fossils, so that at first glance, the impression is of a poorly fossiliferous interval. In East Anglia, the zone appears to be genuinely lacking in fauna (Peake & Hancock, 1970). Specimens of Uintacrinus socialis are nearly always preserved as disarticulated calyx plates and brachials rather than as whole calyces. In Northern Ireland, the extreme hardness of the Chalk Group makes it difficult to collect fossils.

Bio-markers: The zone is typically quite thinly developed in all regions, and most of the characteristic fauna can be faound throughout the zone. However, the common occurrence of oysters, particularly Pseudoperna boucheroni, is notable, although these are also common in the M. testudinarius, U. anglicus and basal O. pilula zones. The so-called early forms of Echinocorys tectiformis, recorded in northern England by Whitham (1993), might be conspecific with the flat-topped morphotypes of Echinocorys that occur in the socialis Zone in southern England.

Age: Santonian