Terebratulina lata Zone
Base: The base of the zone is defined by the first appearance of the index species. This is a short distance above the top of the Mytiloides spp. Zone as currently defined, leaving a small gap as yet uncharacterised by macrofauna. However, defining the top of the Mytiloides spp. Zone by the first occurrence of T. lata rather than the last occurrence of common Mytiloides would remove this problem.
Top: The top of the zone has traditionally been defined at a lithostratigraphical feature, namely the base of the Bridgewick Flints or their lateral correlatives, following Jukes-Browne & Hill (1904). The latter was based partly on the apparently greater abundance of the index species of the next youngest zone (i.e. Sternotaxis plana) in and above the flints than below, and partly because the Bridgewick Flints appeared to mark the junction between the traditional Middle and Upper Chalk subdivisions (Mortimore & Wood, 1986).
Range of index species: The index species ranges through the nominate zone and into the basal part of the overlying S. plana Zone (Mortimore, 1986).
Key fauna:
| Foraminifera: | Labyrinthidoma |
| Bryozoa: | Bicavea rotaformis |
| Brachiopoda: | Orbirhynchia dispansa |
| O. herberti | |
| Terebratulina lata | |
| Bivalvia: | Inoceramus apicalis |
| I. cuvieri | |
| I. lamarcki | |
| I. lamarcki stuemckli | |
| Mytiloides hercynicus | |
| M. incertus | |
| M. subhercynicus | |
| Echinoidea: | Conulus subrotundus |
| Epiaster michelini | |
| Gauthieria radiata | |
| Infulaster excentricus* | |
| Micraster corbovis (lata Zone type) | |
| Sternotaxis plana |
(*: occurs in the T. lata Zone in northern England)
Faunal abundance & preservation: The zone is rather poorly fossiliferous, except in the upper part, where the interval corresponding with the basal Lewes Nodular Chalk of southern England (and its lateral correlatives) has a more abundant and diverse fauna.
Bio-horizons: Mytiloides ex gr. hercynicus - subhercynicus, preserved as chalky moulds (rather than shell covered), and large terebratulid brachiopods are characteristic of the basal T. lata Zone. Conulus subrotundus and Inoceramus lamarcki occur in the lower part of the zone, and Inoceramus cuvieri, Mytiloides incertus, Micraster, Epiaster and Sternotaxis in the higher part (Mortimore, 1986). In the top of the zone, an acme of the large agglutinating foraminifer Labyrinthidoma (= Coskinophragma of Mortimore, 1986), coincident with Southerham Marl 1 in southern England, is closely overlain by an acme of Bicavea rotaformis (Mortimore & Wood, 1986).
Age: Turonian