Acanthoceras rhotomagense Zone
Base: The base of the zone is immediately below the first appearance of the index species, which in expanded successions is within couplet B43 of Gale's (1995) cyclostratigraphical scheme for the Cenomanian.
Top: The top of the zone is immediately below the first appearance of the ammonite Acanthoceras jukesbrownei (Gale, 1995).
Range of index species: The index species ranges through the lower and middle part of the nominate zone (Robaszynski et al., 1998).
|Bivalvia:||Entolium orbiculare (locally abundant)|
|Sciponoceras baculoides (locally abundant)|
Faunal abundance & preservation: Some of fauna found at the base of the zone may be reworked and phosphatised from the underlying C. inerme Zone. Some components of the fauna are extremely abundant and vertically restricted in their occurrence (see Bio-markers).
Bio-markers: At the base of the zone, Inoceramus tenuis is abundant in a hard limestone band (= top of couplet B43 of Gale, 1995; also known as the Tenuis Limestone), although this bed has been removed by erosion in basin-margin settings (see: standard macrofossil zonation and correlation of the UK Chalk Group). Immediately above this, there is an abundance of small brachiopods (Capillithyris squamosa, Grasirhynchia martini, Kingena concinna, Modestella geinitzi) and Entolium orbiculare in a bed of silty chalk. In expanded successions, this horizon was identified as the lower part of couplet C1 in Gale's (1995) cyclostratigraphical scheme for the Cenomanian, but it has traditionally been referred to as the Cast Bed. In basin-margin settings, the fauna of the Cast Bed and some adjacent horizons is caught up in an erosive sandy deposit, named the Totternhoe Stone. Towards the middle of the zone there is an acme of the brachiopod Orbirhynchia mantelliana, representing the highest of three widely developed acmes of this brachiopod in the Cenomanian. The ammonite Sciponoceras baculoides is common in the lower part of the zone and has an acme in a limestone bed coincident with the top of the acme of O. mantelliana. Concinnithyris subundata is locally common in the higher part of the zone, where Inoceramus atlanticus is also characteristically common.
Age: Mid Cenomanian