Mytiloides spp. Zone

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Base: The base of the zone is immediately below the first appearance of the inoceramid bivalve Mytiloides, which is approximately coincident with the top of the N. juddii ammonite zone (Mortimore & Pomerol, 1996).

Top: The top of the zone is coincident with the top of the highest occurrence of common Mytiloides shell fragments, below the appearance of Terebratulina lata (the index of the next youngest zone). However, there is a small gap between the highest acme of Mytiloides and the lowest occurrence of T. lata (Mortimore & Pomerol, 1996).

Range of index: Mytiloides has a sporadic range through most of the Turonian, making the nomenclature of this earliest Turonian zone quite inappropriate. As currently understood, the Mytiloides spp. Zone equates with that part of the early Turonian range of Mytiloides below the entry of Terebratulina lata, in which various species and subspecies of this inoceramid are abundant. As so defined, Mytiloides ranges throughout the Mytiloides spp. Zone.

Key fauna:

Annelida: Filograna avita
Brachiopoda: Orbirhynchia cuvieri
large terebratulid brachiopods
Bivalvia: Mytiloides puebloensis
M. hattini
M. labiatus
M. mytiloides (abundant)
Ammonoidea: Fagesia sp.
Lewesiceras peramplum
Mammites nodosoides
Metasigaloceras rusticum
Pseudaspidoceras spp.
Watinoceras aff. amaduriensis
W.devonense
Crinoidea: Roveacrinus communis
Echinoidea: Hemiaster minimus

Faunal abundance & preservation: Fragmentary Mytiloides shells are abundant through the greater part of the zone, mostly preserved in pinkish calcite. There are local acmes of Filograna avita and Roveacrinus communis.

Bio-markers: Mytiloides hattini and M. puebloensis characterise the lower part of the zone, at which level there is also the lowest of two Turonian acmes of the micro-crinoid Roveacrinus communis. A unique acme occurrence of the serpulid Filograna avita, encrusting Mytiloides shells, occurs just above the middle of the zone (Gale, 1996).

Age: Early Turonian