The Stratigraphy and taxonomy of the Upper Triassic bivalve Monotis in New Zealand.

Some 2500 specimens of Monotis from more than 500 localities within New Zealand were studied by statistical and traditional descriptive methods. From this analysis 20 taxa are recognised as constituting the New Zealand Monotis fauna, with the future possible addition of two further taxa. M. richmondiana Zittel, claimed by some overseas workers as a junior synonym of M. ochotica (Keyserling), is established as validly separable and is seen to include Trechmann's (1918) Pseudomonotis ochotica var. acutecostata as a subspecies. The presence of the New Caledonian M. routhieri and M. ochotica gigantea (Avias, 1953) is confirmed and the latter is transferred to M. subcircularis Gabb. Monotis salinaria var. hemispherica and var. intermedia (of Trechmann, 1918) are reinstated as valid taxa and given full specific rank. M. calvata Marwick (1953) is retained as a full species. In addition, the following species and subspecies are proposed: awakinoensis, kiritehereensis, maniapotoi, marwicki, murihikuensis, murihikuensis taringatura, pinensis aotearoa, rauparaha, rauparaha aries, rauparaha mokaui, subcircularis discordans, wairakae, and warepana.Only Pseudomonotis richmondiana var. truncata Frech (1908) amongst names previously proposed for New Zealand Monotis is rejected as a nomen dubium; it is probably synonymous with M. richmondiana acutecostata (Trechmann).The species-group concept of Westermann (1973b) is formalised by the proposal of five subgeneric divisions to accommodate the 55 definite and up to 5 possible taxa included in the genus. The subgenera Monotis, Entomonotis Marwick (type: M. richmondiana), Eomonotis n.subgen. (type: Pseudomonotis scutiformis var. typica Kiparisova), Inflatomonotis n.subgen. (type: Monotis salinaria var. hemispherica Trechmann), and Maorimonotis n.subgen. (type: Monotis routhieri) are differentiated on the basis of adult size, prominence and sculpture of the posterior auricle, the degree to which it is separated from the disc, shell inflation, radial ribbing and shell thickness.Eomonotis is the oldest subgenus and the stem-stock for the others; it probably evolved from Otapiria. It is unclear from morphologic evidence whether Monotis (s.s.) and Entomonotis developed one from the other or both from Eomonotis independently. Maorimonotis is confidently believed to have sprung from an Entomonotis similar to M. pachypleura (Teller) and Inflatomonotis is most likely to have evolved from an Eomonotis with strongly inflated left valve (e.g. M. jakutica (Teller) or M. iwaiensis Ichikawa).Eomonotis and Entomonotis are geographically the most widespread, the former being absent only from the Andean area of Monotis distribution and Entomonotis not having been recorded in the western or central Tethys. Monotis (s.s.) is common only in the western Tethys. Inflatomonotis and Maorimonotis are confined to New Zealand-New Caledonia except for one record of the former in British Columbia.In many regions the stratigraphic sequence of Monotis is too inadequately known for evolutionary sequences to be deduced but some examples of chronologic and geographic subspeciation are pinpointed. The only clearly defined lineage is that within Maorimonotis from maniapotoi through awakinoensis and routhieri to calvata. The genus can be seen to have a high taxonomic diversity and rapid rate of evolution. Apart from three questionable lower Triassic and lower Jurassic species, Monotis is confined to the Norian Stage, and possibly to the Columbianus and Suessi Zones of the mid and upper Norian.Monotis is postulated as having been epibyssate throughout life and normally epiplanktonic on drifting or floating marine vegetation, with coexisting taxa occupying different microenvironments on different parts of the plants. A serious objection to this mode of life is the massive algal production necessary to provide a substrate for the tremendous numbers of individuals involved in Monotis shellbeds. Reduction of the byssal ear in Maorimonotis is seen as a significant event and coupled with the trend to an equivalved condition is interpreted as indicating change to an endobyssate benthic existence for M. routhieri and calvata; this adequately explains the restricted distribution of this subgenus.Detailed stratigraphic analysis of Monotis in Murihiku sequences necessitates rejection of the Rocky Dome section as stratotype for the Warepan Stage on the grounds that although M. richmondiana has been cited as the marker fossil for the stage, past usage has clearly equated the base of the stage with the incoming of the genus, and richmondiana is not the oldest member of the genus to be found in Murihiku sections. Earlier members are absent from the stratotype which therefore must be relocated in order to retain the stage in its accepted scope. The Kiritehere coastal section suffers the disadvantage of penecontemporaneous submarine slumping, but is proposed as stratotype for a new Marakopan Stage because it is the thickest, best exposed, clearest and best known sequence of Monotis beds and adjacent strata in New Zealand. Within this stage a sequence of five formal and three informal chronozones is defined. The lower Marakopan Mokaui and Murihikuensis Chronozones can be grouped together as an Eomonotis chronozone when faunas are too poor for better correlation, and similarly the overlying Acutecostata, Richmondiana and Calvata Chronozones can be grouped as an Entomonotis chronozone. In addition a topmost Marakopan chronozone can be recognised from the absence of Monotis and basal Otapirian indicators and the presence of a fauna which at present seems common to the two stages.Monotis is the most common fossil in the Torlesse Super-group and the lower four Chronozones, and probably also the Calvata Chronozone, are all represented. Most forms present in Murihiku rocks are also present in Torlesse, but two common Torlesse forms are unknown in Murihiku sequences and this is believed to indicate both communication and barriers between the two regions in Marakopan times. Monotis-bearing Torlesse lithologies are more varied than Murihiku and indicate different depositional regimes. Whilst Murihiku Marakopan strata are all of normal clastic lithologies and show a southwards increase in grain-size, the Torlesse picture is more complex. Firstly, an increasingly deeper and offshore situation is suggested by some deposits with Murihiku faunas: inner or middle neritic origin for Monotis-bearing clasts in a (?) late Jurassic conglomerate at Morrinsville which possibly accumulated in the outer neritic zone or slightly deeper, outer neritic for in situ strata near Aria, slightly further offshore accumulation for ? middle Jurassic mélange in the Oroua Valley and Marakopan slump deposits in the Otaki Gorge, and bathyal or even abyssal deposition for argillites with scattered Monotis in the Mt Arthur-Bealey area. The Aria Monotis beds lie west of the Waipa Fault, yet fit best into Kear's (1967, 1971) Morrinsville Facies, and indicate that the Hakarimata Anticline, Waipa Fault and the facies relations postulated by Kear date from the mid-Jurassic and that there was no feature like the modern continental edge and slope during upper Triassic times in the region. Close paleogeographic relations between these areas and the Murihiku region in Marakopan times is indicated and is not disproven by any sedimentologic evidence yet known.One lithologic association not represented in and showing distinct geographic separation from the Murihiku area is the shell limestone-submarine lava-chert association, which is that containing Monotis species endemic to the Torlesse. This is postulated as having been formed in localised offshore shoal areas (guyots) formed by submarine volcanism; even these regions had some connection with Murihiku seas.