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TitreAn exceptional conodont succession from the Carnian-Norian boundary of the Western Canada Sedimentary Basin, northeastern British Columbia
AuteurOrchard, M J
SourceNew developments on Triassic integrated stratigraphy, Program and Abstracts; par Di Stefano, P (éd.); Balini, M (éd.); 2010 p. 39
Séries alt.Secteur des sciences de la Terre, Contribution externe 20100155
RéunionNew Developments on Triassic Integrated Stratigraphy; Palermo; IT; Septembre 12-16, 2010
Sujetsassemblages fossiles; distribution de fossiles; fossiles; Trias supérieur; Carnien; Norien; Bassin sédimentaire de l'ouest du Canada; paléontologie; Mésozoïque; Trias
Programmebassins sédimentaires du Yukon, GEM : La géocartographie de l'énergie et des minéraux
Résumé(disponible en anglais seulement)
Abundant and diverse conodonts from the Carnian-Norian boundary (CNB) of northeastern British Columbia (NE BC) record remarkable progressive morphogenesis documented in a ~85 m stratigraphic succession. Several new groups, formerly combined by this author as 'Metapolygnathus', are differentiated on the basis of platform outline, lateral profile, and ornamentation; within each group, taxa show evolutionary trends involving a reducing platform, anterior migration of platform constriction, increased parapet or node development on the anterior platform margins, and/or anterior migration of the pit. New genera are proposed for some of these groups and a case is made for abandoning much of the present nomenclature, which is compromised by missing stratigraphic context and/or woefully inadequate definition.
Eight conodont faunas are recognized spanning mostly the Macrolobatus and Kerri ammonoid zones: the oldest three are based on changes in Carniepigondolella, the next four on progressive changes in several lineages including 'Metapolygnathus' primitius, and the youngest on Epigondolella quadrata sensu stricto.
Fauna 1 (beds P-1: ~46 m) is characterized by Carniepigondolella samueli and related species and by relatively unornamented metapolygnathids exhibiting various platform shapes but lateral profiles that are flat or anteriorly down-curved. Most of these specimens have short free blades and a posteriorly located pit, although specimens of one uncommon group have a medial pit.
Fauna 2 (beds 1c-2a: 3.5 m) is marked by elements with longer free blades and reduced platforms in both Carniepigondolella and in smooth 'metapolygnathids'. The latter also exhibit the beginning of an important trend in elevated and noded anterior platform margins. A group of slender ¿metapolygnathids¿ have a marked platform constriction in the posterior 1/3 of the platform.
Fauna 3 (beds 3-4: 1.5 m) comprises Carniepigondolella elements that show the culmination of the trend in reduced platforms with the appearance of several forms, including some that resemble the diminutive forms such as Metapolygnathus echinatus and M. pseudoechinatus. Strongly noded (not denticulated) 'metapolygnathids' appear and some also exhibit anterior migration of the pit.
Fauna 4 (beds 5-13: 7 m) is marked by the appearance of the 'Metapolygnathus' primitius stock with subgroups having very narrow, expanded, quadrate, and pointed posterior platforms. Other groups have a more anteriorly located platform constriction, and others have raised anterior platform margins.
Fauna 5 (beds 13a-17: 7 m) includes 'M.' primitius s. s. and similar forms, including some questionably included in Orchardella. Several groups show an anterior shift in the pit position, and one has the platform divided into two equal halves by a medial constriction. Curved, linguiform, and pointed groups are differentiated, some with nodose anterior platform margins and others with remarkable, sometimes pointed anterior parapets.
Fauna 6 (beds 17a-17c: 1 m) occurs in a narrow interval just before the preferred boundary level and marks a latest Carnian speciation event leading successively to forms with quadrate platform and medial pits, forms with very high parapets, and forms with both very narrow or very expanded posterior platforms.
Fauna 7 (beds 18a-18h: 1.2 m) includes new nodose forms including those close to 'Epigondolella' orchardi and the first typical examples of the Metapolygnathus parvus. In particular, bed 18h marks the appearance of abundant parvus group representatives, and the last occurrence of many typical Carnian taxa. This fauna immediately precedes the turnover event.
Fauna 8 (beds 20-31: ~18 m) is distinguished from all the older faunas by the virtual absence of typically Carnian smooth 'metapolygnathids'. The parvus group remains common in bed 20 but is much less so above, where Metapolygnathus communisti and parapet-bearing species are also rare. Norigondolella navicula appears suddenly in bed 25 and continues through bed 31.
Fauna 9 (bed 32- up) is marked by the appearance of Epigondolella quadrata sensu stricto and similar forms. No smooth metapolygnathids have been recovered from this fauna, which passes up-section both here and in several others sections nearby into the E. triangularis fauna.