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1

Variability changes in the ammonite family Dactylioceratidae during the Lower Toarcian

Morard A., Guex J.

Volumina Jurassica

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2006

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Vol. 4, no. 1

194-195

EN

The comparison of successive dactylioceratid samples collected in the Moroccan Middle-Atlas, the Spanish Betic Cordillera, the Lusitanian Basin and the Causses Basin, allowed us to better characterize the range of variability of these forms from the very base of the Toarcian to the Lower-Middle Toarcian transition. From a biostratigraphic point of view, the Tethyan subgenus Eodactylites (Schmidt-Effing 1972) clearly precedes the NW European subgenus Orthodactylites – as recorded in the Lusitanian Basin succession for example, the latter then giving rise to the Dactylioceras s.s. Both early subgenera can be distinguished morphologically, and separated in a number of “species”, although isolated specimens may be difficult to identify. The main point is that Eodactylites, as well as early Orthodactylites form complete series of continuous covariation, the latter progressively branching into two main lineages in the Semicelatum Subzone. Following the occurence of some rare forerunners, known from the Middle-Upper Domerian, the evolution of the macroconch representatives of this ammonite family can be summarised in three main steps: 1. A sudden “mass apparition” of Eodactylites defines the base of the Toarcian. Their variability spectrum is immediately quite large, with a covariation series between a densely ornamented, somewhat involute and compressed pole (E. mirabile) and a more robust very evolute form with distant primary ribs and possible tuberculation (E. pseudocommune). 2. Ornamental variability tends to disappear in the outer whorls of Orthodactylites, whereas their inner whorls still display a wide covariation series (from slightly compressed forms with dense simple ribs to stout pantuberculate ones), as already noted by Howarth (1962). Intermediate forms seem to disappear in the Semicelatum Subzone. 3. Two lineages are then clearly distinct, one leading from isocostate to variocostate non tuberculate serpenticones (Dactylioceras s.s.), the second developing depressed whorls prone to tuberculation, particularly in inner whorls (Nodicoeloceras). A further diversification occurs at the Lower-Middle Toarcian transition (Guex 1971). In this new evolutionary frame, divergent views on the taxonomy of the Toarcian Dactylioceratidae can eventually be reconciled, as different authors in the past based their classification schemes only on a limited portion of the succession presented here.

2

Proposal of the Muller Canyon section (New York Canyon area, Nevada, USA) as stratotype for the Triassic/Jurassic boundary

Guex J., Taylor D., Rakús M., Bartolini A., Atudorei V., Carter E., Lucas S.

Volumina Jurassica

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2006

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Vol. 4, no. 1

283-285

EN

Completeness of the sedimentary and biostratigraphic record is the most important characteristic of a global stratotype intended to record the boundary between two geological stages. Such a reference section must also contain fossils with proven chronological value. In the Jurassic, ammonites are usually considered the ideal chronological marker and all high-resolution zonations concerning that period are based on this fossil group. Figure 1 demonstrates that the Muller Canyon section (New York Canyon area, Nevada, USA) provides by far the most complete biochronological ammonite sequence known in the world for this interval of time. The oldest Jurassic ammonites found in the area are the smooth Psiloceras tilmanni and P. spelae, which are characterized by nodes in their earliest stage of development (Knötchenstadium). Odoghertyceras, a genus found also in Peru and Canada, occurs just above that fauna. Other smooth ammonites found between the first occurrence of P. spelae and the last occurrence of Choristoceras crickmayi belong to a group that we call "intermediate phylloceratids". Such forms lack the nodose juvenile stage and are geometrically intermediate between rhacophyllitids and psiloceratids. In our region, C. crickmayi (younger than C. marshi) co-occurs with the last Arcestes, Placites and abundant Rhacophyllites. In our sections we have found the first Jurassic-type bivalves below the first occurrence of Psiloceras, in beds that we assign to the topmost Rhaetian. In our view, the occurrence of bivalves is strongly controlled by the nature of the sediments and by local ecological conditions and this group does not seem suitable for use as index for the base of the Jurassic. The absence of Triassic ammonoids in the classical NW European sections precludes the possibility of defining the stratotype for the Triassic/Jurassic boundary (TJB) in that region and the ammonite faunas are not well diversified in the classical Tethyan Alpine sections. At present, there are three potentially good GSSP candidates: Chilingote (Peru), Kunga Island (BC, Canada) and Muller Canyon (Nevada). As far as ammonite record is concerned, Nevada ranks foremost. However, an interesting solution would be to propose the Kunga Island section (British Columbia) as parastratotype because it records a complete sequence of radiolarians across this interval, and this group is not preserved around the TJB in the New York Canyon area.

3

Pliensbachian to Aalenian radiolarian biochronology and global correlation

Carter E., Gorican S., Guex J., De Wever P., Dumitrica P., Hori R., Matsuoka A., O'Dogherty L., Whalen P.

Volumina Jurassica

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2006

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Vol. 4, no. 1

151-152

EN

A new UA radiolarian zonation for the Pliensbachian to Aalenian interval is established using 145 distinctive, widely-distributed species. The data are from biostratigraphic sections in: Queen Charlotte Islands, NE British Columbia, Baja California Sur, Japan, Oman, Turkey, Slovenia and Austria. A catalogue of 280 species (with revised taxonomy) is completed. For the zonation, about half these species were eliminated from the total dataset, because they are either rare (e.g. Danubea, Farcus, Pseudopoulpus, Rolumbus), long-ranging (e.g. Pseudocrucella, Orbiculiformella, Paronaella) or non-diagnostic with wide limits of variability (e.g. some species of Bagotum, Droltus, Parahsuum). Rich well-preserved radiolarians from thick continuous stratigraphic sections in Queen Charlotte Islands provide the most detailed record for this stratigraphic interval, and all collections are tied with North American ammonite zones or assemblages. An initial sequence of 25 UAs (including ammonite data) was determined from this material only. Subsequently, data from other areas were added and a global sequence of nine radiolarian zones was obtained. These zones can be correlated worldwide and link previously established UA zonations for the Hettangian-Sinemurian (Carter et al. 1998) and the Middle to Upper Jurassic (Baumgartner et al. 1995).

4

Criterion for definition of the Triassic/Jurassic boundary

Lucas S., Guex J., Tanner L.

Volumina Jurassica

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2006

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Vol. 4, no. 1

291-291

EN

The criterion for definition of the Triassic/Jurassic boundary (TJB) should be a marker event of optimal global correlateability. Only an ammonite event meets this criterion; other potential marker events for definition of the TJB have less correlation potential. Since the 1960s, the LO of the ammonite Psiloceras (usually the species P. planorbis) has provided the working definition of the TJB. However, other criteria for boundary definition have been advocated, including a change in the bivalve fauna (LO of Agerchlamys), a sudden negative excursion of carbon isotopes and the LO of Psiloceras tilmanni, which precedes the LO of P. planorbis. Other criteria that can be advocated include the supposed TJB mass extinction, the HO (highest occurrence) of conodonts or a significant evolutionary turnover of radiolarians. Distinction of the Triassic and Jurassic systems in marine biostratigraphy has a long tradition rooted in ammonite biostratigraphy. This is because the ceratitedominated ammonite faunas of the Triassic virtually disappeared across the system boundary and were totally replaced by the smooth-shelled psiloceratids of the Early Jurassic. Because of the long history of study of this ammonite turnover, its details are extremely well documented on a global scale, especially in western North America, South America and Western Europe. This ammonite turnover thus provides wide-ranging correlations that are intensively studied, extensively published and documented. No other bio-event associated with the TJB can claim such investigation, and no bio-event is comparable to the ammonite turnover to provide a globally correlateable criterion for boundary definition. Using the LO of Psiloceras tilmanni as to define the TJB thus has these advantages: 1. it maintains longstanding tradition of placing the boundary so that all smooth-shelled psiloceratids are Jurassic; 2. it is a boundary above all bio-events traditionally considered Triassic (Fig. 1); 3. it provides an ammonite-based definition of broad correlation potential (P. tilmanni has a distribution from Nevada to Chile); 4. it places the boundary close to (just above) other marker events that can be used to identify the TJB in sections that lack ammonites (Fig. 1). The LO of P. tilmanni thus defines a TJB of optimal correlation potential.