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1 2006

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Criterion for definition of the Triassic/Jurassic boundary

Lucas S., Guex J., Tanner L.

Volumina Jurassica

2006

Vol. 4, no. 1

291-291

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.