The Triassic in the “Médianes rigides” Nappe of the Swiss Prealps, belonging to the Briançonnais realm of the Western Alps, consists of a several hundred meters thick carbonate succession. At the localities Wiriehorn and Rothorn, 650 m above sequence base, the Costatoria goldfussi limestone, a remarkable coquina and widespread transgressive marker bed, yields the key conodont Sephardiella truempyi (HIRSCH), a proxy for the Curionii - lower Gredleri ammonoid Zones (Early Ladinian). This new discovery has deep implications on the age of the different Triassic formations in the Swiss Prealps and for its correlation with other tectonic units. The newly established Pralet Formation in the Swiss Prealps comprises the Balmi Member with the Lower Ladinian Costatoria goldfussi limestone at its base, followed by the dolomitic breccias of the Ladinian Erpilles Member. Below the S. truempyi level, both the mighty platform carbonate sequence of the Wiriehorn Formation in the Swiss Prealps, and of the coeval Champcella Formation in the Brianconnais realm, are reassigned here to the Late Anisian. In our revised correlation, the Costatoria goldfussi limestone corresponds to the main flooding surface (MFS) of the large marine transgression that correlates biostratigraphically the now well dated Upper Muschelkalk transgression in Provence, Sardinia and Spain with the basal Ladinian type locality at Bagolino in the Southern Alps.