pdf: http://stratigraphy.org/forum/download/file.php?id=97

The earliest ammonite species of Jurassic type, Psiloceras spelae GUEX, was unknown till 1998. This species bridges the long gap between the latest Triassic and the earliest Jurassic ammonoids. It was discovered in North America, then in South America and finally in Europe. The species is proposed as boundary marker for the base of the Jurassic system. It is known in two forms which are regarded as subspecies. The European subspecies is here introduced as P. spelae tirolicum n. ssp. This taxon occurs in the Northern Calcareous Alps of Tyrol as the oldest European Jurassic type ammonite. Both subspecies characterize the first Jurassic ammonite biohorizon and are closely related. Their slight differences may be due to biogeographic separation. The preservation admits a complete diagnosis of the new subspecies. Besides psiloceratid characters still phylloceratid characters are present indicating the origin of the psiloceratids. Additionally, slightly younger psiloceratids are described which were known only from South and North America thus far. They are older than the oldest psiloceratids hitherto known. All these forms are elements of the first biochronological ammonite unit of the Jurassic, the Tilmanni Chronozone which is situated below the Planorbis Chronozone. It is assumed that during the end-Triassic biotic crisis only those ammonites survived which lived in the open parts of the oceans. A repopulation of neritic shelf areas took place from the oceanic realms in line with the gradual Hettangian transgression. This may explain the global similarity of the first Jurassic ammonites and their later biogeographic diversification. A time calculation based on the sedimentation rate of the P. spelae tirolicum n. ssp. bearing sections leads to the conclusion that the earlier Triassic-Jurassic ammonite gap of about 250 to 300 ka now can be reduced to approximately 100 ka.