One, Two or Three Connecting Links between Triassic and Jurassic Ammonoids?

Abstract

IT has long been recognized that the Jurassic and Cretaceous ammonoids are very different from those of the Triassic and it is generally concluded that the whole order Ammonoidea nearly became extinct at the Triassic–Jurassic boundary. More than a century ago Suess¹ recognized that one group, the phylloceratids, was common to both systems. Most authors^2–5 have accepted this view and have further concluded that the phylloceratids provide the only link between the faunas of the Triassic and the Jurassic. In taxonomic terms this is expressed by Arkell et al.⁵ as follows: Jurassic and Cretaceous ammonoids comprise suborders Phylloceratina, Lytoceratina and Ammonitina. Of these only Phylloceratina extend back into the Triassic. In the Lower Triassic, Phylloceratina merge with Ceratitina, the dominant Triassic group, which became extinct at the end of the Triassic. It was the opinion of Arkell et al. that both Lytoceratina and Ammonitina were derived from Phylloceratina.