LOWER JURASSIC1 (Figure 24.1)

Hettangian

Hettangian ammonites are found on both sides of the Pacific, from northeastern Asia to New Zealand and from the Arctic to central Chile. Localities, especially those with diverse assemblages, are normally less frequent than for the rest of the Lower Jurassic. Assemblages with Badouxia, Paracaloceras, Pseudaetomoceras, and some species of Vermiceras or Metophioceras are included that in part may be latest Hettangian (Bloos 1983, 1988) or even earliest Sinemurian (Guex and Taylor 1976; Guex 1987).

The systematics of Hettangian ammonites include genera whose synonymity remains ambiguous. Some Hettangian ammonites found in the circum-Pacific area are very similar in test morphology to genera described from the northeastern Alps, but the circum-Pacific species frequently show much simpler septal sutures. The same difference exists between northwestern Europe and the northeastern Alps. The significance of this phenomenon is controversial (G. Bloos personal communication; Guex 1982), but a possible reason could be the repeated immigration of genera and species from the open sea to shelf environments.

Faunal differentiation in the Hettangian was still weak. The richest assemblages are known from North America (mainly Nevada), Peru, and northern Chile. They all have strong Tethyan affinities. A proper Boreal Realm did not then exist, because those genera found in the northernmost part of the Pacific also occur in the Tethyan Realm and must be designated as pandemic. Differentiation between the Tethyan and Boreal Realms is achieved mainly because of richer assemblages in genera and species in the first region.