|Alsatites proaries, Photo Source: Wikipedia|
The Hettangian, a rather poorly understood 3 million year time interval followed the Triassic-Jurassic mass extinction event.
During the Hettangian, the new or Neoammonites developed quite quickly. Within a million years, a fairly large, diverse selection of genera and species had risen to fill the void. It is the time in our geologic history that the smooth shelled ammonite genus Psiloceras first appears.
It spans the time between 201.3 ± 0.2 Ma and 199.3 ± 0.3 Ma (million years ago). For my European friends, the Hettangian is the time span in which the marine limestone, shales and clay Lias of western Europe were deposited.
This Hettangian ammonite, Alsatites proaries, is a lovely example of the cephalopods cruising our ancient oceans at that time. They would have been swimming in the same seas, and being eaten occasionally, by Temnodontosaurus, a long, slender, large-eyed ichthyosaur.
Alsatites is an extinct genus of cephalopod belonging to the Ammonite subclass. They lived during the Early Jurassic, Hettangian till the Sinemurian and are generally extremely evolute, many whorled with a broad keel. Or, as described by one of my very young friends, he looks like a coiled snake you make in pottery class.
He does, indeed.
In British Columbia, we see the most diverse middle and late Hettangian (Early Jurassic) ammonite assemblages in the Queen Charlotte Islands (Haida Gwaii), an archipelago about 50 km off BC’s northern Pacific coast.
In total, 53 ammonite taxa are described of which Paradasyceras carteri, Franziceras kennecottense, Pleuroacanthites charlottensis, Ectocentrites pacificus and Curviceras haidae are new.
In general, North American Early Jurassic ammonites are of Tethyan affinity or endemic to the eastern Pacific. For this reason a separate zonation for the Hettangian and Sinemurian of the Western Cordillera of North America was established. Taylor et al. (2001), wrote up and published on much of this early research though, at the time, very little Canadian information was included.
Since then, Dr. Louise Longridge (Longridge et al. (2006) made significant changes to the upper Hettangian and lower Sinemurian zones based on a detailed study of the Badouxia fauna from Taseko Lakes. As part of her thesis, she studied the Queen Charlotte fauna to help draw comparisons and update the literature.