FOSSIL HUNTRESS: August 2018

Alsatites proaries, Photo Source: Wikipedia

At the end of the Rhaetian (part of the Triassic period), most of the ammonites had died out.

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.

Ammonites were prolific breeders that evolved rapidly. If you could cast a fishing line into our ancient seas, it is likely that you would hook an ammonite, not a fish. They were prolific back in the day, living (and sometimes dying) in schools in oceans around the globe.

We find ammonite fossils (and plenty of them) in sedimentary rock from all over the world. In some cases, we find rock beds where we can see evidence of a new species that evolved, lived and died out in such a short time span that we can walk through time, following the course of evolution using ammonites as a window into the past.

For this reason, they make excellent index fossils. An index fossil is a species that allows us to link a particular rock formation, layered in time with a particular species or genus found there. Generally, deeper is older, so we use the sedimentary layers rock to match up to specific geologic time periods, rather the way we use tree-rings to date trees.