Wednesday, 2 September 2020


In the early 1980s, Canadian palaeontologist Tim Tozer of the Geological Survey of Canada took a good long look at the marine invertebrate fauna in the Triassic of North America. 

Born in Britain and lost to us in December of 2011, at the age of 82. He was a colleague, friend and mentor. Tozer explored and mapped much of the high Arctic — one of the most remote and inhospitable areas of our planet. The work suited his rugged nature and natural curiosity. 

He loved to learn, solve puzzles and share his knowledge and enthusiasm with colleagues. His vast knowledge of the Triassic — both the biostratigraphy and ammonite taxonomy gained through lived experience — will never be repeated.  

Tozer mapped much of the Arctic Archipelago and was a renowned expert on the Triassic — our world 250 to 200 million years ago. He had a particular fondness for ammonoids, our cephie friends who reveal so much of our ancient past to us. Over his forty-plus year career, Tozer named and published on more than 200 species of Triassic ammonoids. 

Tozer collaborated with the American palaeontologist Norman J. Silberling, to define stratotypes for all the recognized North American biozones. Their North American zonal scheme is now accepted as the standard for Triassic global biostratigraphy and allows Alpine (western Tethyan) and Boreal (Siberian) zones to be placed in their proper chronological sequence.

Isolated occurrences of marine Triassic rocks in western North America were known by 1890, but discoveries of several hundred new localities from the Western Canada Sedimentary Basin and the Sverdrup Basin of Arctic Canada between about 1955 and 1980 added much information to the biochronology of the region. It also was recognized that more than half the world’s known genera of ammonoids occurred in North America, testifying to the cosmopolitan nature of the group. 

Tim Tozer, GSC, Mapping the Arctic Archipelago
Unhappy with the challenges of using the Alpine succession as a standard for the Triassic, Tozer and Silberling proposed their new zonal scheme based on relatively complete and in-place sequences in Arctic Canada, northeastern British Columbia, and the western United States.

Because of the endemism — restriction in the geographic distribution — of most ammonoid species, it is often difficult to correlate faunal assemblages between widely separated regions. Because ammonoids and conodonts are found together, a conodont biochronology can often be accurately intercalibrated with the ammonoid zonation, as established for North America by Michael (Mike) Orchard, from the Geologic Survey of Canada (GSC), Vancouver branch, whom Tozer mentored when Mike first joined the GSC as a post-doc.

Additional tools for correlation include the development of a Triassic sea-level curve for the Sverdrup Basin of Arctic Canada and a Triassic magnetic polarity timescale derived from paleomagnetic studies of mainly sedimentary sequences. Correlating rocks by means of polarity time units imprinted on rocks at the time they form is known as magnetochronostratigraphy and is likely to become more important in the future.

In the western terranes of the Cordillera, marine faunas from southern Alaska and Yukon to Mexico are known from the parts that are obviously allochthonous with regard to the North American plates.

Lower and upper Triadic faunas of these areas, as well as some that are today up to 63 ° North, have the characteristics of the lower paleo latitudes. 

In the western Cordillera, these faunas of the lower paleo latitudes can be found up to 3,000 km north of their counterparts on the American plate. This indicates a tectonic shift of significant magnitude. There are marine triads on the North American plate over 46 latitudes from California to Ellesmere Island. For some periods, two to three different faunal provinces can be distinguished. The differences infaunal species are linked, not surprisingly, to their paleolatitude. They are called LPL, MPL, HPL (lower, middle, higher paleolatitude).

I had the opportunity to head to Nevada last year to look at the Triassic ammonoids and ichthyosaur remains in the West Humboldt Mountains. Nevada provides the diagnostic features of the lower (LPL); northeastern British Columbia that of the middle (MPL) and Sverdrup Basin, the large igneous province on Axel Heiberg Island and Ellesmere Island, Nunavut, Canada near the rifted margin of the Arctic Ocean, that of the higher paleolatitude (HPL).

A distinction between the provinces of the middle and the higher paleo-situations can not be made for the lower Triassic and lower Middle Triassic (anise). However, all three provinces can be seen in the deposits of Ladin, Kam and Nor.

In the early 2000s, as part of a series of joint UBC, VIPS and VanPS fossil field trips (and then Chair of the VanPS), I explored much of the lower faunal outcrops of northeastern British Columbia. It was my first time seeing many of British Columbia's Triassic outcrops. The Nevada faunal assemblages are a lovely match. The quality of preservation at localities like Fossil Hill in the Humboldt Mountains of Nevada, perhaps the most famous and important locality for the Middle Triassic (Anisian/Ladinian) of North America, is truly outstanding. Aside from sheer beauty and spectacular preservation, the ammonoids and belemnites are cosied up to some spectacular well-preserved ichthyosaur remains.

Tozer's interest in our marine invert friends was their distribution. How and when did certain species migrate, cluster, evolve — and for those that were prolific, how could their occurrence — and therefore significance — aide in an assessment of plate and terrane movements that would help us to determine paleolatitudinal significance. I share a similar interest but not exclusive to our cephalopod fauna. The faunal collection of all of the invertebrates holds appeal.

This broader group held an interest for J.P. Smith who published on the marine fauna in the early 1900s based on his collecting in scree and outcrops of the West Humboldt Mountains, Nevada. He published his first monograph on North American Middle Triassic marine invertebrate fauna in 1914.

N. J. Silberling from the US Geological Survey originally published on these same Nevada outcrops in 1962 then teamed up with Tozer — bringing two delicious minds together to tease through the larger Triassic picture. Just prior and after Tozer's passing, Siberling went on to collaborate on papers with Haggart, Orchard and Paul Smith through to 2013. His work included nearly a dozen successive ammonite faunas, many of which were variants on previously described species. Both their works would inform what would become a lifelong piecing together of the Triassic puzzle for Tozer.

If one looks at the fauna and the type of sediment, the palaeogeography of the Triassic can be interpreted as follows: a tectonically calm west coast of the North American plate that bordered on an open sea; in the area far from the coast, a series of volcanic archipelagos delivered sediment to the adjacent basins. Some were lined or temporarily covered with coral wadding and carbonate banks. Deeper pools were in between. The islands were likely within 30 degrees of the triadic equator. They moved away from the coast up to about 5000 km from the forerunner of the East Pacific Ridge. The geographical situation west of the back was probably similar.

Jurassic and later generations of the crust from near the back have brought some of the islands to the North American plate; some likely to South America; others have drifted west, to Asia. There are indications that New Guinea, New Caledonia and New Zealand were at a northern latitude of 30 ° or more during the Triassic.

If you fancy a read, pick up a copy of Tozer's seminal publications: A Standard for Triassic Time (1967); and The Trias and Its Ammonoids: The Evolution of a Time Scale (1984). These are the culmination of over forty years of research. 

Fire and Ice: From the Arctic Archipelago to the Scorching Belly of Nevada. Fossil Huntress.