Wednesday, 10 March 2021

LOWER CAMBRIAN EAGER FORMATION

This specimen of a mature olenellid trilobite, Wanneria dunnae, is from the Lower Cambrian Eager Formation of British Columbia. 

He is from the Rifle Range outcrop near Cranbrook where you can find numerous fragments and complete specimens of the olenellid trilobites Ollenellus sp. and the larger, more robust Wanneria sp. you see here. 

The site outcrops at a few locations as you head east out of Cranbrook towards Fort Steele. 

The first trilobites were discovered with the building of the Kootenay Highway connecting Cranbrook to Fort Steele and beyond. Several other localities, including the outcrops at the Silhouette Rife Range — which is literally on a Rifle Range where folks go to shoot at things — is a shade older than the Middle Cambrian Burgess Shale but the fauna here is much less varied. 

The site has been known and collected since the 1920s. Back in the day, fossil collecting was a family affair with folks heading out in their lightly coloured finery to picnic and surface collect the eroding exposures. Cranbrook local, Clement Hungerford Pollen was an engineer and avocational palaeontologist. He promoted collecting the exposures of the Eager Formation around 1921. As a pedigreed Englishman of considerable means, he had invested in the Kootenay Central Railway, revitalizing the town by opening up railway access within the region.

Olenellus ricei, Eager Formation
Olenellus is an extinct genus of redlichiid trilobites, with species of average size (about 5 centimetres or 2.0 inches long). He lived during the Botomian and Toyonian stages, Olenellus-zone, 522 to 510 million years ago, in what is currently North-America, part of the paleocontinent Laurentia.

These extinct arthropods are common in Early Cambrian rocks — 542 million to 521 million years old — and thus a useful guide fossil for the Early Cambrian. 

Olenellus had a well-developed semi-circular head, large and crescentic eyes, and a poorly developed, small tail. Telltale features are his fifteen body segments with the third being much longer than the others. The fellow you see had a bit of his tail crushed as he turned to stone.

Trilobites were amongst the earliest fossils with hard skeletons. While they are extinct today, they were the dominant life form at the beginning of the Cambrian and it is what we find as the primary fossil fauna in the Eager Formation. The Eager Formation has produced many beautifully preserved Wanneria, abundant Olellenus and a handful of rare and treasured Tuzoia. The shale matrix lends itself to amazing preservation. The specimens of Wanneria from here are large. Some are up to thirteen centimetres long and ten centimetres wide. You find a mixture of complete specimens and head impressions from years of perfectly preserved moults.