Past lives: Chronicles of Canadian Paleontology |
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Past lives: Chronicles of Canadian Paleontology Nahanni glass trilobites
To many, the Nahanni is a river of legend and myth, gold and greed,
death and murder, and headless corpses. Our Nahanni is a little more
benign, but still concerned with the dead -- sublimely beautiful glassy
corpses of trilobites
![Collage of silicified trilobites etched out of limestone of the Esbataottine Formation using hydrochloric acid. These trilobites come from a single collection in Nahanni National Park. Twenty-one species are shown here. (Photo by RL (c).) Collage of silicified trilobites etched out of limestone of the Esbataottine Formation using hydrochloric acid. These trilobites come from a single collection in Nahanni National Park. Twenty-one species are shown here. (Photo by RL (c).)](/web/20061103054609im_/http://www.gsc.nrcan.gc.ca/paleochron/images/nahanni1.jpg) Collage of silicified trilobites etched out of limestone of the Esbataottine Formation using hydrochloric acid. These trilobites come from a single collection in Nahanni National Park. Twenty-one species are shown here.
(Photo by RL (c).) |
The South Nahanni River arises near the Yukon border, flows past hot
springs, cascades over Virginia Falls through the deepest canyons in
Canada cut into fossil-bearing rocks that are painted in vivid pastels of
bright red and orange. This is the river that justified the establishment
of the Nahanni National Park in the Mackenzie Mountains and the
designation of a World Heritage Site by UNESCO. It is also a river of myth
fuelled by fevered stories about the corpses of Willie and Frank McLeod
found in the valley in 1908, robbed of their gold, lashed to trees and
missing their heads. The legend is seemingly corroborated by a set of
sinister place names for features along the Nahanni -- Headless Range,
Deadmen's Valley, Funeral Range, Broken Skull River, and Hell's Gate.
Sunblood Mountain towers above Virginia Falls as a succession of pink,
red and orange limestones of Middle Ordovician age (460 Ma). We have
studied these rocks and their trilobites at this and many other localities
in the Mackenzie Mountains. The collection of trilobites is generally a
time-consuming process which involves many hours and even days breaking
rock at a single locality to obtain sufficient specimens. The strata on
Sunblood Mountain include trilobites, but the Nahanni fossils are
preserved like few others -- the calcite of these exoskeletons has been
replaced by fine-grained silica. This quirk of preservation is a
paleontological windfall because, by slowly dissolving lumps of limestone
with weak acids in the lab, we were able to recover thousands of specimens
of exquisite trilobites, many of them minute, and each preserved in full
relief.
None of the trilobites is preserved intact as a complete animal -- each
consists of cranidia, free cheeks, hypostomes, rostral plates, thoracic
segments and pygidia of different sizes jumbled together with different
elements of more than twenty other species of trilobites. After sorting
these silicified specimens (under a microscope),
we were able to prepare a comprehensive description of all elements of
each species. Moreover, we were able to analyze the growth history of each
species from larvae to adult. And we were able to allocate individual
trilobites to each of four major ecological communities. We found that the
brittle silicified trilobite elements could safely be handled for picking
and sorting with insect tweezers. For photography, however, each element
had to be affixed with water-soluble glue to the tip of a toothpick which
could then be used to orient the specimen underneath the
vertically-oriented camera. The larval trilobites, some measuring less
than half a millimetre, had to be photographed with a scanning electron
microscope.
![Silicified specimens of the trilobite Calyptaulax callirachis from the top of Sunblood Mountain, Nahanni National Park. (Photo by BDEC (c).) Silicified specimens of the trilobite Calyptaulax callirachis from the top of Sunblood Mountain, Nahanni National Park. (Photo by BDEC (c).)](/web/20061103054609im_/http://www.gsc.nrcan.gc.ca/paleochron/images/calypt.jpg) Silicified specimens of the trilobite Calyptaulax callirachis from the top of Sunblood Mountain, Nahanni National Park.
(Photo by BDEC (c).) |
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