![Geological Survey of Canada Geological Survey of Canada](/web/20061103031630im_/http://www.gsc.nrcan.gc.ca/esst_images/gsc_e.jpeg) Natural Resources Canada > Earth Sciences Sector > Geological Survey of Canada > Past lives: Chronicles of Canadian Paleontology
Past lives: Chronicles of Canadian Paleontology Tyndall Stone
Canadian author Carol Shields
described Tyndall Stone in her recent best-selling novel, The
Stone Diaries, "Some folks call it tapestry stone, and they prize, especially,
its random fossils: gastropods, brachiopods, trilobites, corals and snails.
As the flesh of these once-living creatures decayed, a limey mud filled
the casings and hardened to rock"
![Slab of Upper Ordovician Tyndall Limestone at the quarry in Garson, north of Winnipeg showing a large Receptaculites (probable calcareous alga) and mottles produced by Thalassinoides burrows. (Photo by BDEC (c).) Slab of Upper Ordovician Tyndall Limestone at the quarry in Garson, north of Winnipeg showing a large Receptaculites (probable calcareous alga) and mottles produced by Thalassinoides burrows. (Photo by BDEC (c).)](/web/20061103031630im_/http://www.gsc.nrcan.gc.ca/paleochron/images/tyndal.jpg) Slab of Upper Ordovician Tyndall Limestone at the quarry in Garson, north of Winnipeg showing a large Receptaculites (probable calcareous alga) and mottles produced by Thalassinoides burrows.
(Photo by BDEC (c).) |
The small village of Garson
north-east of Winnipeg bills itself as The Limestone Capital
of North America. This is no empty boast. The 450 million
year-old (Late Ordovician) Tyndall Stone quarried here is
probably the most frequently used building stone in Canada. It is
used in the House of Commons, old Eaton's stores, the
Canadian Museum of Civilization and in the venerable Empress
Hotel in Victoria.
Two major types of fossils occur
in Tyndall Stone. The first are body fossils. These are the
calcite shells of a variety of marine animals and plants that lie
scattered through the stone -- like raisins suspended in a
pudding. The second are trace fossils in the form of a network of
burrows that extends through the entire rock.
Body fossils are evident as
shells in random cross-section on surfaces of Tyndall Stone.
Brachiopods and trilobites are present but difficult to identify.
With their thicker shells and distinctive internal septae, small
solitary rugose corals are easier to see. Large mollusks, such as
gastropods and cephalopods, are preserved as internal moulds. The
most conspicuous fossils are mounds of colonial tabulate corals
and stromatoporoids. The largest and most enigmatic of the
Tyndall fossils is the so-called "sunflower coral"
which occur as circular "colonies" the size of a
basketball. These fossils are assigned to Receptaculites
which is thought to be an extinct type of calcareous algae.
The shelly fossils of Tyndall
Stone are certainly intriguing, but it is the trace fossils that
make this limestone an attractive building stone. The trace
fossils are evident as mottling, and the mottled surface is simply
a random section though a three-dimensional branching network of
burrows that permeates the rock. These branching and bifurcating
burrows, extending as deep as a metre below the surface, are
well-known to paleoichnologists -- those paleontologists studying
trace fossils -- who have given them the name Thalassinoides.
During the Cretaceous and at the present time, Thalassinoides
tunnels were excavated as dwelling and feeding burrows by mole
shrimp. However, it is unlikely that mole shrimp or any other
decapod crustacean made these deep burrows in the Ordovician
because these arthropods have a well-documented fossil record
that does not extend below the Jurassic. So what animal made
these burrows? Unfortunately, there are no body fossils in or
near the burrows to give even a suggestion. One is tempted to say
"worms", but when paleontologists attribute a trace in
sediment to the activity of "worms" it is generally an
expression of ignorance rather than an actual identification. We simply
don't know what animal is responsible for the deep burrows
and the mottles in Tyndall Stone.
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