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Geological Survey of Canada

Past lives: Chronicles of Canadian Paleontology
Tyndall Stone
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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).)

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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