Look for fossils of large cone-shaped cephalopods.
Photo by Asia Reid.
Display rock 16 is a block of limestone from the Cavanagh quarry at the Burnt Lands east of Almonte. How many fossils can you find?
This rock represents shallow sea conditions during the Ordovician Period, about 450 million years ago. During this time the land mass that would become North America was part of the paleocontinent Laurentia, situated close to the equator. Much of the continent was under water, including present day Ontario.
Image from Chapter 7 of the book “Four Billion Years and Counting” published by the Canadian Federation of Earth Science and Nimbus (available at the Almonte branch of the Mississippi Mills Library. 557.1 FOU). Image downloaded from http://www.fbycbook.com/chapter-7. Original by Ron Blakey ( http://jan.ucc.nau.edu/rcb7/globaltext2.html), with additions by Asia Reid.
During the Ordovician much of the North American protocontinent was under water. Land is shown in green and brown on this these paleographic maps. The lighter blue areas represent coastal or nearshore areas, and the darker blue indicates deeper ocean waters. Aspects of modern geography have been added for orientation.
How did this rock form?
This display specimen of fossiliferous limestone formed from the slow accumulation of sediments on the bottom of a shallow Ordovician ocean. It was a tropical environment rich in marine life with hard shells. As the animals died, their shells were added to the sea sediment. Over time the layers of mud were buried, compressed, and cemented to form sold rock.
Cephalopods were the dominant predators of the Ordovician seas. They were tentacled squid-like animals with cone-shaped shells. The gas-filled chambers of their orthocone shells enabled them to rise or sink like a submarine. They were good swimmers too, propelling themselves by jetting water through the body cavity. Some cephalopods were giants with shells up to 4 meters long!
Photo by Asia Reid
Red lines have been added to this close-up photo to highlight the cone shape and chambered structure of the fossilized cephalopod shell.
Image © The Manitoba Museum, Winnipeg, MB.
On display at the Manitoba Museum is a diorama of an ancient seafloor environment during the Ordovician Period. Orthocone cephalopods were the dominant predators.