Region 1: Fossils of the Superior Upland

The Superior Upland region of the Midwest contains the largest surface exposure of North America’s ancient core, the Canadian Shield. The shield is composed almost entirely of metamorphic rock that is 1.6 to 2.6 billion years old. Its composition would normally preclude it from containing fossils, as the heat and pressure associated with metamorphism would typically destroy any original fossil remains, but this region contains significant evidence of ancient life! The Earth’s first photosynthesizers have been implicated in aiding the creation of the banded iron formations throughout the region. Stromatolites provide some of the earliest direct fossil evidence for life on Earth, and they can be found in northern Minnesota and Michigan’s Upper Peninsula (Figure 3.2).

Stromatolites are formed by mats of single-celled cyanobacteria—as the bacteria reproduce, new generations form new layers on top of the older mats, also trapping sediment. After many generations, the layers form a dome above the surface of the seafloor. The sediment stuck in these structures makes them fairly robust and easily preserved, some of them surviving despite the fact that even the youngest rocks in this region are 1.6 billion years old.

Figure 3.2. Precambrian stromatolites, about 1–2 billion years old, from northern Minnesota. About 4.5 cm (1.75 inches) wide.

Figure 3.2. Precambrian stromatolites, about 1–2 billion years old, from northern Minnesota. About 4.5 cm (1.75 inches) wide.

Figure 3.3. <em class='sp'>Grypania</em>. Possibly one of the oldest known multicellular fossils. About 2.1 billion years old from Marquette County, Michigan. Each fossil is about 1.5 cm (0.6 inches) across.

Figure 3.3. Grypania. Possibly one of the oldest known multicellular fossils. About 2.1 billion years old from Marquette County, Michigan. Each fossil is about 1.5 cm (0.6 inches) across.

The Upper Peninsula of Michigan is home to fossils of Grypania spiralis (Figure 3.3), which many experts think may be filaments of algae appearing as spirals on rock that is about 2.1 billion years old. These fossils are very important because if they are the oldest examples of multicellular organisms, then they are also the first eukaryotes. Before eukaryotes, all life consisted of single-celled, prokaryotic organisms, which were similar to bacteria in that their cells were small and had no organelles. This means that if Grypania is indeed the first eukaryote, it is a very early relative of all plants, fungi, animals, and protists.

See Chapter 1: Geologic History for more about banded iron formations.

Banded iron formations consist of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shales and cherts, often red in color, of similar thickness, and containing thin layers of iron oxides. They are not fossils in the strictest sense, yet they are important evidence for the Precambrian life of the region located around the northern edge of Lake Superior.

Although the details of their formation are still not completely understood, it is thought that banded iron formations formed when the oceans on the early Earth were anoxic and contained significant quantities of dissolved iron. With the evolution of photosynthesis, increasing amounts of oxygen were released as waste products. This oxygen combined with dissolved iron to form insoluble iron oxides, which precipitated out of solution to form layers. Eventually, all the readily available iron was used up, and oxygen levels became too high for the cyanobacteria to survive, so their population plummeted. In their absence, iron was allowed to build up in the water again, creating a band of iron-poor rock to be deposited. With a renewed buffer against oxygen, the photosynthesizers could rebound, and the cycle repeated itself, creating the regular bands that give the rock its name. Today, banded iron formations are a crucial source of iron ore.

No younger rock is preserved in the Superior Upland portion of the Canadian Shield, but ice age fossils found in this region are discussed near the end of this chapter.