Illustrations of Selected Ordovician fossils Appendix 1 Brachiopods Cephalopods Brachiopods (brack'-i-oh-pods) are a group of marine animals with two shells. They all have an upper and lower shell. The shells range in size from less than a quarter of an inch to several inches. Fossils of cephalopods (sef'-al-oh-pods) have been found in rocks of many ages, and there are many types alive today. Squids and octopuses, are the most well known modern day cephalopods. Most brachiopods live attached to the sea floor. Some forms attach themselves directly to the sea floor or lie loose in the mud or sand. Other members of this group have spines that serve as anchors. Cephalopods are the most advanced of all animals without backbones. They have a highly developed nervous system and have phenomenal eyesight. During the Ordovician Period, about 460 million years ago, some cephalopods grew to be as long as 19 feet, although most were much shorter. Anthozoans Trilobites Anthozoans (an-th-oh'-zo-ans) became important members of shallow sea ecosystems during the Ordovician Period. Trilobites (try'-low-bites) have been extinct for around 250 million years. Two grooves extending down the back of the animal divide it into three lobes; hence, the name "trilobite". They were ancestors of modern corals. While early horn corals lived as individuals, others built up larger colonies of organisms in the shallow tropical Ordovician seas. Trilobites had a head with eyes and a mouth, a jointed body, and a tail. The animals were cousins of crabs and lobsters. Trilobites were were among the most important animals on Earth during the Ordovician Period. They became extinct during Permian time.
Illustrations of Selected Ordovician fossils Appendix 1 Continued Pelecypods Pelecypods (peh-les'-i-pods) include oysters & mussels. They have been found in some of the oldest marine rocks known and still are very numerous in the seas and rivers today. Most pelecypods have two shells (right and left). Gastropods Gastropods (gas'-troh-pods) are commonly known as snails. A snail carries its shell on its back. As a snail grows larger, the shell expands and lengthens. Most commonly the shell is coiled in a spiral. The outside of the shell generally is ornamented by ribs, spines, and growth lines. Many pelecypods burrow into the mud or sand and even into wood or rock. Some oysters attach themselves to rocks, and others creep about the sea floor. There are many types of gastropods. Some live in the sea, some live in rivers, and still others live on land. Generally, the fossil gastropods of the Ordovician Period are less than an inch and a half long. Bryozoans Bryozoans (bry'-oh-zoh'-ahns) generally build stony skeletons of calcium carbonate. These small animals live in colonies of many individuals. Bryozoans grow in a variety of interesting shapes and patterns. The skeleton has numerous tiny holes, each of which is the home of a minute animal. Bryozoans spend their lives attached to the sea floor, to stones, or to other animals. Bryozoans are among the most common fossils in Ordovician fossil beds. Crinoids Crinoids (cry'-noids) are called "sea lilies," but they are animals - not plants. They look like plants because their body skeleton is on the end of a stem made of button-like discs and held on the sea floor by root-like arms. The mouth is on top of the body and is surrounded by arms that sweep food into it. Pieces of Crinoid stems or stem discs are common in most Ordovician fossil beds. Some crinoids live today, mainly in deep parts of the ocean, but they are not nearly as common as they were in the past.
The Midwest s Tropical Beaches Millions of years ago a tropical seashore extended across the upper midwest of the United States part of a vast and shallow sea that covered much of N. America. The sandstone, shale, and limestone rock layers that you can see exposed across much of the river valleys in the area tell the story of the conditions that existed hundreds of millions of years ago. To understand how the bedrock of the upper midwest was formed, you need to have a quick understanding of a few geologic processes: deposition, weathering, and erosion. Deposition is the dropping of those particles by the wind or water into layers. These layers are also called beds, which is why they are called bedrock after they have been cemented and squeezed into rock over time. You can see how different types of sedimentary rocks are formed by looking over figure 1. Small to large grains of sand settle close to the shore and form sandstone. Very small particles settle in calm deeper water to form shale. The chemicals (like carbonate) that form the shells of clams and other ocean organisms can form rock called limestone. figure 1 Where are different Sedimentary Rocks Formed? - Deeper Water - - Shallow Water / Shoreline - Limestone Shale Sandstone - Deeper water is more calm - fine particles can settle to the bottom here. - Shells from marine animals settle here, as well. - Particles of sand (from weathered and eroded rock) settle out near the shoreline. Weathering describes how the earth is broken down by wind, water, sun and ice into smaller particles. Erosion is the loosening and movement of those particles. The dust we see in the air, the muddy water in streams, and even the potholes in roads are evidence that the land is being weathered and eroded.
The Midwest s Tropical Beaches Continued The rocks can tell us about the conditions during the time that they were formed. The earliest sedimentary rocks found in the midwest are from the Cambrian period are sandstone. Picture a sandy coast like the Gulf of Mexico but without the palm trees & tourists. The waves continually roll back and forth, sorting the sand grains and rounding them until they are almost all the same size. At the same time, smaller grains and clay-sized particles could float much further out into deeper water before they fell and settled to the bottom (see Figure 1). The sandy shoreline layers became sandstone, while the deep water layers became shale. Sea level changed during the Paleozoic, like it is changing now with global climate change and glaciers melting. As the sea over what is now the American Midwest got deeper, nearly all of the area was flooded and limestone was deposited at the bottom of these deep seas. By looking at a cross-section diagram of typical sedimentary rock deposits, you can see how the water level over the area changed over time. Sandstone was deposited when the shoreline of the ocean moved over the area (Figure 2). The water became deeper as sea level went up, and shale was deposited. When the water was deeper and the area was more like a coastal shelf, shale was deposited over the sandstone (Figure 2). When sea levels were highest, and the sea water was deepest over the area, sand was not able to go all the way from the shore out to the deepest sea floor. The rocks that were deposited in the area when sea levels were highest were made of carbonate chemicals that fell out of the sea water and the remains of the animals that fell to the bottom and became fossils. These rocks formed in the deepest water became limestone and were deposited over the shale (Figure 2).
The Midwest s Tropical Beaches Continued The rocks can tell us about the conditions during the time that they were formed. The sea became shallower again shale was deposited, and when sea levels rose again and the sea got deeper again, the limestone was deposited. There is no bedrock in areas like the Twin Cities (Minnesota) area that represent the time period after limestone (newer than 350 million years old). This is because the oldest rocks (Figure 2, dolomite) are buried the deepest, and the youngest rocks are closest to the surface (Figure 2, limestone). Rock that was even younger (closer to the surface) than the limestone got worn down and scraped off or left as a mixed up layer called glacial till (Figure 2). Fossils are evidence of organisms buried and preserved in sediments. They are hard parts like shells and bone usually, but may also be tracks, imprints, and burrows. The shallow sea that covered the area during the Ordovician period contained a variety of salt water organisms including corals, clams, snails, bryozoans, brachiopods, cephalopods, and trilobites (see the included Appendix 1 for fossil images/descriptions). Many of these groups of organisms can be found in today s oceans, but many of the individual species from the Ordovician period are extinct or very rare in today s oceans. Some groups, like trilobites, are extinct altogether. The shale is the best bedrock in the area in which to collect fossils. Life was abundant in the calm water environment in which these sediments were deposited. This type of environment was also good for preserving fossils, and many can be found whole. The sandstone has very few fossils because it was a shoreline deposit. Waves and oxygen broke down organisms very quickly and didn t preserve them well. The limestone layers were deposited in clear water under calm conditions, and both contain many fossils. They are difficult to get out of the hard limestone, but can be found on the bedding planes where the rock can be pried apart. Descriptions of the Ordovician fossils that can be found in the Midwest are in Appendix 1. Drawings of what the fossils look like can also be found in Appendix 1.
The Midwest s Tropical Beaches Continued Many of the sedimentary rocks (rocks made of parts or other rocks/shells, etc.) of the midwest formed under a shallow sea hundreds of millions of years ago. In earliest Paleozoic time, North America was on the equator, with the area that is now the midwest of the United States was a little south of the equator! Although the climate was tropical, land plants and animals had not yet evolved! The land had nothing but some primitive algae and bacteria living on it. As sea levels began to rise to levels much higher than today, most of North America became covered by ocean waters. Due to this sea level rise, around 520 million years ago a marine ecosystem existed with isolated islands throughout the area. Over the next 200 million years or so, sediments (sand, smaller particles, and carbonate chemicals) eroded and settled into the sea into more or less flat layers. These layers were buried by other layers and were eventually cemented into layers of bedrock (Figure 2). Shells of snails and evidence of other organisms were preserved in this bedrock and became the fossils that you can still see today in some of the local layers of Paleozoic rock. figure 2 Typical Layers of Sedimentary Rock found in the Midwest Elevation above Current Sea Level 950 900 850 800 750 700 650 Glacial Till (sediment left over from melted glaciers) Limestone (compacted & cemented animal shells) Shale (small particles that settled in deeper calm water) Limestone (compacted & cemented animal shells) Sandstone (larger particles that settled near the shoreline) Dolomite