Ordovician to Devonian Periods and the Paleozoic marine fauna
|
Information to know |
The basis for the information |
Explanatory comments |
|
Brachiopods, trilobites, sea lilies, horn (rugose) and colonial (tabulate) corals are typical skeletonized marine invertebrates from the Ordovician to Devonian |
Central and western NY bedrock is Ordovician to Devonian-age sedimentary rock from a shallow sea that contains millions of these fossils; such fossils are common in many other parts of the world as well |
Of course, not every kind of animal is found in every kind of rock (shale, sandstone, limestone), because different animals have different environmental preferences |
|
Ordovician to Devonian fish are mostly “armored” |
Plates of armor from around the heads of Ordovician to Devonian fish are sometimes common, but the rest of their skeleton is was cartilagenous and is rarely preserved |
Cartilaginous fish are poorly known from the fossil record. |
|
Many fish in the Orodovician and Silurian are “jawless” |
Armored head shields had an opening for a mouth, but no moveable jaw |
These fish presumably filtered particles from the water (filter feeders) or digested organic matter from mud (deposit feeders) |
|
In the Devonian, enormous armored fish with jaws known as placoderms, are the first known great fish predators |
Dunkleosteus[1] is the most famous example, known from head plates in the midwest and northeast US |
Placoderms did not have “teeth”, but instead used the sharp jagged edges of armored jawline |
|
The Devonian is know as the Age of Fish, because of the great diversity of fish at that time |
Devonian fish are present (though usually not common fossils) in central NY, but their records are known from around the world |
The diversity was so great in part because it combined the end of the days of the armored fish with the early expansion of sharks and bony fish and heydey of the lobe-finned fish |
|
Land life is evolving, including simple plants and some invertebrates – most arthropods |
Records from Ordovician to Devonian, in many cases microscopic remnants; a good record is known from Gilboa, NY (Devonian) |
This fossil record has only recently expanded as people have looked more carefully with special for tiny parts of plants and arthropods. |
|
Early plants were very simple and small, with no leaves and reproductive organs at their tips. |
Very records around the world contain such plants. These plants were probably near water and needed water for the reproductive part of their life cycle. |
|
Central and Western New York were covered with a shallow sea in the Ordovician to Devonian, and in this sea accumulated thick layers of sediment that we can now find as sedimentary rocks. The rocks contain information on life in this area, and the rocks have been well studied over the past 150 years. At one time there was discussion of naming the Devonian the New Yorkian.
The biggest part of the fossil record accessible on land is from continental seas like the one that covered central NY and from coastal seas. We hear a lot about dinosaurs, but far more paleontological problems (such as under what conditions organisms are likely to form new species or go extinct) are solved by studying abundant invertebrate fossils from marine (sea) sediments and sedimentary rocks.
A fossil is any evidence of past life. For a fossil to form, it usually has to be buried (there are a few exceptions in which fossils that are not very old are mummified in caves), or else forces at the surface will destroy it – if it is organic matter it will rot, if it is skeletal matter is will weather, dissolve, or otherwise be destroyed by physical processes.
The way nearly all fossils get buried is in mud and sand. Mud and sand don’t accumulate just anyway, as erosion ultimately carries such particles downhill until they get trapped. The particles generally get trapped in aquatic bowl-shaped settings geologists call “basins” – once the particles reach a water body without flowing water to carry it further, it settles to bottom.
Such water bodies can be lakes or large rivers, but in area the biggest water bodies are the coasts of oceans and continental seas. During most of Earth history, sea level has been high enough to flood parts of many continents, creating broad shallow seas, and these accumulated large amounts of sediment. The fossils we find in central New York are from such a sea. It just so happens that enough water is tied up in glacial ice today that sea level is relatively low and extent of continental seas is fairly small.
[1] Genus and species names are italicized; in the case of hand-writing, they are underlined. Technically, many of the kinds of fossil animals you here about are genera: Tyrannosaurus, Triceratops, Archaeopteryx. Tyrannosaurus rex is a particular species of Tyrannosaurus, Homo sapiens is a particular species of hominids, Mammut americanum (American mastodon) is a particular species of mastodon. Species are “real” biological grouping, in that all individuals within a species can mate and produce fertile offspring (dogs, for example, are one species in spite of the variety of form); genera and other categories are groups of evolutionarily related organisms, but opinions often vary on which species to include in which genus, etc.