Fossil Ecosystems of North America: a preamble

A brief guide to the first, introductory quiz preparing your for the reading of "Fossil Ecosystems of North America"

PREAMBLE

This quiz is mostly about learning the subdivisions of the Geological Time Scale.

The Geological Time Scale subdivides the entire length of time during which Earth existed as a planet, that is 4.6 billion years, in smaller time units. At this point in the semester it is too early to be looking at how or why this was done (we will do that later on), but you need to start memorizing the names, their hierarchy, the numerical ages, and the chronological order in which these time intervals follow each other.

We have discussed the concept of Geologic Time in class and we mentioned the fact that we cannot find a rock or a fossil and immediately tell its numerical age by looking at it. Most likely, and not always, a scientists will be able to assign a relative age to the find.

What does “relative age” mean?
It means that we are able to put geological events, rocks, and fossils, in a relative order, from the older to the younger one. That is, we can reconstruct a sequence of events.

In order for us to be able to do that, we need a way to tell history based on events that cannot be repeated; if an event is repeated, then we cannot really tell when exactly within a sequence it happened because we cannot build a reference time frame (review PowerPoint notes from the first day of class and also your textbook, pages 3 through 5, then pages 19 through 23).

If it is true that rocks cannot completely serve to this purpose, it is also true that we observed that the natural laws of chemistry and physics do not change over time (Principle of Uniformitarianism, by James Hutton). If we only use rocks to depict the passing of time and the sequence of events that happened during that time, Earth History could be represented by a cycle, or a series of different cycles (albeit far from perfect ones)

What really has changed for good over time is life: because of evolution by natural selection, life moves as an arrow. Life forms have transitioned from a fauna of very simple chemosyntehtic, prokaryotic, single-celled, mitotic Archaea and Bacteria to today’s complex ecosystems that rely on energy coming from the Sun (through photosynthesis performed by algae and plants) and include plenty of eukaryotic, mostly multicellular, meiotic Protists, Fungi, Animals and Plants.

The remnants of these creatures, and of their activities, found in the rock record are called fossils. Fossils outline the path that life forms took over time on Earth. Many important fossils (index fossils) or fossil assemblages (associations of fossils during certain time periods) are used to “characterize” specific time intervals of Earth History.

These time intervals are organized hierarchically, and always listed from older (oldest) to younger (youngest): there are four Eons (Hadean, Archean, Proterozoic, Phanerozoic), and Eons are further subdivided into Eras, Eras into Periods, Periods into Epochs, and Epochs into Ages.

For instance, the Phanerozoic Eon is subdivided into the Paleozoic, Mesozoic, and Cenozoic Eras. The Cenozoic Era is subdivided into the Paleogene and the Neogene Periods. The Paleogene Period is further subdivided into the Paleocene, Eocene, and Oligocene Epochs, and each of these Epochs into smaller time units called Ages.

How do you prepare for this first quiz?
You will start by studying your PowerPoint Introduction slides and your main textbook (Earth System History), pages 3 through 5 and 19 through 23; memorize the subdivisions (and the names) of the Geologic Time Scale from the figure above. Please refer only to this illustration and NOT to the ones that you will find in any of your three textbooks.

What do you need to know in particular?

The information on the Introduction PowerPoint notes
All of the names from the figure above, Eons, Eras and Periods, and the Epochs of the Cenozoic.
Notice that in the illustration above only the Phanerozoic Eon is subdivided in smaller time intervals, and only the Cenozoic Era is further subdivided into Epochs with a specific name. You need to know all of those names.
Some of the names might appear very minuscule on a small screen, so I am copying them here for clarification
- The Carboniferous Period is split into two distinct Periods in North America only, named (older to younger) Mississippian and Pennsylvanian
- The Paleogene Period includes three smaller rank Epochs, called Paleocene, Eocene, and Oligocene
- The Neogene Period includes two smaller rank Epochs, called Miocene and Pliocene
- You do not need to know other unit names you might have heard of, and that are not listed here, such as Quaternary, Pleistocene, and the recently introduced Anthropocene
The order in which these units follow each other (which is older, which is younger, what Period is part of what Era, etc.)
The numerical age of the subdivisions between Eras (in bold in the figure)

You will be quizzed on the basic concepts from the PowerPoint notes and the book, on these names, their relative order (for instance, is the Jurassic older or younger than the Paleocene? Is the Paleocene an Eon, a Period, or neither, etc), what is the age at the contact between Archean and Proterozoic, etc.

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