Through plate tectonics, Earth's continents move around, merging together and rifting apart. We see evidence of two supercontinents – that's where all the continents are stuck together in one big landmass, right here in the eastern United States. Most recently, Pangea separated 200 million years ago. Its predecessor, Rodinia, separated some 6 to 7 hundred million years ago, forming the Iapetus Ocean, in the same relative spot as the modern Atlantic.
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Locally, Rodinia's breakup is recorded in a package of rocks called the Chilhowee Formation, which stretches from Alabama to Pennsylvania.
What we're seeing with the Chilhowee is the sediments coming off the continents, filling that coastal line there, and then sea level rising.
As we go higher in the sequence, we would go through several places where we could track the beach, seeing where the sea level was right at that place in time in that part of North America.
If you could slice the the rock layers of Chilhowee - like a 200 kilometer thick cake - you would see this line of sandstone, made from ancient, pristine coastal beaches, cutting diagonally and rising toward the west, tracking sea level rise.
When sea-level rises, that shoreline would move inland, and you would have a shallow ocean. When you don't have any more sediments coming off the continents in a warm shallow ocean, you'll start to get limestone reefs forming.
Just like modern coral reefs. But there was something else going on at that time.
This is not just the breakup of Rodinia, it's also crossing an important zone of evolution of multicellular life. So, in the oldest parts of the Chilhowee, we have some of our very first multicellular, soft-bodied organisms. By the time we get all the way up to where this carbonate bank (the reef) is forming, life is getting more complex. Also they start to form more and more skeletons, and it's more like that that those fossils will be preserved. So, it's kind of a double signal here. It's life evolving, but also life becoming more preservable, and so it's those limestones (Again, the reefs) above where we start to get a well-known fossil record.
But the one they found was special.
It's not what we expected to find. It's smaller and a bit flatter. And the family name is called Nevadiidae, named after Nevada. One of the questions that this trilobite is helping to answer is whether or not this part of the Chilhowee Group had always been part of North America. Our continent has been growing on both sides through geologic time, so through plate tectonics, when you have collisions with other, either micro-continents, or islands, or sometimes you'll collide with a continent, and not break up on the same line, and you'll leave part of the continent with you. And this trilobite goes a long way to say “yes, it has been.” In other words, we find it way on the west coast, and it's not similar to ones we could have found over in South America or Africa or something like that. So, it's not an exotic form that's been added on. It looks like this whole form has been part of North America this entire time.
A fossil of an organism that lived during in a very short and specific period of time, like the Nevadiidae trilobite for example, is called an index fossil. If you have one, you can safely put a rough age to the rock unit you found it in.
This helps place us precisely in the sequence of time. The big part of it is trying to put an age date on these rocks that don't have many fossils in it at all. This trilobite fossil is just a very nice punctuation that helps us nail down a time correlation. With using other radiometric age dates, not on the trilobite, but on rocks that are above and below that, we can correlate that this comes out to right about 517 million years ago.