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On Monday, I said that the mystery fossil this week takes us back to the beginning and ties us to the present. I also said the answer was written in stone. Could you figure it out? Katharyn D. was the first to get it this week.
The picture shows a graptolite. This particular one is likely a species of Cyrtograptus. Most specimens of this type of graptolite are from Europe, they do appear in Canada and parts of the United States, including Oklahoma, according to Fossilworks.org. Did it appear in Arkansas? We don’t know. Other graptolites did, but then the book of Arkansas graptolites has not yet been written, so the true diversity of graptolites within the state is not really known.
So what are graptolites? The name means “written in stone” because they reminded people of hierogylphs or petrographs, writing or pictures scratched into stone. The first graptolites I remember seeing in the Quachita Mountains reminded me of nothing more than pencil scratches. Others, better preserved, look like saw blades or little tubes. For a long time, no one really knew what to make of them, regarding them as cnidarians, or plants, or even inorganic mineral formations. But with the advent of electron microscopy, most workers have come to the conclusion they are actually members of the group Hemichordata. This conclusion puts them at the very beginnings of all vertebrates. Vertebrates evolved from a group called urochordates, the first animals with a stiffened rod for support, an ancestral spine. Today, urochordates are tiny animals called tunicates, or sea squirts. But even before these animals evolved, there were the hemichordates, meaning “half-chordate.” They share branchial opening, or “gill slits,” a collar-like pharynx, and the beginnings of a notochord, called a stomochord. The main hemichordates alive today are called enteropneusts, or acron worms. If you go any earlier than this, you find yourself in echinoderms.
Graptolites appeared in the Cambrian Period over 500 million years ago and were thought to have died out in the Mississipian Period (Early Carboniferous), roughly 320 million years ago. They are useful as index fossils, meaning they can be used to date rocks with a fair degree of precision, because they have a worldwide distribution and are common in the rocks, as well as evolving quickly so they have many species, many of which did not last long.
Graptolites started off growing on the ocean floor, but later ones floated freely in the ocean. When they died, they would sink to the bottom, becoming entombed in the deep ocean sediments. So today, graptolite fossils are usually found in shale formed from the deep ocean mud. Sometimes they can be found in marine limestone or chert, but they are less commonly found there. As a result, the best place to find them is in the shales of the Ouachita Mountains in west-central Arkansas. The best place to find them is in the Womble Shale, a black shale with thin limestone layers and a few small silty sandstone and chert layers. The Womble Formation was named after the town of Womble, which is now Norman, in Montgomery County, AR, so that makes that area a fine place to look. You can also find conodont fossils commonly in those rocks, which are tiny, tony, early chordates. Conodonts were the first animals in the vertebrate lineage with mineralized tissue. They had teeth, but no bone. Other places you can find them are the Arkansas Novaculite in the Ouachitas, although you will have better luck in the older rocks, such as the Mazarn or Collier Shales. You can also find them in any of the Ordovician or Silurian aged Ozark limestones in northern Arkansas, but they will be harder to find as those rocks were formed in shallower water, with many more fossils of many other animals which are far more common, whereas the graptolites will be much rarer than in the Ouachitas. For a full listing and description of the appropriate rocks and maps to their location, try the Arkansas Geological Survery website here.
I mentioned that graptolites were thought to have died out in the Mississippian Period. That is because no fossils are found after this date. However, it is thought by most workers that graptolites may still be living today. We just call them pterobranchs, another type of hemichordate and are acorn worm-like animals with plant-like fronds used to filter out plankton from the water.
Despite the snow, we didn’t get a chance to have any other posts this week other than the Monday Mystery fossil. We did, however, have three different school trips in the past couple of weeks to talk to kids about fossils, dinosaurs, and the skeletal system, as well as giving talks on the fossils telling us about the origins of crocodiles and dinosaurs, as well as attending a talk on the origins of birds. So a lot of paleo work, just not much showing up here. Fortunately, some of you had some time to examine our mystery fossil and congratulations to Laurenwritesscience for coming up with the correct answer.
It is indeed a stromatolite. Bruce Stinchcomb has a video on Youtube showing several examples of Ozark stromatolites and providing a good explanation of what they are.
Essentially, stromatolites are microbial ecosystems, built up of layer after layer of microbial mats. The general description is that of blue-green algae, which forms a sticky layer over the surface of a rocky surface in a shallow marine or coastal environment. Blue-green algae are not actually algae and are better referred to as cyanobacteria. These bacteria are photosynthetic, just like plants, so they need sunlight, thus limiting the depth at which they can be found. Actually, they are typically found right at the water’s edge in the tidal zone. This sticky substance, while maintaining their hold on the rock, also tends to collect sand, clay, and organic debris. Over time, all the stuff that sticks to the mat blocks the sunlight from the cyanobacteria and they migrate above the layer and build another mat, which collects more debris, which causes them to build another mat, etc. Stromatolites form much the same way as piles of laundry. By the time you finish washing one set, there is another pile forming in a neverending stream. The life of a cyanobacteria in a stromatolite is a depressing condition of always digging themselves out from under a pile just to get dumped on again. I am sure most people can empathize.
The sticky mucus (properly referred to as extrapolymeric substance, or EPS for short, but we can go with mucus here) forming the mat does more than just cause things to stick to it. The mat protects the bacteria in from ultraviolet radiation. It also allows the bacteria to control the microenvironment around them, keeping such things as pH levels in a good range. It also has an unfortunate aspect for the bacteria. The mucus allows the levels of calcium and carbonate ions to build up until they precipitate out of the water as calcium carbonate, also known as calcite (when referring to the mineral), or limestone (when referring to the rock). So not only are the poor bacteria constantly getting buried, they are getting turned to stone in their very own medusa nightmare. Life is hard as a cyanobacteria. But just wait, it gets worse.
These microbial mats are not just cyanobacteria, though. There are lots of other organisms that live in and on them. There are many other types of bacteria. There are sulfate reducing bacteria, which use sulfur like we use oxygen, only they release hydrogen sulfide instead of carbon dioxide, causing a nice rotten egg smell. There are purple sulfur bacteria that eat the hydrogen sulfide, as well as colorless sulfur bacteria that eat both the hydrogen sulfide and the oxygen released by the cyanobacteria, thus free-loading off of everyone. In addition to bacteria, there are plenty of prokaryotes (organisms without nuclei that holds their DNA) and eukaryotic (with nuclei) single-celled and multi-celled organisms living in the mat. Diatoms, single-celled photosynthetic organisms that grow their own shell, live on top, while nematodes burrow through the mat. In addition to all this, a wide variety of animals love to chow down on the mats. Everything from snails, sea urchins, crabs, crawfish, and just regular old fish happily eat them. As a result, there are not a lot of places left in the world you can find stromatolites growing. The Bahamas and Shark’s Bay, Australia are the best areas to find them.
They may be rare now, but at one time, they ruled the earth. As some of the oldest living communities in the world, they have been around for at least 3.5 billion years (that’s 3,500,000,000, or roughly 600,000 times the length of human civilization) and for more 2/3 of that time, they were the only game in town and in all probability served as the cradle for all eukaryotic and multi-cellular organism on the planet. These days, if you live in Arkansas, the only places you can find them are as fossils in the Cambrian age Cotter Formation and Ordovician age Everton Formation in the Ozark Plateau.
For further information (and the source of the images shown here), check out the stromatolite page at the Arkansas Geological Survey and the Microbe Wiki stromatolite page, as well as the Microbes.arc.nasa.gov site, which supplies a nice teacher’s guide to teaching all about microbial mats, designed for grades 5-8.
Welcome back! I hope everyone had a great holiday to mark the end of a great year. 2013 marked the inaugural year for Paleoaerie. Version 1 of the website was set up, providing links to a wealth of online resources on fossils, evolution, the challenges of teaching evolution and the techniques to do it well. The blog had 26 posts, in which we reviewed several books and websites, discussed Cambrian rocks in Arkansas and the dinosaur “Arkansaurus,” and went to the annual meeting of the Society of Vertebrate Paleontology. We looked at geologic time and started a series on dinosaur misconceptions. We also had several Forum Fridays, recapping the many news stories reported on the Facebook page. One of the recent things we’ve started is Mystery Monday, posting a fossil of the week for people to try to identify. Speaking of which, to start off the new year, the first mystery fossil will be posted early. look for it at the end of this post.
In the upcoming year, we hope to expand the site, providing many more resources, along with continuing posts on Arkansas geology and fossils, including many more mystery fossils. Stick with us and you will learn about the history of Arkansas in a way that few people know. The site will be revamped to be more user-friendly and enticing to visitors. If plans materialize, we will be adding interactive activities, animations, and videos, many of which will be created by users of the site. Materials from workshops and talks will be posted for people to view and use. More scientists will be posted that have offered their services to teachers and students. We encourage you to contact them. They are there as a resource.
Of course, all of this does not come free. it takes money to provide quality services. Thus, more avenues of funding will be pursued, including other grant opportunities and likely a Kickstarter proposal. You may soon see a small button on the side of the website for Paypal donations. Any money donated will go first towards site maintenance. Other funds will go towards a student award for website design, a 3D laser scanner to put fully interactive 3D fossil images on the site, and materials for review and teacher workshops. If grant funding becomes available, additional money will be spent on research into the effectiveness and reach of the project. But even if no more funding becomes available, you can still look forward to continuing essays on Arkansas fossils, reviews of good books and websites, and curation of online resources suitable for teachers, students, and anyone else interested in learning about the endlessly fascinating history of life on planet earth.
I mentioned at the beginning about the latest mystery fossil. Here’s the first hint: it is a very common fossil found in Arkansas and lived during the Mississippian period roughly 330 million years ago. More hints and photos to come. Leave your guesses in the comments section. Don’t worry about getting it wrong, every success has lots of failures behind it. Errors are only stepping stones to knowledge.
Clue number 2: Many people think I’m a coral, but I’m not.
Clue number 3: I am named after a famous Greek mathematician and inventor.
What am I?
Our tour of Arkansas fossils and geology should begin, like any tour, at the beginning. The oldest rocks found in Arkansas in which fossils may be found were formed in the Cambrian Period, the earliest part of the Paleozoic Era. When the Paleozoic Era was first named, it began with the rocks containing the oldest known fossils. We now know of fossils far older than that. Nevertheless, it marks a good starting point for rocks in which fossils become commonly found and are easily recognizable. So while Arkansas does not have the earliest fossils, we do have fossils dating back through most of the history of life once hard parts developed.
The Cambrian Period started about 540 million years ago and lasted until 485 million years ago. During that time, while the land was mostly barren, the seas were full of life. Much of what people know about the Cambrian comes from the Burgess Shale in Canada, possibly the best known example of a lagerstätten, a fossil site rich in either fossil diversity or exceptional preservation, of which the Burgess Shale has both. From the Burgess Shale and other localities, we know that the Cambrian saw the rise of most of the major groups of animals we see today. In addition to the comb jellies, sponges, algae and anemones, brachiopods and bristle worms, velvet worms and crinoids; the Cambrian also arthropods of several kinds, most in particular the trilobites, the first chordates like Pikaia, and bizarre creatures like Anomalocaris and Hallucigenia.
The rise of such a diversity of animal life during the Cambrian has been termed the Cambrian Explosion, leading some people to assume it appeared suddenly and without precedent. In truth, the Cambrian “explosion” took tens of millions of years and was preceded by a diverse fauna known as the Ediacaran or Vendian fauna, which first appeared almost 100 million years earlier. The end of the “Garden of Ediacara” and the rise of the Cambrian fauna is thought to have come about due to the evolution of the first predators, necessitating hard shells for defense and hard claws and teeth to kill prey.
The only place in Arkansas to find Cambrian rocks is in the Collier Shale, which was formed in the Cambrian through the Lower Ordovician.
Outcrops for the Collier Shale are limited to a small set of ridges in the Ouachita Mountains, within Montgomery County between Caddo Gap and Mt. Ida, just to the east of state Highway 27. However, most of this area is part of the Ouachita National forest and is ILLEGAL TO COLLECT anything without a permit.
The Collier Shale is a large unit at least 1000 feet thick formed mostly of gray to black clay shale that was intensely crumpled during the formation of the Ouachitas. Interspersed within the shale are thin layers of black chert, which together indicate a deep water environment. However, there are also thin layers of dark gray to black limestone, which contain pebbles of chert, limestone, quartz, and even sandstone. It is thought that these layers initially formed in shallower water on the continental shelf before some event caused them to slide off the continental slope into the abyss.
The Collier Shale is not known for abundant fossils, but it does have some. In the Cambrian section of the formation, several genera of trilobites have been found, chiefly of the groups known as Asaphida and Ptychopariida. For more information on trilobites and the different types, try the Fossilmuseum.net and Trilobites.info websites. The trilobite genera found in the Collier Shale have been from what is known as the Elvinia and Taenicephalus Zones. These are specific groups of trilobite genera that, when found together, allow the age of the rocks to be determined using correlative dating. These groups, or assemblages, of genera have been found in other parts of the world in rocks that have been able to be dated using rigorous and independent methods, such as radiometric dating. We know that rocks elsewhere in the world containing these fossils are roughly between 490 and 500 million years old, indicating the rocks forming this part of the Collier Shale are the same age. This conclusion is supported by fossils in the rock units overlying this part of the Collier matching those found in rock units over similar rock units of known age elsewhere. The trilobites in the Collier are found in the lower part of the formation. The upper part of the Collier contains fossils known as conodonts, but they are Ordovician in age and will be discussed later.
Trilobite images from www.fossilmuseum.net and www.trilobites.info. The Cambrian painting by Miller can be found at http://paleobiology.si.edu/burgess/cambrianWorld.html, along with more Cambrian information. The map of the Collier Shale can be found at www.geology.ar.gov and the continental shelf image is from kids.britannica.com.
Hart, W. D., J. H. Stitt, S. R. Hohensee, and R. L. Ethington. 1987. Geological implications of Late Cambrian trilobites from the Collier Shale, Jessieville area, Arkansas. Geology 15:447–450.
Hohensee, S. R.; Stitt, J. H. 1989. Redeposited Elvinia zone Upper Cambrian trilobites from the Collier Shale, Ouachita Mountains, west-central Arkansas. Journal of Paleontology 63(6): 857-879
Loch, J.D. and J.F. Taylor. 2004. New trilobite taxa from Upper Cambrian microbial reefs in the central Appalachians. Journal of Paleontology 78(3):591-602. Online publication date: 1-May-2004.
UPDATE: I thought I would add a little more information about the “Cambrian Explosion,” or as Dr. Donald Prothero calls it, the “Cambrian slow fuse.” The reason for this is because of how long it really took for multicellular life to develop. We have evidence for the earliest life going back over 3.5 billion years, but the earliest agreed upon multicellular life appeared in the Ediacaran fauna (Grypania is a possible multicellular organism dating back 2.1 billion years, but may not be a true multicellular organism and really a colonial organism). The diagram to the right (click to enlarge) is from Prothero’s book, Evolution: What the fossils Say and Why it Matters, and reproduced on a review he wrote of another book. In the diagram, he shows the Ediacaran as starting about 600 million years ago, but now most researchers peg that to about 635 million years ago, so the slow fuse is actually even longer than he shows. The Collier Shale in Arkansas is in the late Cambrian, so as you can see, several other groups are already present. The fact that we have thus far only found trilobites means that we may yet find more diverse types of fossils, so keep looking (and if you find anything, let us know)!