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Cabinets of Curiosities at the Old State House
Back in the old days, before the idea of museums being educational centers became commonplace, most museums were more like some rich person’s trinkets collected during vacations packed into glass cases so people could see them. They rarely had signs telling people what anything was, nor was there oftentimes any attempt at organization. Because of the rather haphazard and all-encompassing nature of the displays, they got the name Cabinets of Curiosities.
As time went on, they became more thoughtful and organized, with more attention being spent on telling people what the objects were, teaching people about them, and properly preserving the objects for posterity.
The Old State House Museum in Little Rock honors that tradition with an exhibit called “Cabinets of Curiosities: Treasure of the University of Arkansas Museum Collection.” I am sad to say that the exhibit has been up since March 11 of 2017, but I have only recently learned about it and have gone to see it. Truth be told, the only reason I found out about it was because I went to the University of Arkansas at Fayetteville to see the Hazen Mammoth fossils and found that the lower jaw of that mammoth was on display in Little Rock.
The reason I am sad to say this is because the exhibit is well worth your time to go see. If I had known about it earlier, I would definitely have discussed it here by now. I was told it will still be at the museum for another year, so you still have time. It is free, so go. Spend a Saturday downtown and explore the exhibit. Then go see what else is down there, which is plenty to keep you busy over the weekend.
What will you see when you are there? To begin with, you might want to ask directions from their helpful and friendly staff, because the exhibit is upstairs in a side gallery that you might miss if you didn’t know it was there.
The objects are from the University of Arkansas Museum at Fayetteville, which, sadly, closed its doors in 2003. As a result, if you want to see their collections, you have to make an appointment to see the warehoused collections. Or you can see a small part of them in this exhibit. The museum had collections from all over the world covering many subjects, which the exhibit honors by having selections of a wide range. It is like a mini-museum of anthropology and natural history.
I am a fossil guy, so I am going to focus on the fossils you can see. The fossils are mostly in the first two rooms that you enter in the exhibit. In the first room, under the horns of a large cervid (the family containing deer, elk, and moose), are two cabinets of invertebrate fossils and minerals. There are some nice fossils to see. You can see a crinoid with the calyx, or body, and pinnules, or “fronds”. These are much rarer than the pieces of stalks we normally find throughout the Ozarks. There is also a starfish, ammonoids, and nice plant fossils, among others.
To the right of this display is a cabinet containing the skulls of a musk ox and Smilodon (saber-toothed tiger), a couple of large, straight-shelled nautiloids, and one of the bones of Arkansaurus, which up until very recently, was the only known dinosaur found in Arkansas.
On the other side of the room, you can see the bones of an elephant leg and a cabinet full of geology specimens, including one enormous quartz crystal that would not be allowed as a carry-on for some planes because it is too big to fit.
The next room contains case with the leg bones of a camel, lion, dog, and cat. It also contains the pieces of mammoth that I came to see. But before you get to that case, you have to pass a giant clam that could fit a child, or even a small adult if they curled up.
The other side of the room contains the skulls of a bison and rhino, a whale rib, and an entire icthyosaur from Germany. The end of the room has the skulls of a bear and walrus, complete with tusks, and the skeletons of a bat and snake.
The final part of the fossil and biology section that I was most interested in was an old display of horse evolution. If made today, it would probably look substantially different because of all the new data we have gotten since that display was made; but except for a few very minor details, the essential facts would be the same.
Of course, there is much more to the exhibit than what I have presented here, such as this cool whalebone armor and cavalry sword, so spend some time checking out the rest of the exhibit and the museum.
There is one thing that the exhibit could have capitalized on, but didn’t (mainly because they didn’t know about it, but also because they were trying to stay with the whole cabinets of curiosity motif), was that many of the fossils and biology specimens have fossil counterparts that have been found in Arkansas. For instance, we have fossil Smilodons, musk oxen, whales, several mastodons, giant deer and giant snakes, and much more.
So here is my question to you. How many people would pay for a special tour of the exhibit that discussed all the cool Arkansas fossils that matched up with the ones on display here. How much would you pay? $10/person? More? Less?
Buffalo National River: Boone Formation
If you are looking for a great place to begin your canoeing experience, or just a quiet river to float down with great views, you can’t go wrong with the Buffalo National River in Arkansas as it flows through the Ozark and Boston Mountains. In 1972, Congress declared the Buffalo to be a National River, the first river to be so designated in the Unites States, which protects it from industrial use and any construction that might change the natural character of the river. It is renowned for its clean water and spectacular bluffs. People come from all over to camp in the park, hike its trails, and float the river. Much of the river is easy to float, so a welcome adventure for novices, although the upper reaches can be challenging.
This is the first of many posts about the geology of the river and the fossils that can be found in the park. Please note that this is a national park, so collecting fossils within the park boundaries is strictly prohibited. However, many of the formations discussed herein can be found throughout large portions of the Ozarks, so if you want to collect fossils, consult a geologic map and find a road that runs through the formation outside the park to find suitable roadcuts. Fossil collecting is allowed on state land, so just make sure you are not in a national park, national forest, or on someone’s private land (unless you have their permission).
The Buffalo river cuts through several formations which are mostly Ordovician or Mississippian in age (~470 to 320 Mya). You can find geologic maps in pdf format of the Buffalo National River here and here.In the western reaches, the primary formation is the Everton Formation, but in the central and eastern portions of the river, the Boone Formation dominates. There are numerous bluffs displaying thick sections of the Boone.
The United States Geological Survey describes the Boone Formation as “mainly finely crystalline limestone with some cherty limestone and interbedded chert and minor shale. Approximately 400 ft. maximum thickness.” There is a lot of limestone in the Ozarks, but the nodules and thin beds of chert make the Boone stand out from the others.
It is early Mississippian in age, although exactly how old is a bit debateable. The USGS lists it as being in the Meramecian/Osagean stages, which places it mostly in the Middle Missippian. However, the Arkansas Geological Survey says it is in the Kinderhookian/Osagean stages, which are mostly early Mississippian. These stages are regional North American names, so you won’t find them on standard geological time scales meant to be used globally. At any rate, the Boone formed approximately 340-359 million years ago.
During the Paleozoic Era, the ocean had several cycles of raising and lowering sea levels. During the time the Boone Formation was forming, the region was a near shore marine environment, which explains the limestone and shale. The chert has typically been ascribed a biogenic origin, possibly the result of blooms of diatoms and radiolarians, both of which are single-celled organisms that make shells from silica, rather than the more common calcium carbonate which helped form the limestone. These organisms have also been presumed to form the Arkansas novaculite, a formation of metamorphosed microcrystalline quartz that reaches up to 900 feet in thickness. However, recent work indicates that both the Boone chert and the novaculite were formed from volcanic ash, created by an island-arc volcanic chain that existed about where the Ouachita Mountains are today.
Northern Arkansas is known for its widespread karst topography, meaning it has a lot of sinkholes and caves, most of which are in the Boone Formation. The cave systems are so extensive that at periods of very low flow, the entire Buffalo River is swallowed up and becomes subterranean in a few areas. The Boone forms the ceiling of the most famous cave in Arkansas, Blanchard Springs Caverns, which are well worth visiting if you find yourself in northwest Arkansas. On a side note, you may find references to the Boone in Blanchard Springs being as young as 310 million years old, but with better refinement of dating techniques and better dating of the rocks, that date has been pushed back.
The next posts in this series will cover the fossils that have been found in the Boone Formation. Stay tuned.
Holiday Fun in Arkadelphia
Wow. I can not believe that I have not posted anything here since Halloween. My New Year’s Resolution is to not let that happen again. I have no excuses. But as was said on the Syfy show The Expanse, “We can not change the things we’ve done, but we can all change the things we do next.”
For this post, I want to relate the trip I took to Arkadelphia just before the Christmas holidays to visit the Goza Middle School on the invitation of one of their science teachers, Trent Smith. That trip will benefit many people in the future, and it also provided a chance to see some Arkansas geology and paleontology that may prove interesting to fossil enthusiasts.
This all started with an email I got from Trent Smith, who had found some fossils he wanted help identifying. After looking at the attached photo, I tentatively identified most of them as specimens of Exogyra ponderosa, a common oyster from the Cretaceous Period. There also appeared to be a goniatite ammonoid, a Cretaceous Period cephalopod, or squid relative. I could not be sure just from looking at the pictures, so I offered coming down to take a look at them in person. Trent was amenable to that and after a few emails back and forth, we arranged to not only look at his fossils, but talk to his eighth grade science class while I was there. It turned out that the school was interested in me talking to multiple classes, which all told was about 160 students. They suggested I could either give one talk to all of them at once, or I could do multiple talks to individual classes. I much prefer the smaller groups where people can get a more hands on experience with the fossils and have more opportunity for students to ask questions, so I opted to give several talks. I wound up giving seven talks, with two of the talks to combined classes. So I had the opportunity to speak with a lot of students.
When I got there, Trent helped me bring in my boxes and took me to his room to start setting up. Goza Middle School students are fortunate to have great science teachers who are passionate about science and education. Trent’s classroom fossil collection was by far the largest fossil collection I have ever seen in a public school classroom. They have a good variety of most of the invertebrate fossils that can be found in Arkansas. They also had a fabulous nautiloid ammonoid 4″ across or more. I had a shell of a modern Nautilus, a genus of the only extant ammonoids, so the students were able to compare a modern version with one over 70 million years old.
For each class, I gave a short introduction to the fossils that can be found in the state, which is much more diverse than most people realize. I also gave them a quick demonstration of the immense expanse of time we were discussing. I have a timeline that stretches eighteen feet and covers 600 million years. People are usually suitably impressed with that timeline, but when I tell them how much space our civilization represents on the timeline, they are stunned. At that scale, all of human recorded civilization is approximately the width of one human hair. Afterwards, we let the students look at the fossils I brought and ask questions. The students were more reluctant to get out of their seats and approach the front table than the younger kids I usually talk to, which I found interesting and speaks to how quickly we train our students to sit and listen without interaction. But once they got over their training, they enjoyed being able to handle the fossils and examine them close up. The students were uniformly polite and well behaved and were a pleasure to talk with. Midway through, the teachers treated me to a tasty potluck lunch.
If everything was left at that, it would have been a great trip and I would be happy to return, but they really went above and beyond. In addition to lunch and a small donation (I have generally not asked for payment for classroom visits in the past and as a result, getting paid for it almost never happens, but getting paid means I can go to more classes so is greatly appreciated), they provided me with even more. They gave me my first two Paleoaerie shirts, which they designed and they did a fantastic job. On the front of both shirts is a dinosaur foot that looks like the foot of Arkansaurus, the only dinosaur bones ever found in the state, and my name, Dr. Daniel. On the back of one shirt, it has the dinosaur foot with the words PALEONTOLOGY above it and DIGGING UP KNOWLEDGE below it. On the back of the second shirt, it says PALEOAERIE.ORG followed by my three statements of what guides my efforts: The universe is endlessly amazing, knowledge is useful only when it is shared, and you can’t really know something unless you understand how and why we think we know it. The shirts are going to be my uniform for future talks.
After school was over, Trent showed me a spot he has collected fossils from on Wp Malone Road, just west of I-30. According to the Arkansas Geological Survey’s geologic map of the Arkadelphia quadrangle, the area is listed as being in the Nacatah Sand, an Upper Cretaceous formation consisting of a mix of unconsolidated sediments deposited in a nearshore marine environment. However, the marl, a limey clay, we found in the creek looked more like it came from the Marlbrook Marl, a formation that lies underneath the Nacatah and separated from it by the Saratoga Chalk formation. The Saratoga Chalk is not thick in this area, so it is quite easy to go from the Nacatah to the Marlbrook in a very short distance. In this particular locale, the Marlbrook is close by and it is likely that what we found was washed downstream to where we found it. As I recall, Trent mentioned that fossils were more common the farther upstream one went, which would support this idea. The Marlbrook Marl, when fresh, is a blue-gray lime clay, or marl, laid down in nearshore, shallow marine environments, just like the Nacatah Sand, but without the sand contribution. The upper part of the Marlbrook is also famous for being extremely fossiliferous and this site was no exception. I initially attempted to collect what I found, but very quickly realized there were so many shells that it was impossible to carry them all. The great majority of what we found were shells of Exogyra ponderosa, but the numbers would have allowed us to quickly fill a crate with specimens. We also found a few snail shells (of what type I am not sure) and a terebratulid brachiopod, but the numbers of everything else did not begin to compare with the shells of Exogyra. On other trips, Trent collected numerous Exogyra shells and gave me two boxes full of shells. Thanks to him, I will be able to supply many Arkansas classrooms with actual Arkansas Cretaceous fossils.
This area is a nice place to collect. As long as one is on public land (or with the permission of the land owner), you can collect any of the invertebrates you want, so you can feel free to collect Exogyra shells here. But the Marlbrook also contains more than just oysters, brachiopods, and snails. It has also yielded mosasaurs and even the occasional elasmosaur. There is even the possibility that a dinosaur was washed out to sea and could be found there. So if you collect in this area and find some bones, give me a call.
Many thanks to Trent Smith and the whole of Goza Middle School, not just for your hospitality, but for living the statement of Dr. Scott the Paleontologist on Dinosaur Train: “Get outside, get into nature, and make your own discoveries.”
Mystery Revealed: A Common Coral in Arkansas
It is the unfortunate fact of life that volunteer efforts are all too often derailed by other pursuits. Such is the case for last week’s Mystery Monday fossil. Nevertheless, the answer shall be forthcoming. If you have been paying attention to the Facebook feed, you will know that the fossil presented last Monday was identified. Were you able to figure it out?
This is a large, very well preserved piece of tabulate coral. Corals are colonial species that are very important in modern ecosystems. A fourth of all ocean species live within these reefs. They form the backbone of reefs that are among the richer areas of biodiversity on the planet. Billions of dollars each year are pumped into local economies across the world.
What we think of as coral is mostly the calcareous homes they form, within which the animals live. The actual animal is a tiny animal in the Phylum Cnidaria. Cnidarians are soft-bodied animals, the best known of which are the jellyfish and sea anemone. Cnidarians take two general forms. Medusae are free-floating forms like the jellyfish. Coral and sea anemones are polyps, mostly stationary, or “sessile”, forms that remain in place their entire lives. Corals, like other cnidarians, are predatory, catching their prey with tentacles armed with nematocysts, cells containing potent poisons to immobilize or kill their prey. Of course, since corals are tiny creatures themselves, they prey on even tinier prey. The tentacles surround an opening which serves as both mouth and anus, basically making the animal a living, carnivorous sack. This is not the only way corals get food though. Most modern corals also have a symbiotic relationship with single-celled algae called zooxanthellae, which provide essential nutrients for the coral in which they live. Unfortunately, when the coral gets too stressed from increasing temperatures or other causes, they tend to respond by evicting the zooxanthellae. Because the zooxanthellae are what gives corals their bright colors, this is known as coral bleaching.
While there are several different kinds of coral, most of the coral people are familiar with are the stony corals, or Scleractinia, because these are the ones that build the reefs. They are part of the larger group of corals known as Hexacorallia (at least, if you are talking to modern biologists, paleontologists often restrict Hexacorallia to scleractinians), known for often having the individual coral homes partially divided with six partitions, or septa (although you may be hard pressed to identify the three axes forming the six partitions even if they are present in that number).
The scleractinians have only been around since the Mesozoic however. They did not build the coral reefs of the later Paleozoic Era. That distinction goes to the rugose, or horn, corals and the tabulate corals, such as the example above. Tabulate corals are known for the corals being aligned in horizontal stacks. The image above should really be rotated 90 degrees to get the life position. This stacking always reminds me of apartment building, particularly cheap tenement housing, or wire mesh. According to phylogenetic studies on modern corals, it appears that the earliest scleractinians did not have zooanthellae, the symbiotic relationship evolving later, so it seems likely tabulate corals didn’t either. Tabulate corals appeared in the Ordovician Period roughly 450 million years ago. They started dying out in the Permian and finally succumbed to extinction at the end of the Permian period 252 million years ago, along with most other life on the planet. However, it is a bit misleading to say they went extinct. It is thought that the modern scleractinians that arose in the early Triassic are descended from tabulate corals, so they appear to have evolved, rather than just died out.
If you want to find corals such as this in Arkansas, one need only travel anywhere in most of the northern part of the state. The Ozark Mountains are predominantly formed from shallow marine Paleozoic rocks. Anywhere you find limestone in the Ozarks, keep your eyes peeled for samples of this type of coral. They are invertebrates, so as long as you are not collecting in a National Forest or private property without the owner’s permission, you are free to collect them.
Fossil Friday, it’s a crabby day
Were you able to figure it out? Congratulations to Showmerockhounds for getting it right.
This picture shows the carapace of a decapod crustacean, the group that includes crabs, crayfish, lobsters, and shrimp. More specifically, it is Imocaris tuberuculata, a crab generally considered to be in the group Dromiacea, within Brachyura. The name means crab from the Imo Formation, which is where it was found by Frederick Schram and Royal Mapes in a roadcut along I-65 near Leslie, AR. The rocks around Leslie are a great place to hunt for invertebrate fossils, numerous specimens have come from there. Imocaris is very rare, but quite distinctive, with a carapace that looks like a frog-headed bodybuilder wearing enormous sequined parachute pants.
Imocaris is an intriguing fossil in that the Imo Formation is thought to be Carboniferous in age, in the Upper Mississippian Period roughly 320-330 million years old. Even though the fossil record of decapods goes back to the Devonian Period, few exist in the Paleozoic, not really hitting their stride until the Mesozoic Era. The fossil record of crustaceans as a whole go all the way back to the Middle Cambrian over 500 million years ago, with specimens found in the Burgess Shale. Thus, the true origin of the crustaceans must be even earlier than that, probably some time in the early Cambrian or the Ediacaran, the latest stage of the preCambrian Era.
The Arkansas Geological Survey calls the Imo Formation as a member of the Pitkin Limestone Formation. The Imo is a shale layer interspersed with thin sandstone and limestone layers found nearthe top of the Pitkin Formation. The Imo, and the Ptikin in general, demonstrate a shallow marine environment indicated by the limestone and an abundance of marine fossils. Most of the fossils are invertebrates showing off a thriving coral reef system, but you can also find conodonts and shark teeth as well. The Pitkin Limestone sits on top of the Fayetteville Shale, itself well known for fossils, particularly cephalopods. The boundary between the two can be seen along I-65 closer to Marshall.
Frederick Schram & Royal Mapes (1984). “Imocaris tuberculata, n. gen., n. sp. (Crustacea: Decapoda) from the upper Mississippian Imo Formation, Arkansas”. Transactions of the San Diego Society of Natural History 20 (11): 165–168.
Last Week’s Mystery Monday Revealed: Getting to the Root of the Problem
Were you able to figure out last Monday’s mystery fossil? For a reminder, here it is again.
Two people correctly identified it as a plant fossil. While both guesses were fossils that are found in Arkansas in similar places and times, the Natural Historian identified this as a Stigmaria. Technically, Stigmaria is a “form taxon”, meaning that it is named for the shape and not the actual organism, but in general, the only ones that really get called Stigmaria are root casts of lycopsid trees. The two main ones are Sigilaria and Lepidodendron. This particular one is Lepidodendron, which is the typical one found in Arkansas.
Lycopsids like Lepidodendron lived during the Carboniferous Period from about 300 to 360 million years ago, so named because this was the time of extensive coal swamps. Coal swamps, as the name suggests, were responsible for most of the coal we find. During this time, organisms capable of digesting lignin, a chief component of wood, had not yet evolved and spread sufficiently to make a dent in the decaying logs. Lignin is a tough fiber, so without organisms capable of breaking it down, it tended to last for a long time, so decaying plant matter built up, eventually being compressed into coal. Genetic studies indicate that the enzyme to digest lignin first appeared around 300 million years ago, which likely not coincidentally marks the beginning of the end for coal swamps, which by and large died out in the Permian Period, not long after the end of the Carboniferous.
Lycopsids today include the quillworts, spike mosses, and club mosses, although Lepidodendron is most closely related to the quillworts. Today, these plants are small and serve mostly as ground cover. In the Carboniferous, they formed towering trees reaching over 40 m tall (just for comparison, the average oak tree is no more than 20 m, although they can get up to 30 m tall). They also grew very quickly, reaching maturity in only a few years, which likely also contributed to the massive buildup of decaying plant matter. Lepidodendron literally means “scale tree”, so called for their scaly appearance. They have occasionally been mistaken for fossils of snake or lizard skin. Personally, they remind me of giant pineapples.
Lepidodendron can be found in most of the Pennsylvanian age rocks in Arkansas, although the most common place is in the upper Atoka Formation in the Boston Mountains and the Arkansas River Valley through the northern section of the Quachitas. The Atoka Formation is a series that represents deep marine sediments at the base of the formation gradually turning into deltaic deposits in the upper sections. There are several layers of coal, coally shale, and oil shale. I have even seen a few spots in which the amount of oil in the shale is enough to smell it and the rock can catch fire. This region has not been extensively used by the coal and oil industries because it is prohibitively expensive to extract the oil from the shale and the coal is high in sulfur, making it less than optimal for use. But if you are looking for plant fossils in Arkansas, it is the place to go.
The Cambrian of Arkansas
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)!