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Yearly Archives: 2014
This will be the last post of the year. This year we have looked at four books, a couple of movies, 12 websites, three evolution myths, 34 fossil organisms, and a few miscellaneous educational topics. Next year, we will start off with the second set of Aliki books. More reviews will be done, including books, videos, and websites. If you have something you want reviewed, examined, or discussed, please let me know and it will put put in the queue. We will also get back to the geology posts and get more myths covered, as well as a few posts on the best ways to teach hostile topics.
In the meantime, enjoy the holidays, whether that holiday is Christmas, Hanukkah, Yule, or just a Festive Festivus. Whatever your particular stripe, have a great time for the last week of the year and we will see you in 2015. We’re halfway out of the dark (to shamelessly steal a line).
Next week is Christmas, Hanukkah started this week, there is Boxing Day, Yule, Kwanzaa, even Festivus and Hogswatch, not to mention the old classic Saturnalia and a whole host of others. Busy week for those wanting to celebrate. In honor of that, I came as close as Arkansas fossils allow to a well-known, traditional, seasonally-associated animal. Were you able to figure it out?
If you guessed reindeer, you were wrong. Sadly, there is no record of reindeer ever having lived in Arkansas that I can find. If there were, I would have used it. So no skeletons of Rudolph for us. The closest thing to a reindeer that has been found in Arkansas are fossils of the common white-tailed deer, which is so common in the state that it not infrequently becomes one with motor vehicles, much to the dismay of both deer and driver.
So what could this be besides a deer, reined or not? Reindeer are in the genus Rangifer, which are in the family Cervidae, along with deer. Cervids are artiodactyls, mammals best known for having cloven hoofs, thus the term “even-toed ungulate.” Unfortunately, Arkansas is not really known for artiodactyls either, other than pigs, and somehow, pigs did not seem an appropriate holiday animal. So what to do?
There is another animal that is often associated with the holidays, especially in the Christmas tradition, that being the donkey. I am sure you’ve heard of the story of Joseph leading a donkey upon which rode Mary to Bethelem and what Nativity scene is complete without a donkey? Donkeys, of course, are in the same genus as the horse, Equus, which are perissodactyls, the odd-toed ungulates. So allow me to introduce you to Scott’s Horse, Equus scotti, named after the paleontologist William Berryman Scott, a Princeton paleontologist known for his work on Cenozoic mammals.
Almost everyone is familiar with horses today. They stand as an iconic symbol of the Wild West, an integral image of the American cowboy and the Native Americans that roamed through the plains. Horses are also one of the most commonly used examples of evolution, with the line from Hyracotherium to Equus in virtually every evolution textbook ever written. All the discussions talk about how they got bigger and lost most of their toes as adaptations for running, and grew higher-crowned teeth to deal with the tough grasses they started to eat that replaced the softer, lush forest plants.
What is less well known outside of those who study evolution and paleontology is that this process was not a straight chain from tiny, forest-dwelling horse ancestor to the modern horse. The horse lineage diversified, evolving into multiple niches. This shouldn’t really be too much of a surprise, considering the diversity seen in horses today, with everything from burros and Shetland ponies to Clydesdales and zebras. Most of them died out before the modern horses we see today arrived. Scott’s horse was one of these extinct forms.
Another thing that is not well known outside of paleontologists is that the modern horse originated in North America, but are not the ones living here today. Horses evolved during the Pliocene, five million years ago. Adaptations allowed them to survive the change from forests to more open, grassy plains, driving their evolution. From North America, they spread into South America through Central America and into Asia and Europe across the Bering land bridge. The Bering Sea Straits were dry at this time because the ice ages during the Pleistocene lowered water levels, allowing passage between the continents. Horses, along with many other animals, like mammoths and camels (also originally American), crossed through the land bridge to populate lands on either side.
At the end of the ice ages about 11,000 years ago, every species of the American horse, including E. scotti, died out, along with all of the other megafauna. Horses continued to thrive in South America and Eurasia, but for over 10,000 years, their North American homeland was barren of horses. It was not until the Spanish conquistadors brought them back that horses once again thrived in North America. Thus, we can thank the Spanish for bringing back a quintessentially American product.
This will be the last mystery fossil for the year. After this, I, like hopefully everyone else, will be enjoying the holidays. Arkansas does not have any fossils that are terribly well associated with any of the late December holidays, but I got as close to one as I could. It is closely related to animals living here today, but it died out long before its modern relative was reintroduced. Put your guesses in the comments section and check back Friday for the answer.
Aliki Brandenberg, known mostly simply as Aliki, has written several popular books for children in the Let’s-Read-And-Find-Out Science series published by Harper Collins. Among these books are ones about fossils and dinosaurs written for 5 to 9-year -olds (I think 4-8 would be a better range, as many 4-year-olds will like the books and most nine-year-olds will have moved on to books with more information). When they came out in the 1980s, they were widely regarded as excellent books for children. The books were voluminously illustrated with colored pencil drawings of fossils and people studying them. The main text was supplemented with word balloons for the human characters, supplying interesting tidbits and additional information, so should not be ignored. Unlike many books of the time, these were about as accurate as one could expect to get without going into so much detail that a person of that reading level would feel overwhelmed.But it has been 25 years or more since then. We’ve learned a lot since then. How have they held up? Surprisingly well, for the most part, better than the majority of books published at the same time. I will review four of them here, two in this post and two in a following post. Some people might find the reviews a bit lengthy, so here they are in a nutshell: still good reads for kids, even better with a few additional comments to update them and correct a few misconceptions that kids might get from the simplicity needed to pare down complicated subjects into something that would fit the space constraints and interest levels.
Fossils Tell of Long Ago
Publication Date: 1972, revised 1990.
Harper Collins Publishers. ISBN: 978-0-06-4455093-5
AR Book Level: 3.6
Fossils Tell of Long Ago endeavors to explain what fossils are, how they form, and what they can tell us. In a quick 32 pages, Aliki provides a wealth of information well written for the intended reading level of early elementary kids.
Fossils starts off describing what fossils are and how they are formed. The description of the fossilization process is simplistic and doesn’t get into the microbes precipitating minerals around the bones during decomposition, but that was not known when the book was written and the description in the book is sufficiently accurate for the level of reader at which the book is aimed. I do like the use of the famous Xiphactinus fossil as the lead example as it is a fascinating fossil in its own right and thus a good fossil with which to hook readers. Aliki’s description of coal as a fossil is great. She does a good job of introducing different types of fossils, even including different pieces of information that may be gleaned from fossil footprints.
Aliki then goes on to talk about mammoths in ice, amber, and how fossils can tell us about the environments when the rocks were deposited, introducing many more types of fossils along the way. She ends the section by reinforcing the utility of fossils to tell us about past environments and organisms that no longer exist, even putting in a plug for museums.
The book ends with showing how to make your own fossil track and thoughts about how people in the future may interpret it. Best of all, she ends on a positive, encouraging note that anyone can find fossils, even the kid reading the book, and discover something no one else in the world knows. And that is a powerful motivator.
All in all, the Fossils book stands up very well and can still be recommended as a great book for kids.
Publication Date: 1988 (Amazon lists the publication date as 1990, which differs from what is printed in the book).
Harper Collins Publishers, ISBN: 978-0-06-445077-5.
AR Book level: 3.7
“Dinosaur Bones” tells about the early history of the study of dinosaurs and briefly discusses dinosaurs and the world of the Mesozoic. This book does not hold up quite as well as the “Fossils” book and shows its age by being out-of-date in some places, but is still reasonably accurate a good read for young kids. It provides an interesting glimpse at the beginnings of the modern studies of dinosaurs (dinosaur bones have been found for millenia, but modern scientific study is much more recent) and a very brief introduction to dinosaurs and their world.
This book, like almost every other book, has a European bias. On the very first page, it says the Dr. Robert Plot was the first person to describe a dinosaur bone in 1676. He had no idea what it was and he described it as possibly a giant human thigh bone or some other such animal. He was hardly the first to find dinosaur fossils and try to describe them though. Native Americans, ancient Greeks, and many others found them far earlier. They just did not recognize them as dinosaurs. Fossil Legends of the First Americans and The First Fossil Hunters:Paleontology in Greek and Roman Times, both by Adrienne Mayor, are filled with accounts of early fossil hunters.
The book begins by describing how she was introduced to dinosaurs and her curiosity about how scientists know what we do about past life, which she begins to answer by talking about people finding fossils. The book provides an excellent short history of the early scientific study of dinosaurs by Europeans, hitting all the famous highlights. The best part of this section is her emphasis on early ideas changing with new fossils and new data. She presents dinosaur paleontology as a dynamic process, with ideas being revisited and revised in the face of new evidence, which is a great thing to put into a book for kids.
The book then delves into the world of the dinosaurs, showing that the world was much different than it is today. I like that this was included and I realize there were space limitations, but I have a small problem with this section. The Mesozoic Era, what is commonly known as the time of the dinosaurs, lasted for over 200 million years. That is a huge time. In general, the description of the continents being joined together into one land mass was accurate for a good bit of that time, but it broke up during the Mesozoic,which had important effects on the evolution of the dinosaurs. The temperatures were also only warm everywhere, as stated in the book, if one considered temperatures warmer than current “warm.” Neither the Arctic nor Antarctic were covered in glaciers, but it was still cold enough to snow and reach frigid temperatures at night in the poles. Basically, it is not possible to compress the diversity of climate and landforms of 200 million years across the entire world into two pages and six sentences. But given that constraint, she did the best that could be done. At the very least, she presented the concept of great changes in the globe over great expanses of time, which is a substantial achievement for a book aimed at elementary kids.
Following this section are two pages describing how fossils are formed and geologic time. She mentions the important concept of dinosaurs evolving. For the space available and the intended audience, the book does remarkably well. For the purpose of just introducing the concepts to kids, they are handled succinctly and clearly. The biggest place where it falls down is saying that scientists tell time by looking at the order of the fossils. This is indeed one way, but if that were the ONLY way, it would be a circular argument. You can’t use the fossils to date the rocks and the rocks to date the fossils at the same time without additional evidence. This method also only provides relative dating, there is no way to really tell how old the rocks and fossils are this way, only the order they were laid down. There are some rocks though, such as ancient lava flows or ash beds, for which we can get absolute dates using radiometric techniques. Between the two dating methods and comparing rock units from different areas to each other, we can get reliable dates for all the rock layers. Having said that, the major geologic time units were devised by looking at the order of fossils. It was only later that we learned how to provide the absolute dates, which told us how old the rocks really were. I would have preferred a simple change of wording to say that finding fossils is ONE of the methods scientists use to tell time and not make it look like it is the only way. The change may not look like much, but it really does make a big difference and many kids will pick up on the distinction so long as adults don’t give them misinformation.
The final few pages describe the history of the dinosaurs in a few sentences. The Triassic Period is done well for the allotted three sentences and the illustrations provide examples of some of the dinosaurs. The only problem here is the description of Heterodontosaurus, which is out of date (for cool information on this unusual animal, go here and here… no, really, check it out).
The Jurassic Period is a bit problematic in that it has the giant, long-necked sauropods tromping around what look to be swamps and dragging their tails, which is no longer considered accurate. Interstingly, all the carnivores are shown in dynamic, tails-up poses. The Cretaceous Period starts with saying “dinosaurs had taken over.” Dinosaurs were dominant throughout the Jurassic Period, long before the Cretaceous. The dinosaurs are also drawn too much in the old, upright positions. More than any other page, this one looks like a throwback to an earlier artistic era. In the entire section, the dinosaurs are drawn very simply and generically, despite the fact that they are named with specific names.
The final page starts with “Then suddenly, they all died out. No one knows why.” This is followed by several things scientists don’t know about dinosaurs. By and large, it is true, but we have made great progress and can now provide at least partial answers to all of them now. We now have some very good ideas about why they died out. There is also considerable debate about how “suddenly” it was. Most notably, dinosaurs didn’t all die out, just most of them. Birds are directly descended from the Mesozoic dinosaurs and are the most diverse group of vertebrates that live on land. Scientists are also making strides to answer the final questions the book states about their colors, what sounds they made, and their metabolism. While there are still many gaps, we have made much progress on those questions. As a result, I would recommend that anyone reading this book to kids mention how old the book is and that a lot of work has been done since then to find answers to those questions, but there is still much more to do.
Were you able to figure out what this ancient skull belonged to?
It looks for all the world like a bird, but birds don’t have teeth, do they?
Certainly not now, but early in avian history, they did. Teeth are but one of the many pieces of evidence that connects them to theropod dinosaurs like Velociraptor and Deinonychus. This particular bird was named Hesperornis regalis, the “royal western bird”. It lived in the Late Cretaceous, at the same time as such famous dinosaurs as Tyrannosaurus rex and Triceratops and marine reptiles like mosasaurs and elasmosaurs.
The picture of the skull above was published by Othniel Marsh in 1880. The skeleton Marsh described and several other specimens show that Hesperornis was a diving bird, much like grebes, loons, and some rails and cormorants. Like the flightless cormorant, Hesperornis had very small wings and lacked the ability to fly. Some diving birds, like penguins, use their wings to “fly” through the water, but Hesperornis, like its modern counterparts, used their feet to propel themselves. Its feet were likely lobed, like grebes, rather than fully webbed like most aquatic birds.
Hesperornis was a large bird, standing close to a meter (3 feet) tall and 1.8 meters (6 feet) in length. Its beak was long and pointed, with teeth on its maxilla and all but the tip of the mandible, or lower jaw. According to work by Tobin Hieronymus, the parts of the jaws with teeth were covered in feathers, with keratin covering the toothless portions. It lived in coastal waters, diving for fish and trying to avoid the aquatic reptiles that were the apex predators of the time.
In Arkansas, Hesperornis has been reported from the Ozan Formation in Hempstead County, a series of mostly sandy, limey mudstone, typical of warm coastal marine areas. As one would expect, given Hesperornis‘s aquatic nature, all the other fossils found with it represent marine animals, including sharks, bony fish, turtles, mosasaurs, and pliosaurs. During the Late Cretaceous, when these sediments were deposited, southwest Arkansas was at the eastern edge of the Western Interior Seaway, a marine environment created by high sea levels that flooded much of the central United States.
This find represents the southernmost extent of Hesperornis’s range, which extended up into the Arctic. It was a lot warmer then, but still cold at the poles. It should be kept in mind though, that the find consists of one partial bone, the left tarsometatarsus, part of the lower leg. It is easily recognizable as avian and has been identified as Hesperornis due to its age and size, although Dr. Larry Martin stated it could be a new taxon. We will just have to wait until more fossls turn up to know for sure. Parts of the Ozan Formation are quite fossiliferous, so there is a chance that more will be found.
I hope you had a Happy thanksgiving, filled with all the food and family you could stand. Get back into the swing of things (before you take another break for Christmas) with another Mystery Monday fossil. Here’s a hint to get you going: while it may pose a passing resemblance to a modern animal, this one hasn’t lived since dinosaurs were walking around Arkansas. Come back Friday for the answer.
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.
If the cold and the ice (or lack of ice for those of you hoping for a day off from school) has gotten you in a bad mood, see if you can distract yourself with solving this week’s mystery fossil. This week I am presenting something totally unlike anything I have presented before. The scale bar in the photo is 5 mm.
I was talking with someone at an educator’s conference of all places that asked me how, if evolution was true, could monkeys still exist. Surely evolution meant that if we evolved from monkeys, that monkeys should no longer be around, right? This is a common misconception. I might even say it is the most common misconception I run across. It is so common, in fact, that paleontologists really get tired of hearing it because it means the person does not understand how evolution works at all. Matt Bonnan, a well respected paleontologist that mostly works on the giant, long-necked dinosaurs called sauropods, in a period of frustration, penned this cartoon. In it, he explains the first thing I ask people: Did your parents die when you were born? Did your parents become you? No, of course not, that’s silly. So why would you think that an entire species has to go extinct when a new one evolves?
Evolution generally works the same way. Just as your parents and their siblings continued to exist after you were born (barring tragic occurrences of course), species seldom evolve all at the same time in the same way. There are almost always some populations of a species that do not noticeably evolve, at least, not in the same way. As a result, one can usually find populations of the parent species long after new species evolve. We see the same thing in TV shows. Just because CSI spun off CSI:New York and CSI: Miami, that did not mean the original show was cancelled. All three happily continued to air alongside each other.
This is pretty obvious when you think about it in other contexts. So why do people think evolution works differently? It all has to do with an old belief called the Great Chain of Being. While it had its origins as early as Aristotle’s Scala Naturae, it reached its heyday in the medieval Catholic Church. This chain organized all of existence into a hierarchical system with rocks and minerals at the bottom and God at the top. Some of them got very detailed, even putting different minerals in order. Of course, all of them had humans as the pinnacle of earthly creation, with the nobility and kings above all as the highest order of Man. This type of classification still holds sway to this very day for many people for a very powerful reason.
For people with a hierarchical worldview, this is very appealing. Everything has its place and the Great Chain of Being strictly lays out who has authority over who. It is neat, tidy, and ordered. It also feeds into normal human desire to feel special. It enshrines human exceptionalism into the very soul of the worldview.
And this is where people run into a brick wall with evolution. Evolution tears apart this neat, ordered hierarchy. Life becomes messy, without clear boundaries and order. Most importantly, evolution, as they see it, makes no distinction between humans and the rest of the system of life. It puts humans as just another branch on the great family tree, with nothing inherently special putting humans on top and that simply will not do. I do not mean for this to be taken as derogatory of any particular religious belief or religion in general. The need for order and security and to feel important and with a purpose are understandable human desires with a rational basis at their core that we all feel to some extent. Unfortunately, when applied to natural history, all available evidence indicates it is wrong.
So if evolution does not work this way, how then does it work? How should evolutionary lineages be considered and how do species evolve? Let’s tackle the latter question first and then see how it looks on a grander scale.
There are many ways organisms can create new species, or speciate. But there are two main patterns. One is similar to the incorrect view of evolution shown above, in which a species gradually evolves into another species as a unit due to changing environmental conditions, such that the original species and the new species are only in existence together for a brief transitional period. This is called “anagenesis”. This sort of speciation has been proposed most commonly in groups like ammonoids (Mesozoic shelled squid-like creatures) and foraminifera (a large part of plankton in the oceans). On a side note, this sort of speciation plays havoc with interpreting the fossil record because it creates what is known as “pseudoextinctions”, in which the original species appears to have gone extinct when in reality it just changed.
The other pattern, and the one that is considered far more common, is called “cladogenesis.” In this case, different populations of the same species evolve into two or more species. Much of the time, populations of the original species stick around and can do so indefinitely, so long as the environment allows.
There are several ways cladogenesis can happen. Most commonly, populations get separated by some sort of barrier, like a river or mountain, possibly a volcano. It can be anything, so long as it prevents breeding and thus gene flow between the populations. This can even happen if populations diverge from each other by specializing in different parts of the same area. Probably the most famous examples of this are the cichlid fish in Africa. They have formed numerous species by specializing in different microhabitats within the same geographic area.
Another way cladogenesis can occur is by what is known as “peripheral isolates”. In this situation, you have a generalist species that has a wide range due to its ability to fit into multiple niches. However, subpopulations can evolve to specialize into the different niches to the point they no longer breed with the main ancestral species. This form of cladogenesis is very similar to the ones listed above. In a way, it can be considered a combination of them, with subpopulations adapting in different geographic areas.
The important point in all this cladogenesis talk is that at no time is it ever required that the original species go extinct. They can and often do at some point, but cladogenesis does not involve a species changing into another, it involves a species splitting up into multiple species. Much like parents are free to live their own lives once the children leave the nest (and even while the kids are still at home), once a species splits, each new species follows its own evolutionary path that is separate from the original species. In short, there is no linear chain of species going from one to the next.
So what does this look like over geologic time? It looks something like a branching tree (or, if it happens over a short time, like a bush). The ancestral species forms the trunk at the bottom and each branch represents another split, until finally, you reach modern day, represented by the leaves.
It is often argued that this does not get rid of the line of species from distant ancestor to descendant. If you look at the family tree of horses, for example, you can still draw a line straight from Hyracotherium all the way to the modern Equus. It is true, you can. However, it is not a straight line. It proceeds in fits and starts, with many branches, most of which die off. It follows environmental changes and interactions with other organisms. Here is the big kicker though. All the species sharing a common ancestor that survive at any given point of time have all evolved equally. It may not be apparent, some species may show far more changes than others, but all of them have experienced the same evolutionary time. There is no hierarchy of dominance in evolutionary terms. Every bacteria has experienced the same amount of evolutionary time as the lineage that led to humans. One can draw a straight line from a root of a tree to any leaf, but one would be hard-pressed to claim that single leaf is more important than any other leaf. So if you are looking for a reason to support human superiority, evolution will not help you. As a result, evolution can be a serious blow to the human id.
To those people who have problems with evolution on this basis, I ask you to consider that simply because bacteria have experienced the same time for evolution, that does not diminish human accomplishments. We have built communities that span the globe, we have explored the edges of the solar system and beyond. We have accumulated vast stores of knowledge that we have preserved for our descendants. We have glimpsed the inner workings of life itself. We have witnessed the awe-inspiring glories of the universe. If you need something to take pride in humanity, do not look to our evolutionary heritage, look to what we have achieved that is unmatched (as far as we know) by any other organism. As we so often tell our children, it is not where we came from that determines our worth, it is who we make ourselves to be.
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.