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The After Thanksgiving Mystery Monday
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.
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.
Fuxianahuia. 520-million-year-old crustacean with a heart. https://www.sciencenews.org/article/ancient-crustacean-had-elaborate-heart
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 contact (black line) between the Pitkin Limestone (above) and the Fayetteville Shale (below) along highway 65 near Marshall. Arkansas Geological Survey.
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.
Life in the Carboniferous oceans.
Mystery Monday
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.
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.
Image by Dennis Murphy. http://www.devoniantimes.org/who/pages/lycopsid.html
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.
Upper Atoka Formation in Arkansas. Courtesy of the Arkansas Geological Survey.
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.
Mystery Monday
Halloween is over, time to get back to work for a few weeks before we take off for Thanksgiving. To get your brain back into gear after eating all that candy and waking up from sleeping like a log in your sugar coma, can you figure out what this week’s fossil is?
Check back in next Monday for the answer. I will be at a conference on Thursday and Friday and will likely not be able to post an answer until after the conference is over. Or you can check the Facebook page for a quicker answer.
Mysteries Revealed at a Slothful Pace
What happened to revealing the answer on Friday, you ask? If you follow the Paleoaerie Facebook feed, the answer was given there. Sadly, unexpected illness (as if one can schedule being sick) delayed me from getting a more complete post on the blog. Thus Fossil Friday has preempted Mystery Monday.
With that being said, were you able to figure out what this was?
If you thought they looked like teeth, you are quite right. But the chisel-like teeth are rather peculiar. To give some sense of scale, the teeth are roughly 10 cm long and 3 cm wide. So they are not small. They are hypsodont, meaning high-crowned, like that seen in the molars of horses, which is a good indication of herbivory. The cusps are simple, with only two cusps, unlike that seen in horses and other mammalian herbivores and totally unlike anything seen outside of Mammalia. The teeth are distinctive for particular group of animals, those known as sloths.
Megatherium, Wikipedia.
No, not that type. These teeth belonged to a giant ground sloth. Unlike their modern cousins, giant ground sloths are so named because they lived on the ground rather than in the trees and were very large, capable of standing upwards of 6 m (20 feet) high and weighing, depending on who you ask, 6-8 tons. Ones of this size are known as Megatherium and are commonly found in the La Brea Tar Pits, although they ranged throughout the Americas. The ones found in Arkansas are of the genus Megalonyx (“Great Claw”) were somewhat smaller than Megatherium, only about half the size of its larger kin, and ranged throughout North America.
Megalonyx jeffersonii, or Jefferson’s Ground Sloth, was so named because Thomas Jefferson gave a lecture on this animal in 1797 which is sometimes said to have marked the start of vertebrate paleontology in the United States.
Great American Interchange. Richard Cown http://mygeologypage.ucdavis.edu/cowen/
Giant ground sloths, as a group, originated in South America about 35 million years ago and made their way into North America during the Great American Interchange about 8-9 million years ago (this is often not considered to start until about 3 million years ago, but fossil evidence indicates at least some organisms began the march between the continents much earlier). The Great American Interchange was so named because this marked the time that North and South America became joined, creating a wide corridor through which many plants and animals passed. Interestingly, ground sloths were unusual in that they migrated northward. Most animals migrated south from North America. This was most likely due to the fact that Central America at the time was mostly scrubland, whereas the northern part of South America was covered in lush forests. This meant that those on the southern continent had little incentive to leave. The North American animals, on the other hand, were in areas of limited resources and would have viewed the abundant resources in the south as highly enticing.
Xenarthran joint. Dr. Kaci Thomspon. http://www.life.umd.edu/classroom/bsci338m/Lectures/Xenarthra.html
Another animal that went north during this time was one well known to many people in the southern Midwest, the armadillo. Interestingly, both sloths and armadillos are in the group called Xenarthrans, so-called for an extra articulation in the vertebra. Xenarthrans include sloths, armadillos, anteaters, and an interesting animal called a glyptodont, the mammalian equivalent to the ankylosaur. It had a bony carapace, a tail club, and was the size of a subcompact car. For some reason, the xenarthrans were far more successful moving north than almost any other animal, with the main exception of the giant, flightless terror birds in the group Phorusrachidae, such as Titanis, a bird towering 2.5 m (8 feet) or more tall.
Giant ground sloths lived in a variety of environments, ranging from arid savannas to jungles. Most of them, like Megalonyx, appear to have preferred lush forests along rivers and lakes.
I will end this with a statement of the end of the giant ground sloths. What drove them to extinction? They went extinct 10-12,000 years ago, at the end of the last ice age. This was also shortly after the arrival of humans, who, evidence suggests, hunted them on a regular basis. As large, slow-moving herbivores, they made for easy prey. Unlike other predators, humans could easily avoid the sharp claws and powerful forelimbs of the sloths by attacking with spears. Was it climate change or human overhunting, known as the overkill hypothesis? Many researchers fall on the side of human overkill due to the fact that the sloths had survived several similar periods of climate change without issue and the pattern of extinctions globally does not match well with local climate changes. However, others claim climate change due to the loss of plant diversity after the last ice age, which was not seen in the previous climatic shifts. Which is correct? Perhaps the answer is both. It is not unreasonable to think that the climate change lowered population levels, which ordinarily would have been survivable, but the added pressure of human hunting proved an insurmountable challenge.
Mystery Monday
Time for another Mystery Monday. This one should be easy if you haven’t been taking things too slow lately. The answer will be revealed Friday.
Mystery Monday
It’s Mystery Monday again. Can you tell what large, but delicate creature to which this belongs? If you would like to see it and a whole bunch more in person, come to the Museum of Discovery in Little Rock on October 11th as we celebrate National Fossil Day.
Fossil Friday
Were you able to identify Monday’s fossil? Allie Valtakis was. Find out after the picture what it is. I hope it doesn’t make your weekend too crappy.
These two things are coprolites, otherwise known as fossil poop. Always a hit with kids when I show them in schools, but I always get the same questions. Do they smell? Will I get poop on my hands if I touch it? Most are tentatively reassured when I inform them that to be considered a coprolite, the poop has to be replaced with mineral. After a long period of time, there isn’t any actual poop left.
Dr. Karen Chin. Paleoportal.org
Coprolites can be quite informative. Coprolites preserve traces of what the animal that left it ate, so they can be useful for looking at the diet of prehistoric animals. Karen Chin, a curator at the Colorado University Museum at Boulder, is the leading expert on coprolites, particularly dinosaur coprolites. She found wood In some coprolites found in the Two Medicine Formation in Wyoming, which is unusual for two reasons. One, most coprolites are from carnivores, so herbivore coprolites are relatively rare. Secondly, most herbivores don’t eat wood except as a last resort when no other food sources are available. She was able to tentatively attribute these coprolites to the hadrosaur called Maiasaura (mainly due to the size and content of the coprolites, and the abundance of maiasaur bones in the area), making this the first dinosaur known to eat wood, as well as giving a unexpected perspective on the lifestyles of these “duck-billed” dinosaurs.
Probably the most famous coprolites known are also from the Two Medicine Formation and were also studied by Dr. Chin. They were uncommonly large and clearly from a carnivore. The only known carnivore from that formation big enough to create such a ponderous poop was Tyrannosaurus rex himself.
Tyrannosaur scat
These coprolites told a fascinating story. The coprolites were readily identified as being from a carnivore due to elevated levels of phosphorus, which results from eating a high protein (i.e. meat) diet. The coprolites contained numerous bone chips, indicating that T. rex was not a dainty eater. T. rex had a massively built skull with powerful jaws, providing T. rex with the most powerful bite of any terrestrial animal. It put these jaws to use chomping through a carcass, bone and all. If one compares the thick, broad teeth of a tyrannosaur with the flatter, blade-like teeth and lighter skull of an allosaur, it is clear they had fundamentally different niches and eating styles.
There was bigger surprise found in the tyrannosaur coprolites. Dr. Chin found traces of undigested muscle. Obviously, it was not original muscle left in the coprolites, but mineralized remains. Why is this important? Modern reptiles have a slow metabolism. Food takes a long time to go through the digestive tract. As a result, digestion is phenomenally thorough. Crocodilians can take the enamel off teeth. Mammals, on the other hand, have notoriously inefficient digestive tracts. It is not uncommon to find recognizable bits left in the feces. Because of the elevated metabolism, food simply passes through too quickly for digestion to be complete. Meat is far easier to digest than plant matter, so carnivores, even mammalian carnivores, typically do a good job of digestion. To have traces of undigested muscle in the coprolite of a T. rex means that either the tyrannosaur was terribly sick with a bad case of the runs, or more likely, tyrannosaurs had short digestive times and a high metabolism to go along with it. It is possible to have thorough digestion with a high metabolism, but it is much harder to have incomplete digestion in a carnivore with a low metabolism.
Thus, coprolites not only tell us about the diet of extinct animals, they can also tell us about their physiology.
On the preservation side of things, one may ask how something as soft and squishy as a poop can fossilize. The answer to that is not easily. The vast majority of poops get washed away. But fecal material does have some advantages that help them get mineralized. As I stated earlier, carnivore feces is enriched in phosphorus. Phosphorus is an important nutrient, eagerly sought after by many organisms because it is not all that common in the environment, making it what is known as a limiting resource.
The other advantage is that feces is mostly made of bacteria, not really waste products. Our intestines are populated with microbes without which we can’t digest our food very well. The richer foods we eat, the more the microbes can grow and meat is a very rich food source.So why is having bacteria in the feces an advantage? Because the waste products they give off during their metabolic processes cause minerals to precipitate around them. Those bacteria are in a phenomenally rich food source in the poop, so they are growing like crazy, which means they are also precipitating minerals like crazy. In effect, they fossilize the poop while they are trying to eat it. If the poop can stay together, is not disturbed, and there is sufficient water around to allow the continued growth of the microbes, you will get a coprolite. The problem with this of course, is that poops are rarely left alone. Other animals eat them, dung beetles carry them off, they get stepped on and spread about, and rain washes them away.
If you have a kid interested in learning more about coprolites, I recommend the book Dino Dung, by Karen Chin. The book is written for elementary school kids, but is packed with a lot of good information on the study of coprolites and provides a great introduction to the study of fossil poop.
Mystery Monday
Time for another Mystery Monday fossil. Spend a little time to digest the image, then tell us what you think it is.
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