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.
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.
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.
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.
Tomorrow is the day to see tons of fossils at the Museum of Discovery. Come out and see what Arkansas has to offer in fossils. We have a far more diverse array of fossils than most people realize. Here are a few dinosaurs you will see.
Here are some casts of Allosaurus bones, a foot and a humerus. Allosaurus itself didn’t live here, but its descendant, Acrocanthosaurus, did.
Here is an ischium of an Apatosaur, which did not live here either. But its titanosaur relatives did.
Here are some hands and claws of Allosaurus and Utahraptor, a giant version of Velociraptor.
Not interested in dinosaurs? How about a Smilodon, one of the biggest and most iconic of mammalian predators?
Of course, what I have shown the past few days is only a few things that the University of Arkansas Earth Sciences Department is bringing. The Anthropology Department will also be there with their own fossils. The Arkansas Geological Survey will be there with a cast of the dinosaur, “Arkansaurus” and more. The Virtual Fossil Museum will be present with more than virtual fossils. Not to be outdone, the Museum of Discovery will have a collection of dinosaur skulls on display, which will be part of an upcoming dinosaur exhibit which you won’t want to miss.
The National Fossil Day event at the Museum of Discovery is almost here.Here are a few more things you might see at the event. Here is a cast of a coelacanth, a lobe-finned fish that is closely related to the fish that started the trek onto land, and a mesosaur, an early reptile before reptiles and mammals went their separate ways.
Want something with a bite to it? How about some teeth? Like Megalodon teeth, and a nodosaurid dinosaur, and how about the tooth of a giant ground sloth? What about primitive horse and rhino teeth?
Lots more to see. Come out and have a look. Bring your fossils and compare.
Here are some brachiopods you will see at the National Fossil Day event. They look much like clams, but are unrelated.
Here are some bryozoans, which look like they should be related to coral, but are thought to be more related to brachiopods because they share the same odd feeding structure.
Speaking of corals…
One of the common fossils you can find in Arkansas are cephalopods, which are all the squids, octopuses, and in terms of fossils, ammonoids. Arkansas has some very large ones. If one goes to Northeast Arkansas and looks in the Fayetteville Shale, one can find ammonoids with shells several feet long. Here is a preview of some of the fossils you will see Saturday.
Come back tomorrow for more fossil previews. Come to the museum to see much, much more.
National Fossil Day is October 15th, but the Museum of Discovery, in conjunction with the Earth Science and Anthropology departments at the University of Arkansas at Little Rock (UALR), the Arkansas Geological Survey, the Arkansas State University Museum, the Virtual Fossil Museum, and others (including of course, me) will be putting on an exhibit on October 11th. If you are in the neighborhood, please stop in. there is much to do and see for everyone from toddlers to grandparents and professional researchers.
This week I will be sharing a few photos of the collections at the UALR Earth Science Department as a preview of things you will see. We will start with echinoderms today. These include crinoids, sea urchins, starfish, and an array of others.
So were you able to figure out what Monday’s fossil was? Congratulations to Herman Diaz for not only correctly identifying it, but the relative that lived in Arkansas as well. If you want to see it in person, along with much more, come out to the National Fossil Day event at the Museum of Discovery in Little Rock.
This is the foot of Allosaurus fragilis, which means “different delicate reptile”. It has gone by a lot of names. Depending on the researcher you ask, Antrodemus, Creosaurus, Labrosaurus, Epanterias, and Saurophaganax are all just versions of Allosaurus.
Because of this, it can be difficult to say just how big Allosaurus was. The most famous specimen called “Big Al” is actually on the small side, measuring only about 7.5 m (25 ft), whereas the American Museum of Natural History has a specimen that is almost 10 m (33 ft.). The fossil called Epanterias is a good 12 m (40 ft.). Thus, Allosaurus may have been essentially the same length as Tyrannosaurus rex, although it was more lightly built, so it would not have weighed as much. We should keep in mind though, that every species has a fairly wide range of sizes, so even when we can measure a bunch of living specimens, stating an average size has to come with wide error bars, so take these measurements with a grain of salt.
Allosaurus itself never lived in Arkansas. But a close relative of it did. Acrocanthosaurus is classified as a Carcharodontosaurid, which is a group that is generally considered to by descended from earlier allosaurs. Allosaurus himself lived in the late Jurassic Period, whereas Acrocanthosaurus lived in the early to middle Cretaceous, so the timing lines up with what we know. No bones of Acrocanthosaurus have ever been found in Arkansas, although they have been found in Texas and Oklahoma. What we do have in Arkansas is their tracks. A large trackway was found in Howard County in 2011. This trackway was mostly footprints of large sauropods such as Sauroposeidon, but it also contained theropod trackways, which were identified as being from Acrocanthosaurus.
Acrocanthosaurus was as big as the biggest allosaurs and was known for unusually long spines on the vertebra, especially over the ribcage. Why did it have the spines? While they weren’t as tall as Spinosaurus, but they were longer than would be necessary for strictly muscle attachment, so the best explanation was that it formed part of a display to make it look bigger and more impressive. It was the biggest predator around, so it likely was not for defense, but to intimidate rivals and impress potential mates.
As the biggest predator around, it preyed upon the main herbivores of the day, which in this case were probably juvenile or elderly sauropods (the healthy adults were likely immune from predation simply on account of size). The skulls were more lightly built than tyrannosaurs, so they probably did not munch through bone like tyrannosaurs. They were apparently more selective in their eating. This would have made them very popular with the scavengers of the time as they would have left more behind.