Today is National Fossil Day™. The National Park Service holds this annual event on the second Wednesday every year to coincide with Earth Science Week sponsored by the American Geosciences Institute. Earth Science Week highlights the important role of earth sciences in our everyday lives and “to encourage stewardship of the Earth.” National Fossil Day is, as NPS says, “held to highlight the scientific and educational value of paleontology and the importance of preserving fossils for future generations.”
In honor of the day, I am going to give you a whirlwind tour of some of our most outstanding fossils from all over the state. People may not think of Arkansas as being rich in fossils, but we have a rich natural history spanning 500 million years. To give you a quick summary of the wide array of fossils, just check out the map on the fossil page, reproduced below.
The most fossiliferous region in the state is the Ozarks, without a doubt. It is a favorite fossil collecting spot for many people, even though much of the area is national forest or national park owned, which prohibits fossil collecting. Nevertheless, fossils may be collected on any roadcut. I-65 near Leslie has several fossiliferous roadcuts. You are most likely to find abundant examples of crinoids, bryozoans like the screw-shaped Archimedes, clams and brachiopods, ammonoids (mostly goniatites), corals such as horn corals and tabulate corals, as well as the occasional echinoid and trilobite, along with many other types of fossils. This list of fossils makes it plain that the Ozarks are dominated by marine deposits, but you can find the occasional semi-terrestrial deposit loaded with plants like Calamites and Lepidodendron.
Top, left to right: Calamites, spiriferid brachiopod, blastoid echinoderm, goniatite ammonoid. Bottom left to right: Archimedes bryozoan, crinoid with calyx and fronds (very rare, mostly you just find pieces of the stalk).
There are a few fossils that particularly stand out. One is Rayonnoceras, a nautiloid ammonoid, which reached lengths of over two meters, making it one of the longest straight-shelled ammonoids ever found. The other is a shark named Ozarcus. While shark teeth are common, it is rare to find one that preserves parts of the skull and gill supports. At 325 million years, Ozarcus is the oldest one like this ever found and it changed the way we viewed shark evolution, indicating that modern sharks may be an offshoot of bony fish, not the other way around.
We can’t leave the Ozarks without talking about Conard Fissure, a spectacular collection of Pleistocene fossils. Barnum Brown excavated the first chamber of the cave in 1906, pulling out thousands of fossils or all kinds, many of which were new to science. Of course, of all of them, the ones that most people remember were 15 skeletons of Smilodon, the largest of the saber-toothed cats. The one pictured to the right is a cast of one from La Brea, California. All of ours are held at the American Museum of Natural History.
The Ouachita Mountains are not nearly as fossiliferous, but they have two important types of fossils that are commonly found: graptolites (below left) and conodonts (below right, not from AR, Scripto Geologica). Graptolites are thought to be closely related to pterobranchs, which are still living today, even though the graptolites themselves are all from the Paleozoic Era. Most of the time, Graptolites look like pencil marks on slate, but if you find a good one, you can see they are often like serrated files that may come branched or coiled. The reason these are important is because they are hemichordates, the closest group to the chordates, all animals with a spine (either a stiff rod or actual bone). Conodonts, on the other hand, are the closest we have to the earliest vertebrates, looking like nothing so much as a degenerate hagfish.
The coastal plain is quite fossiliferous and has attracted the majority of press because it is here where you will find Cretaceous aged rocks and that means dinosaurs and their compatriots. Here you will find thousands of Exogyra oysters. Scattered among them, you can find numerous shark teeth, along with teeth from Enchodus, the saber-toothed herring (although not really a herring), especially if you look in the chalk beds. You can also find the rare example of hesperornithids, extinct diving birds, as well as fossil crocodilians.
But of course, the main draws here are the marine reptiles and the dinosaurs. Mosasaur vertebrae are not uncommon, although the skulls are. More rarely, one can find plesiosaur (the article only mentions elasmosaurs, which are a type of plesiosaur, but most plesiosaur fossils in Arkansas cannot be identified that closely) vertebrae as well.And then of course are the dinosaurs. We only have a few bones of one, named Arkansaurus, but we have found thousands of footprints of sauropods, the giant long-necked dinosaurs. Since the sauropods that have been found in Texas and Oklahoma are titanosaurs, such as Sauroposeidon, it is a good bet the footprints were made by titanosaurs. A few tracks have also been found of Acrocanthosaurus, a carnivorous dinosaur like looked something like a ridge-backed T. rex. Acrocanthosaurus reached almost 12 meters, so while T. rex was bigger, it wasn’t bigger by much.
Top left: Mosasaur in UT Austin museum. Top right: Plesiosaur vertebra from southern AR. Middle left: reconstruction of Arkansaurus foot. Middle right: statue of Arkansaurus (out of date). Bottom left: Sauropod footprints. Bottom right: Acrocanthosaurus footprint, Earth Times.
The eastern half of the state is dominated by river deposits from the Mississippi River, so the fossils found there are mainly Pleistocene aged, with the exception of a few earlier Paleogene fossils near Crowley’s Ridge. Pleistocene deposits can be found all over the state, as they are the youngest, but are most common in the east. In these deposits, a number of large fossils have been found. A mammoth was found near Hazen, but we have almost two dozen mastodons scattered over the state. I already mentioned Smilodon, but we also have , the giant short-faced bear, dire wolves, giant ground sloths, and even a giant sea snake named Pterosphenus. Most unusual of all is a specimen of Basilosaurus, which despite its name meaning king lizard, was actually one of the first whales. Considering the month, I would be remiss not to include Bootherium, also known as Harlan,s musk ox, or the helmeted musk ox.
Top left: Mastodon on display at Mid-America Museum. Top right: Basilosaurus by Karen Carr. Bottom left: Arctodus simus, Labrea tar pits. Wikipedia. Bottom right: Bootherium, Ohio Historical Society.
This is nowhere near all the fossils that can be found in Arkansas, but it does give a taste of our extensive natural history covering half a billion years. After all, we wouldn’t be the Natural State without a robust natural history. Happy National Fossil Day!
Monday I posted a set of pictures showing an Arkansas fossil. Were you able to figure it out. Check below for the answer.
This skull and mandible comes from the Madrean Archipelago Biodiversity Assessment (MABA) website. I couldn’t find a good picture of an actual fossil, so I used this modern example instead. Below is a living version.
The skull is that of Myotis leibii, the eastern small-footed Myotis. Myotis bats are also called mouse-eared bats, the most famous of which is the little brown bat, Myotis lucifugus. The other fossil bat in Arkansas is the big brown bat, which is not in the genus Myotis at all. It is in the genus Eptesicus (E. fuscus specifically).
I have talked about E. fuscus before, where I talked a bit about bats in general. I didn’t go into their phylogeny at all, so I will talk about that here. Bats as a whole belong to the order Chiroptera, which is the sister group to a group called Fereuungulata. That group includes artiodactyls, cetaceans (whales and dolphins), carnivorans, and pangolins. Altogether, Chiroptera and Fereuungulata form the horribly named Scrotifera. Why do I say it is horribly named? Besides the fact that naming such a large group after scrotums is a bit odd, take a look at the simplified mammal phylogeny illustrated by Darren Naish.
Notice what is NOT in Scrotifera. That’s right. Primates, such as us. Yes, we are more closely related to rats and squirrels than we are to bats, dolphins, or cats and dogs. We are also not included in the group named for a feature we possess.
Both Myotis and Eptesicus are Vesper bats, meaning they belong in the family Vespertilionidae, along with over 300 other bat species. When it comes to diversity, mammals could easily be described as rodents, bats and their less common relatives, seeing as how those two groups include 60% of all mammals. Vesper bats are in the suborder Microchiroptera, the micro bats. The other suborder, Megachiroptera, is composed of the fruit bats like the flying foxes. The two suborders are rather lopsided in numbers, with just under 200 species in Megachiroptera and over 1000 in the Microchiroptera. This is the traditional classification at any rate.
There is another phylogeny that splits it up slightly differently and gives them different names. Megachiroptera has become Yinpterochiroptera and includes the horseshoe bats in the group called Rhinolophoidea as well as the lesser and greater false vampire bats in the genus Megaderma. Everything else that was in Microchiroptera is in Yangochiroptera.
So returning to the vesper bats, these include most of the bats people are likely to run into, which is why the bats in this group are sometimes called common bats. Most of the bats in this group have rather plain faces and are insectivores. Myotis leibii itself belongs in the group Myotinae, marked in the red box in the phylogeny below, which was also put together by Darren Naish.
The interesting thing about this is that Eptesicus, the big brown bat, is in the serotine clade, up near the top of the tree and quite a distance away from Myotis, the little brown bat. Eptesicus is also sometimes called a house bat, but the house bats are in the group Scotophini, which while still in Vespertilioninae, is not closely related within the group. This is part of the reason common names can get confusing. just because the common names are similar and overlap doesn’t necessarily mean they are at all closely related.
M. leibii itself lives in forests throughout eastern North America, in spotty patches from Canada to Arkansas and Georgia. It is a small bat, weighing only about 5 grams and with a wingspan of less than 10 cm. Unusually for its size, it is long lived, living as long as 12 years and tolerates the cold better than most other bats, so spends less time in hibernation than other bats.
The fossil record of M. leibii is sparse, although the fossil record for Myotis in general is fairly good for bats. According to molecular data, the genus Myotis first appeared roughly 16 Mya, with the North American clade splitting off no more than 9 Mya. However, the actual fossil data indicates Myotis is far older, with the earliest known Myotis fossil being 33 Mya to the earliest Oligocene, although in North America, the record only extends to the late Miocene no more than 23 Mya. Interestingly, the fossil record for M leibii demonstrates a range far greater than the current distribution, with fossils being found as far as Oregon. In Arkansas, fossils are limited to one spot, which happens to be the same spot Eptesicus has been found: pleistocene deposits within the Conard fissure. If one looks in the original publication of Conard Fissure by Barnum Brown, one will find Vespertilio fuscus and Myotis subulatus, but both of those names have been changed in the intervening 110 years, to Eptesicus fuscus and Myotis leibii.
It is long past time I resurrected Monday Mystery fossils. So to celebrate the season, here is a little animal whose relatives, or at least representations thereof, shall be widely seen over the next month.
If you think you know what this is, please leave your identification in the comments. I will let everyone know what it is and where fossils like this have been found in Arkansas on Friday. Have a great October!