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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.
On Monday, I said that the mystery fossil this week takes us back to the beginning and ties us to the present. I also said the answer was written in stone. Could you figure it out? Katharyn D. was the first to get it this week.
The picture shows a graptolite. This particular one is likely a species of Cyrtograptus. Most specimens of this type of graptolite are from Europe, they do appear in Canada and parts of the United States, including Oklahoma, according to Fossilworks.org. Did it appear in Arkansas? We don’t know. Other graptolites did, but then the book of Arkansas graptolites has not yet been written, so the true diversity of graptolites within the state is not really known.
So what are graptolites? The name means “written in stone” because they reminded people of hierogylphs or petrographs, writing or pictures scratched into stone. The first graptolites I remember seeing in the Quachita Mountains reminded me of nothing more than pencil scratches. Others, better preserved, look like saw blades or little tubes. For a long time, no one really knew what to make of them, regarding them as cnidarians, or plants, or even inorganic mineral formations. But with the advent of electron microscopy, most workers have come to the conclusion they are actually members of the group Hemichordata. This conclusion puts them at the very beginnings of all vertebrates. Vertebrates evolved from a group called urochordates, the first animals with a stiffened rod for support, an ancestral spine. Today, urochordates are tiny animals called tunicates, or sea squirts. But even before these animals evolved, there were the hemichordates, meaning “half-chordate.” They share branchial opening, or “gill slits,” a collar-like pharynx, and the beginnings of a notochord, called a stomochord. The main hemichordates alive today are called enteropneusts, or acron worms. If you go any earlier than this, you find yourself in echinoderms.
Graptolites appeared in the Cambrian Period over 500 million years ago and were thought to have died out in the Mississipian Period (Early Carboniferous), roughly 320 million years ago. They are useful as index fossils, meaning they can be used to date rocks with a fair degree of precision, because they have a worldwide distribution and are common in the rocks, as well as evolving quickly so they have many species, many of which did not last long.
Graptolites started off growing on the ocean floor, but later ones floated freely in the ocean. When they died, they would sink to the bottom, becoming entombed in the deep ocean sediments. So today, graptolite fossils are usually found in shale formed from the deep ocean mud. Sometimes they can be found in marine limestone or chert, but they are less commonly found there. As a result, the best place to find them is in the shales of the Ouachita Mountains in west-central Arkansas. The best place to find them is in the Womble Shale, a black shale with thin limestone layers and a few small silty sandstone and chert layers. The Womble Formation was named after the town of Womble, which is now Norman, in Montgomery County, AR, so that makes that area a fine place to look. You can also find conodont fossils commonly in those rocks, which are tiny, tony, early chordates. Conodonts were the first animals in the vertebrate lineage with mineralized tissue. They had teeth, but no bone. Other places you can find them are the Arkansas Novaculite in the Ouachitas, although you will have better luck in the older rocks, such as the Mazarn or Collier Shales. You can also find them in any of the Ordovician or Silurian aged Ozark limestones in northern Arkansas, but they will be harder to find as those rocks were formed in shallower water, with many more fossils of many other animals which are far more common, whereas the graptolites will be much rarer than in the Ouachitas. For a full listing and description of the appropriate rocks and maps to their location, try the Arkansas Geological Survery website here.
I mentioned that graptolites were thought to have died out in the Mississippian Period. That is because no fossils are found after this date. However, it is thought by most workers that graptolites may still be living today. We just call them pterobranchs, another type of hemichordate and are acorn worm-like animals with plant-like fronds used to filter out plankton from the water.