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.