These are terminal branches from the lycopsid Lepidodendron sternbergii, one of the great scale trees that dominated major global habitats for 40 million years. Forests of these 35-meter-tall plants contributed greatly to the Carboniferous-aged coal deposits of North America. Most scale trees became extinct before the end of the Carboniferous. The scale trees were not typical lycopsids both fossil and extant forms tend to be small and inconspicuous. Find out more about lycopsids or lycophytes here and here. You can see a reconstruction of an entire Lepidodendron (Fig. 5-7) here.
You get bonus points if you noticed that this is not Lepidodendron's first appearance as a Mystery Fossil. Back in 2001 we looked at Lepidodendron's scaly "bark" (Mystery Fossil #7).