I'm a little sat on the fence regards this fossil... can we have other photographs?, try moistening the fossil, with regards increasing its contrast etc? I'd feel happier to think millipede if there were signs of small nodular structures on the surface of raised segments ie. spiracles, marking the entrances to the tracheal system of the 'respiratory tract'. there does not seem to be any indication of limbs (see UK Arthropoda thread of recent) but they do not always fossilise, being so fragile and soft. There are generic millipedes from Mazon creek in the US where no limbs appear in the nodules, and they do have similarities to your fossil (see photo below). As to plant material - Upper Carboniferous sphenopsids like Calamites are a possibility - albeit terminal spikes of material, before the Annularia and Asterophyllites leaves start to appear..???? Try contrast photographs, side lighting etc to see if any indications of raised segments, with spiracles, etc Cheers Aron Mazon creek - generic genus Euphorberia sp.
"I think I could turn and live with animals, they are so placid and self contained..."