Pachytheca.

Pachytheca was a very primitive sphere-shaped plant that lived from the late Silurian to the early Devonian. Pachytheca is related to Prototaxites, so my hypothesis is that it is a fungus. It was up to 6mm long. 


This organism had a prominent outer layer. It's possible that in juvenile specimens this layer was thicker, and on the adults it was thinner. The structure of Pachytheca was made up of tubes. Thicker, stronger tubes on the outer layer, and thinner, more brittle ones on the inner layer. 

Some of the fossils look like marbles, while others look like tiny geodes. At first the organism was thought to have been a piece of a bigger plant or the tooth of a fish. Then it was incorrectly classified as an alga. 

Pachytheca and Prototaxites are now classified as Nematophytes, enigmatic organisms that were either plants or fungi. Pachytheca has been found mostly in western Europe, but they also lived in places such as Canada and Australia. 

References:

http://steurh.home.xs4all.nl/engpach/epachy.html

http://www.chertnews.de/Pachytheca.html

http://de.wikipedia.org/wiki/Pachytheca