Moss among three divisions: Bryophyta, Marchantiophyta and Anthocerotophyta. The current circumscription of Bryophyta includes only the mosses. The mosses, now division Bryophyta (Bryophyta sensu stricto), are divided into eight classes: Six of the eight classes contain only one or two genera each. Polytrichopsida includes 23 genera, and Bryopsida includes the majority of moss diversity with over 95% of moss species belonging to this class. The Sphagnopsida, the peat-mosses, comprise the two living genera "Ambuchanania" and "Sphagnum", as well as fossil taxa. However, the genus "Sphagnum" is a diverse, widespread, and economically important one. These large mosses form extensive acidic bogs in
Moss Antarctica and Russia, and a case is put forwards for Carboniferous mosses. It has further been claimed that tube-like fossils from the Silurian are the macerated remains of moss calyptræ. Mosses also appear to evolve 2–3 times slower than ferns, gymnosperms and angiosperms. Recent research show that ancient moss could explain why the Ordovician ice ages occurred. When the ancestors of today's moss started to spread on land 470 million years ago, they absorbed CO from the atmosphere and extracted minerals by secreting organic acids that dissolved the rocks they were growing on. These chemically altered rocks in turn reacted