Classification
Hornworts were traditionally considered a class within the division Bryophyta (bryophytes). However, it now appears that this former division is paraphyletic, so the hornworts are now given their own division, Anthocerotophyta. The division Bryophyta is now restricted to include only mosses.
Traditionally, there is a single class of hornworts, called Anthocerotopsida, or older Anthocerotae. More recently, a second class Leiosporocertotopsida has been segregated for the singularly unusual species Leiosporoceros dussii. All other hornworts remain in the class Anthocerotopsida. These two classes are divided further into five orders, each containing a single family.
Among land plants, hornworts appear to be one of the oldest surviving lineages; cladistic analysis implies that the group originated prior to the Devonian, around the same time as the mosses and liverworts. There are only about 100 species known, but new species are still being discovered. The number and names of genera are a current matter of investigation, and several competing classification schemes have been published since 1988.
Read more about this topic: Hornwort