There are only a few fossils of liverworts and mosses and there are no fossils of hornworts. This is because the soft tissue of these bryophytes does not fossilize well. The oldest known liverwort fossil is from the late Devonian period, about 350 million years ago. Most botanists believe that they originated long before this.
Some botanists have proposed that there are over 10,000 species of liverworts in the world. A more realistic estimate is about 6,000 species. The number of species may have been overestimated in the past because the morphology of many species is plastic, in that it differs in different environments. This makes identification of liverwort species very difficult, typically more difficult than that of higher plants.
Interestingly, even though liverworts originated several hundred million years before the flowering plants, there are several hundred thousand species of flowering plants but only about 6,000 species of liverworts. The reason for this may be that liverworts rely upon the inefficient mechanism of water-transported sperm for sexual reproduction. Thus, it has been proposed that most species of liverworts rely upon asexual gemmae as a means of reproduction. Asexual reproduction tends to reduce genetic diversity. Since genetic diversity is needed for new species to evolve, the liverworts and other bryophytes may have evolved into a sort of evolutionary dead end.
See also Bryophyte.
Greenaway, T. Mosses and Liverworts. Orlando: Raintree Steck-Vaughn, 1992.
Margulis, L., and K.V. Schwartz. Five Kingdoms. W.H. Free-man and Company, 1998.
Peter A. Ensminger