A few weeks back (My Bryology Bookshelf III) I had a question about when hornworts were moved from being recognized as a type of liverwort to their own independent lineage. I put this question to my lab-mate Juan Carlos and he came up with this response.
Since the flat thalloid gametophyte looks quite similar between hornworts and some liverworts, they were thought to be to be a type of liverwort. They were definitely thought to be unique due to their sporophyte. Hornwort sporophytes grow from the base and open by two longitudinal slits that start from the top and move toward the bottom. The spores line the entire length of the interior of the sporophyte. Whereas liverwort sporophytes consist of a spore filled capsule atop a thin translucent stalk.
One of the first researchers to publicly recognize the hornworts as their own phylum (Anthocerotophyta) was Rothmaler in 1951. So there's the answer to my question. When were hornworts recognized as a separate lineage?
For more information on hornwort classification check out this scientific article.Raymond E. Stotler and Barbara Crandall-Stotler. 2005. A Revised Classification of the Anthocerotophyta and a Checklist of the Hornworts of North America, North of Mexico. The Bryologist 108:16-26.