A study of the Ichthyostega genus of extinct animals This detailed fossil study takes readers back millions of years to the Devonian Period. The Devonian Tetrapod Ichthyostega is Number 40 within the Fossils and Strata series. The series offers monographs and memoirs in palaeontology and stratigraphy. The international Fossils and Strata features systematic and regional monographs with taxonomic descriptions. The series is owned by and published on behalf of The Lethaia Foundation with collaboration among the Scandinavian countries.