Brittagnathus is an extinct genus of four-limbed vertebrate ("tetrapod") from the Late Devonian of Greenland. It contains a single species, Brittagnathus minutus, which is based on a complete lower jaw recovered from an Acanthostega bonebed in the Britta Dal Formation. It is the fourth named genus of "tetrapod" (more precisely a stem-tetrapod or stegocephalian) from the Late Devonian of Greenland, after Ichthyostega, Acanthostega, and Ymeria.
The jaw is only 4.5 cm (1.8 in.) long, with the total length estimated at 25 cm, making Brittagnathus the smallest known Devonian "tetrapod". A phylogenetic analysis places it among Carboniferous stegocephalians, particularly the small whatcheeriid Pederpes, rather than the contemporary Devonian stem-tetrapods. This provides support for an origin for Carboniferous-type tetrapods as early as the Devonian.
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