The so-called "Romer's gap" is a 30 million year interval between the appearance of tetrapods in the Devonian about 365 million years ago and until their adaptation to modern amphibian forms by the middle Carboniferous some 335 million years ago. While the early tretrapods have some skeletal similarlities with land vertebrates, their locomotion apparatus was basically aquatic. In a Letter to Nature (Clack, J. A. Nature 418, 72-76 (2002)) of the July 4, 2002 issue of Nature, J.A. Clack describes the first 'Romer's Gap', tetrapod, from the Tournaisian epoch (354-344 million years ago), a transitional fossil showing the first signs of terrestrial locomotion. According to Clack: "Pederpes is the earliest-known tetrapod to show the beginnings of terrestrial locomotion and was at least functionally pentadactyl. With its later American sister-genus, Whatcheeria, it represents the next most primitive tetrapod clade after those of the Late Devonian, bridging the temporal, morphological and phylogenetic gaps that have hitherto separated Late Devonian and mid-Carboniferous tetrapod faunas."

Coates, M. I. & Clack, J. A. in Studies on Early Vertebrates (eds. Arsenault, M., Lelièvre, H. & Janvier, P.) 373-388 (Bulletin du Muséum National d'Histoire Naturelle, Paris, 1995.

Clack, J. A. Nature 418, 72-76 (2002) [PubMed: PMID: 12097908]