According to Clack et al. 2016
“The term ‘Romer’s Gap’ was coined for a hiatus of approximately 25 million years (Myr) in the fossil record of tetrapods, from the end-Devonian to the mid-Mississippian (Viséan).”
This paper starts to fill Romer’s gap
with five new, incomplete taxa. three stem tetrapods and two stem amphibians, suggesting a deep split among crown tetrapods. That conclusion confirms an earlier one first reported here based on: (1) tetrapod footprints in the Middle Devonian, (2) the first appearance of reptiles in the Viséan and (3) the earlier split of microsaurs + amphibians, evidently before the end of the Devonian or at the very origin of the Carboniferouus following the post-Devonian extinction event.
Unfortunately the phylogenetic analyses
of Clack et al, (Fig. 1) fail to separate basal reptiles from microsaurs, fail to nest Gephyrostegus and Silvanerpeton as basalmost reptiles and fail to split basal reptiles into Archosauromorpha and Lepidosauromorpha in or before the Viséan. These problems are in addition to their inability to find accord in their own two published topologies (Fig. 1).
Koilops (Fig. 2) was a basal tetrapod, smaller than most.
Based on chronological and phylogenetic bracketing
we should expect to find amphibian-like reptiles (with amniote eggs) prior to the Viséan in Romer’s Gap, but they are likely to be a minority component. Utegenia, or similar sister, should be found there, along with other basal seymouriamorphs and reptilomorphs.
Clack JA and 14 other authors 2016. Phylogenetic and environmental context of a Tournaisian tetrapod fauna. Nature ecology & evolution 1, 0002 (2016) | DOI: 10.1038/s41559-016-0002