A previous blog on Devonian tetrapods remarked on their aquatic lifestyles and noted their suite of mosaic characters that make discussion of evolutionary trajectories highly speculative. Few fossils from the lower Carboniferous were known, but the diversified forms from the middle and upper Carboniferous were clearly components of terrestrial faunas. So, we find a group of aquatic amphibians in the Upper Devonian and a diversified group of terrestrial amphibians in the middle-upper Carboniferous. The puzzle is the lack of any terrestrial fossil material in the lower Carboniferous, leaving evolutionary palaeontologists little or no data to work with. The absence of evidence has been so noticeable that this part of the record has been labelled Romer’s Gap (after the distinguished American palaeontologist from the last Century). In 2006, Ward et al. proposed an explanation for the lack of terrestrial fossils that invoked low concentrations of atmospheric oxygen. This, they surmised, inhibited the evolutionary development of ecosystems on land. Since that paper, more discoveries have been made in Scotland in rocks representing Romer’s Gap, and “a wealth of new tetrapod and arthropod fossils” have been recovered. The inference can be made that the Romer’s Gap ecosystems were not impoverished but, for various reasons, only recently have palaeontologists discovered the evidence needed to warrant this conclusion.
“Rather than beginning immediately following “Romer’s Gap”, we can now test the hypothesis that diversification and terrestrialization of tetrapods had been taking place during the 15 or more million years that it represents. Our discoveries and other recent new records from elsewhere certainly suggest that many tetrapod lineages have their origins much earlier than previously appreciated, and their earliest appearances may well be extended back in time as the result of further research.”
For more, go here.