A partial lower jaw of a tetrapod from “Romer's Gap”

The first half of the Mississippian or Early Carboniferous (Tournaisian to mid-Visean), an interval of about 20 million years, has become known as ‘‘Romer’s Gap’’ because of its poor tetrapod record. Recent discoveries emphasise the difference between pre-‘‘Gap’’ Devonian tetrapods, unambiguous stem-group members retaining numerous ‘‘fish’’ characters indicative of an at least partially aquatic lifestyle, and post-‘‘Gap’’ Carboniferous tetrapods, which are far more diverse and include fully terrestrial representatives of the main crown-group lineages. It seems that ‘‘Romer’s Gap’’ coincided with the cladogenetic events leading to the origin of the tetrapod crown group. Here, we describe a partial right lower jaw ramus of a tetrapod from the late Tournaisian or early Visean of Scotland. The large and robust jaw displays a distinctive character combination, including a significant mesial lamina of the strongly sculptured angular, an open sulcus for the mandibular lateral line, a non-ossified narrow Meckelian exposure, a well-defined dorsal longitudinal denticle ridge on the prearticular, and a mesially open adductor fossa. A phylogenetic analysis places this specimen in a trichotomy with Crassigyrinus and baphetids higher tetrapods in the upper part of the tetrapod stem group, above Whatcheeria, Pederpes, Ossinodus, Sigournea and Greererpeton .Itrepresents a small but significant step in the gradual closure of ‘‘Romer’s Gap’’.