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[dinosaur] Tetrapod jaw from "Romer's Gap" from Scotland




Ben Creisler
bcreisler@gmail.com


A new paper


Donglei Chen, Yasaman Alavi, Martin D. Brazeau, Henning Blom, David Millward & Per E. Ahlberg (2018)
A partial lower jaw of a tetrapod from "Romer's Gap".
Earth and Environmental Science Transactions of The Royal Society of Edinburgh (advance online publication)
doi: https://doi.org/10.1017/S1755691018000099
https://www.cambridge.org/core/journals/earth-and-environmental-science-transactions-of-royal-society-of-edinburgh/article/partial-lower-jaw-of-a-tetrapod-from-romers-gap/258CE8B8F24B8D2C0F9741253A9E59F8


The first half of the Mississippian or Early Carboniferous (Tournaisian to mid- VisÃan), an interval of about 20 million years, has become known as "Romer's Gap" because of its poor tetrapod record. Recent discoveries emphasise the differences 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 VisÃan 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. It represents a small but significant step in the gradual closure of "Romer's Gap".