New rhizodontid (Tetrapodomorpha, Sarcopterygii) material from Romer’s Gap (Tournaisian) of the Ballagan Formation (Scotland, U.K.)

The Devonian and Carboniferous periods saw tremendous faunal turnover of early vertebrates. The end Devonian mass extinction, the Hangenberg Event, caused the extinction of ∼44% of jawed fish clades. Sarcopterygians and placoderms were severely affected, with the rhizodont fishes being one of the few sarcopterygian groups that persisted into the Carboniferous. Lower Carboniferous rocks recording the ∼12-million-year period following the Hangenberg Event were historically considered to yield fewer vertebrate fossils than the Upper Devonian. Previous studies have debated whether Romer’s Gap resulted from taphonomic or sampling bias; nevertheless, our knowledge of rhizodont fishes during this critical time period is scarce. This obscures our knowledge of rhizodontid evolution, diversity, and disparity beyond the Devonian. Recent fieldwork on the Ballagan Formation, Scotland, U.K., has identified many new vertebrate taxa, including well-preserved new rhizodontid material. Preliminary results from new material show at least two morphological groups (referred to here as ‘morpho groups’) existed in the Ballagan Formation ecosystem. Herein, we provide initial descriptions of one block of material, but more than 10 large blocks from the site are still being prepared and investigated. In this report we describe preliminary features of the largest and most complete specimen, which will ultimately represent the earliest and most completely known taxon from the Carboniferous once all material is fully described. The results from this preliminary investigation alone illustrate that large predators co-existed in this environment, suggesting that even by the earliest Tournaisian, some ecosystems were thriving relatively soon after the Hangenberg event.