Redescriptions of Birgeria acuminata (Agassiz, 1835) and Saurichthys sp. (Actinopterygii) fossils from the British Rhaetian, and redundancy of the genus Severnichthys Storrs, 1994

The latest Triassic Rhaetian Westbury Formation of the Penarth Group in Britain, with several highly fossiliferous bone beds, has been of interest to vertebrate researchers for two centuries. The bone beds were deposited under high energy conditions, resulting in vast quantities of fragmentary and disarticulated vertebrate fossils, including actinopterygian teeth named by Louis Agassiz in 1835 as holotypes of Birgeria acuminata and Saurichthys longidens. Some of these were later misinterpreted as remains of temnospondyl tetrapods, based on enamel infolding (plicidentine) seen in cross sections of the bases of the teeth. They were recognized later to be fossils of large fish, but the two were merged under the name Severnichthys acuminatus, encompassing isolated conical teeth of Saurichthys and robust dentitions of Birgeria. Here, we present a novel Saurichthys rostropremaxilla from the European Rhaetian. This elongate and gracile rostrum highlights inconsistencies with the diagnosis of Severnichthys, providing a definitive confirmation of the invalidity of the genus. We justify the resurrection of Saurichthys sp. and Birgeria acuminata as valid taxa within the Rhaetian, but note that it is typical for multiple species of Saurichthys to be present within a single deposit so S. ‘longidens’ must be treated cautiously. Further, we demonstrate a form of plicidentine in two early actinopterygians for the first time, interpreted as a morphofunctional adaptation to a predatory lifestyle.