Redescription of †Chondrosteus acipenseroidesEgerton, 1858 (Acipenseriformes, †Chondrosteidae) from the lower Lias of Lyme Regis (Dorset, England), with comments on the early evolution of sturgeons and paddlefishes

Abstract

Synopsis We present a redescription of †Chondrosteus acipenseroides, a basal member of the order Acipenseriformes (sturgeons, paddlefishes and their fossil relatives) from the Lower Jurassic of England. †Chondrosteus is known from several whole or partial skeletons and many isolated fragmentary elements. Even on many of the articulated skeleton and skull specimens, many aspects of its anatomy are not sufficiently preserved to allow for detailed descriptions. However, we were able to describe it more fully than previous authors and to clarify many points of anatomy, such as the presence of elements we interpret as rostral canal bones, the absence of a postorbital bone, the sclerotic ring being composed of dorsal and ventral elements, a deep lateral groove on the dentary and the presence of an ossified basibranchial. We then comment on the systematic affinities of †Chondrosteidae within Acipenseriformes on the basis of the results of a new phylogenetic analysis. †Chondrosteidae has alternatively been regarded as the sister‐group of all other Acipenseriformes (i.e. †Peipiaosteidae + Acipenseridae + Polyodontidae) or the sister‐group of Acipenseridae + Polyo‐dontidae. Our new systematic analysis supports the former hypothesis. All Acipenseriformes to the exclusion of †Chondrosteidae is supported by the following synapomorphies: fewer than seven but more than one branchiostegal, the absence of ossified basibranchials, and the absence of teeth on the gill rakers.