Chondrosteus

Chondrosteus ("cartilage bone" in Greek) is a genus of extinct marine actinopterygian (ray-finned fish) belonging to the family Chondrosteidae. It lived during the Hettangian and Sinemurian (early Early Jurassic) in what is now England. Chondrosteus is related to sturgeons and paddlefishes as part of the clade Acipenseriformes, and is one of the earliest known definitive members of the group. Similar to sturgeons, the jaws of Chondrosteus were free from the rest of the skull (projectile jaw system). Its scale cover was reduced to the upper lobe of the caudal fin like in paddlefish. It is represented by a single species, C. acipenseroides.

== History and naming == The first mention of material that would be assigned to Chondrosteus was in the last paragraph of "Recherches sur les Poisson Fossiles", a paper written by Louis Agassiz in 1884. The specimen (UMO J.3064) was made up of a portion of the caudal skeleton found at Lyme Regis and was named Chondrosteus acipenseroides based on similarities in the shape of the caudal fin. The genus would not be mentioned again until 1858 when Sir Philipde Malpas Grey Egerton would comment that Agassiz had not given a proper description of the material with him describing other specimens attributed to the genus since the original naming. Along with the description of the material, Egerton would also name two new species being C. pachyurus and C. crassior. After this publication, a number of papers were published describing the anatomy of Chondrosteus with the latest one being by Woodward in 1895. One of these papers, by Davis in 1887, argued the validity of C. crassior due to the presence of ossifications that are more likely indicative of an ontogenically based change rather than a difference between species. Though Traquair was critical of the paper by Davis, Woodward would agree with the synonymy of the two species in 1895 though suggested that C. pachyurus was still its own species. It wouldn't be until 2009 that Eric J. Hilton and Peter L. Forey would publish a redescription of the type species. During the redescription, they also looked at the only specimen of C. pachyurus concluding that it was also synonymous with the type species. This paper not only was the first redescription of the material but also agrees with previous ideas that the family Chondrosteidae is sister to all other Acipenseriformes.