Chondrosteus

The skull roof of Chondrosteus was highly ornamented with this ornamentation ranging from a large amount of denticles which make the surface of the bone look like course sandpaper to a texture described as an "irregular honeycomb". This ornamentation is the heaviest on the sides and back of the skull roof along with the back of the opercular bones and the sides of the pectoral girdle. Though the skull roof is usually poorly preserved in specimens, the frontal, dermopterotic, and parietal bones largely make up the skull roof with the dermopterotic bones being the largest. The parietal and dermopterotic bones make contact with the frontals towards their front and the extrascapular series towards the back. Unlike in living acipenserids, Chondrosteus lacked dorsal postorbitals with the dermosphenotic being in direct contact with the L-shaped jugals. The snout region of specimens is usually poorly preserved, due to this, the bones that would have been in front of the frontals have not been preserved in any specimen. There are rostral canal bones present in front of the jugal bone which are flatter than what is seen in other acipenseriforms. Also unlike other acipenseriforms, the eye of Chondrosteus would have been completely surrounded by four sclerotic bones with a few other members only having isolated ossifications near the eye.^[3]

The jaws of Chondrosteus lacked teeth with the upper jaw largely being made up of the palatopterygoid with there being no evidence of the quadrate and autopalatine seen in living acipenseriforms. The lower jaw is largely made up of the edentulous dentary with the whole jaw being robust. Though there are no teeth present, there is a groove on the sides of the dentary which was potentially where the mandibular sensory canal was housed. Gill rakers are preserved in the specimen with a number of small teeth. The only other presence of teeth specimens being on toothplates which are found in association with the ceratobranchials and epibranchial. The exact position of these tooth plates in life is unknown.^[3]

Chondrosteus was a large fish with the largest measured specimen having a total length of around 1,100 millimetres (43 in) though specimens once attributed to C. pachyurus are much larger. Outside of a single specimen (BMNHP.3366), the only preserved parts of the vertebrae are the neural arch and supraneural series. On the other specimen haemal elements near the anal fins and neural arches are preserved over the entire length of the fish with the largest being preserved near the dorsal fin. The pectoral girdle is preserved in multiple specimens though they lack the interclavicle, this being most likely due to the preservation of the specimens. Though it is similar to the pectoral girdles of other acipenseriforms, the postcleithrum of Chondrosteus was triangular rather than stick-like. The pectoral fins of the fish are triangular and made up of 59 fins rays with a small fin spine right in front of them. The pelvic region of Chondrosteus is poorly preserved with the fish lacking an ossified pelvic bone, similar to living acipenseriforms. The pelvic and dorsal fins of the fish are located nearly parallel to one another with the base of the dorsal fin being slightly further up than the pelvic fins. The dorsal fin is triangular though concave in the back and has a long base with it being around the length of the longest dorsal fin length. The anal fin is much further back than the dorsal fin and has a more narrow base, being around 70% of the length of the dorsal fin base. This fin forms a caudal peduncle, being very close to the lower lobe of the caudal fin. The caudal fin is overall similar to other acipenseriforms with the upper and lower lobes being around the same length. Like sturgeon and more basal ray-finned fish, this fin's dorsal margin processes fulcra.^[3]

The body of Chondrosteus was mostly scaleless though the pectoral girdle does share similar ornamentation to the skull. Though not well preserved, the lepidotrichia of the fins could have also been ornamented with spiny tubercules similar to relatives. Like other acipenseriforms, the upper lobe of the caudal fin would have processed a set of rhombic scales.^[5]

Since the original description of the genus, the relationship between it and sturgeon was noticed by multiple authors with authors like Traquair calling it an "Acipenseroid" fish. He would place the family containing Chondrosteus into Ganiods with it being closely related to "Spatularidae" and Palaeoniscidae.^[4]

In the most recent analysis of the genus by Hilton & Forey (2009), the placement of Chondrosteus within Acipenseriformes was tested based on both living and extinct members of the order. Based on a number of features, the fish was found to be sister to all other members of the group. This separation was due to differences in the anatomy and composition of the gills that are most similar to earlier groups of ray-finned fish. Along with this, Chondrosteus lacks the rounded scales near the pectoral girdle that is seen in the modern members and Peipiaosteus.^[3]