Ichthyosaurus communis (Conybeare 1822, Jurassic, up to 2m in length) had a relatively large skull. Other specimens attributed to this genus (at right) had a shorter rostrum and taller cranium.
Ichthyosaurus is a sister to Ophthalmosaurus in the large reptile tree (see below) and was derived from a sister to Mixosaurus and Phalarodon. All had toothy skulls.
The lacrimal contacted the naris. The maxilla did not contact the naris. Extra medial digits appeared on the forefin. The metatarsals and phalanges were compact and tiled. The pelvis was reduced to three slender elements.
Note the differences in the two interpretations of the I. communis skulls in these two images.
Ichthyosaurus had a dorsal fin and a deep caudal fin with bent caudal vertebrae according to skin impressions.
The skulls at right have not been subject to phylogenetic analysis.