Phantomosaurus is an extinct genus of ichthyosaur[1][3] that lived during the late Anisian stage of the Middle Triassic.

[5] Phantomosaurus is known from a partial skull, the lower jaw, some of the vertebrae and ribs (pictured) and the hind fin.

[5] As is common in basal ichthyosaurs, the basioccipital can be divided into two sections, the condylus occipitalis and the area extracondylaris.

In this respect it closely resembles that of Cymbospondylus, and probably had an articulation with the atlas vertebra which was more flexible than some ichthyosaurs.

These are laterally sutured to the opisthotics and extend as far as these bones posteriorly, and about 8 mm in front of the condylus anteriorly.

Part of its base is still attached, indicating that it would have been around 15 mm wide at the most posterior point, and not strongly set off from the basal plate of the parasphenoid.

The basal plate was wider than its length of 27 mm, but the actual width cannot be measured as the right pterygoid is still articulated in the natural position.

They extend anteriorly as canals into the bone and are probably the entrances of the cerebral carotid arteries, although positioned unusually far posteriorly.

As mentioned, they are sutured to the tubera basioccipitalia, and so could not have moved freely, meaning that they were not bones for the transmission of sound in this manner.

The posterior surface of this forms a steeply inclined trough running from the suture to the distal extremity of the paraoccipital process.

The ventral margin of this trough is formed by a narrow ridge which also divides the posterior and anterior surfaces of the bone.

The exoccipital and opisthotic are tightly connected, which means that, in fact, the foramen metoticum is completely enclosed by these two bones.

Apart from this, they were fairly normal for an ichthyosaur, forming two pillars of bone between the basioccipital and the supraoccipital around the foramen occipitale magnum.

This demonstrates the existence of paired ossifications between the supraoccipital and the parietal, which can only be homologous to the postparietals of basal amniotes.

In the articulated state, the pterygoid covered the entire lateral margin of the parabasisphenoid and at least part of the tuber basioccipitale.

[5] In general, the braincase, otic capsules and other parts of the skull more closely resemble basal diapsids such as Youngina than they do other ichthyosaurs, especially later and more derived forms such as Ichthyosaurus and Ophthalmosaurus.