Phantomosaurus is an extinct genus of ichthyosaur that lived during the late Anisian stage of the Middle Triassic. Fossils have been found in southern Germany. It was discovered in 1965 and named in 1997 as a species of Shastasaurus by Sander in the rocks of the Upper Muschelkalk.
More recently, in 2005, the braincase was studied by Maisch and Matzke. They found it to be unique among all known ichthyosaurs in terms of braincase morphology. Despite its close relation to many other ichthyosaurs, in particular Cymbospondylus, Phantomosaurus appears to have a very primitive braincase which resembles other diapsids more than other ichthyosaurs.
Morphology
Phantomosaurus is known from a partial skull, the lower jaw, some of the vertebrae and ribs (pictured) and the hind fin.
Hind fin
The hind fin is very long and slender, with elongated femur, tibia and fibula. These features suggest a long but not particularly powerful limb, which would have generated a reasonable quantity of lift. Presumably the fore fins also generated lift, or the ichthyosaur would have found itself permanently heading downwards.
Vertebrae and ribs
The anterior centrum of each vertebra has two symmetrical hollows in, possibly to aid buoyancy. The vertebrae also have ventro-lateral keels, for reasons unknown. Each zygapophysis is very long and extends a long way posteriorly - their articulatory facets are almost totally horizontal.
Lower jaw
The lower jaw is around 40 cm long and has many conical teeth for spearing fish. On the lateral surface, the articular is anterior to the suprangular.
Skull and braincase
The skull is mostly disarticulated. Bones we have found are: left quadratojugal, postfrontal, supratemporal, postorbital, basioccipital and both tubera basioccipitalia, the parabasisphenoid, the otic capsule containing both opisthotic and prootic bones, exoccipital, supraoccipital and pterygoid.
Quadratojugal
This is very similar in shape to the quadratojugals of other large cymbospondylid ichthyosaurs, hence its classification as related to them. It was originally mistaken for the quadrate bone.
Postfrontal, supratemporal and postorbital
The postfrontal and supratemporal together form the border of the upper temporal fenestra, but unusually, the postorbital has no contact with this fenestra. Again, it is similar to Cymbospondylus and not to Shastasaurus or Mikadocephalus in this respect.
Basioccipital
As is common in basal ichthyosaurs, the basioccipital can be divided into two sections, the condylus occipitalis and the area extracondylaris. The condylus occipitalis is flattened dorsoventrally, and was probably concave, with a rather saddle-like shape. 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. Anterior to this part of the bone, the surface is very flat with a slight concavity. The area extracondylaris has high lateral margins which become well-formed tubera basioccipitalia. 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. Each tuber basioccipitalis is 13 mm in length, 10 mm in width, and rises at least 8 mm above the ventral surface of the basioccipital at the highest point. In life they would have been partially covered by the pterygoid, and also by a layer of cartilage. They would not have been attached to much of the otic capsule.